Organic Functional Groups

This section introduces you to the fascinating world of organic chemistry, focusing on the structure, bonding, properties, and everyday uses of key organic compounds such as: alkanes, alkenes, alkynes, benzene, alkanols, alkanoic acids.

KEY IDEAS

• Alkanes are saturated hydrocarbons with single sigma (σ) bonds; general formula CₙH₂ₙ₊₂

• Alkanoic acids are organic compounds that contain a carboxyl group (− COOH); general formula CₙH₂ₙ₊₁COOH.

• Alkanols are hydrocarbons with one or more hydroxyl (−OH) groups attached.

• Alkenes are hydrocarbons that contain at least one double bond; general formula CₙH₂ₙ.

• Alkynes are hydrocarbons that contain at least one triple bond; general formula CₙH₂ₙ₋₂.

• Benzene a cyclic hydrocarbon with alternating double bonds.

Alkanes are hydrocarbons with carbon atoms connected by single bonds only.

They are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. One common use of alkanes is as fuel.

Naming a Straight-Chain Hydrocarbon

1. Determine the parent name by identifying the longest carbon chain in the compound. Refer to Tables 8.1 and 8.2 to determine the parent name. Notice that these names are made up of a prefix related to the number of carbons in the chain and the suffix -ane, indicating that the molecule is an alkane (Table 8.2).

Table 8.1: Names and Formulae of the First Ten Straight-Chain Alkanes Name Molecular Formula Condensed Formula Alkanes CₙH₂ₙ₊₂ Methane CH₄ CH₄ Ethane C₂H₆ CH₃CH₃ Propane C₃H₈ CH₃CH₂CH₃ Butane C₄H₁₀ CH₃CH₂CH₂CH₃ Pentane C₅H₁₂ CH₃CH₂CH₂CH₂CH₃ Hexane C₆H₁₄ CH₃CH₂CH₂CH₂CH₂CH₃ Heptane C₇H₁₆ CH₃CH₂CH₂CH₂CH₂CH₂CH₃ Octane C₈H₁₈ CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₃ Nonane C₉H₂₀ CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₃ Decane C₁₀H₂₂ CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₃

Table 8.2: Carbon Chain Length and Prefixes Used in the IUPAC Nomenclature System Carbon Chain Length Prefix Alkane Name 1 Meth- Methane 2 Eth- Ethane 3 Prop- Propane 4 But- Butane 5 Pent- Pentane 6 Hex- Hexane 7 Hept- Heptane 8 Oct- Octane 9 Non- Nonane 10 Dec- Decane

2. Number the parent chain to assign the lowest number to the first substituent encountered.

3. Name and number each substituent on the parent chain, indicating its type and position. F example, it may be one of the halogens [F—(fluoro), Cl—(chloro), Br—(bromo), and I—(iodo)] or an alkyl group (Tables 8.3). In the following examples, the parent chain is highlighted in yellow:

Substituent: 2-Bromo 3-Methyl 4-Ethyl IUPAC name: 2-Bromopropane 3-Methylpentane 4-Ethyloctane

Table 8.3: Names and formulae of the first five continuous-chain alkyl groups Alkyl Group Structure Name

-CH₃ Methyl

-CH₂CH₃ Ethyl

-CH₂CH₂CH₃ Propyl

-CH₂CH₂CH₂CH₃ Butyl

-CH₂CH₂CH₂CH₂CH₃ pentyl

4. Use prefixes (di-, tri-, tetra-, etc.) and assign separate position numbers for occurrences of the same substituent:

2,5-Dibromo 3,5,7-Trimethyldecane 2,5-Dibromohexane Not 4,6,8-Trimethyldecane

5. List substituents in alphabetical order before the parent compound name, separating numbers with commas and numbers from names with hyphens;

halogens are listed before alkyl groups.

2-Bromo-3,3- 4-Ethyl-3-methyloctane Not 3,3-Dimethyl-2-bromopentane Not 3-Methyl-4-ethyloctane

Drawing the Structure of Alkanes

1. Determine the Number of Carbon Atoms (Parent Chain) Use the name of the alkane to determine the number of carbons in the longest chain: Methane: 1 carbon, Ethane: 2 carbons, Propane: 3 carbons, Butane:

4 carbons, and so on for longer chains.

2. Draw a straight line of carbon atoms to represent the parent chain.

3. Attach hydrogen atoms to each carbon until it has four bonds.

4. For branched alkanes, identify the substituents from the name and attach them to appropriate carbon atom.

Types of organic reactions Reactivity of Alkanes Alkanes are not very reactive and do not react easily with acids, bases, oxidants, or reductants under normal conditions.

The unreactive of alkanes is due to the following reasons;

1. Alkanes have saturated bonds, leaving no room for additional bonding.

2. The primary bonds in alkanes are strong, non-polar sigma bonds, making them difficult to break.

3. There is little electronegativity difference between carbon and hydrogen, so alkanes do not attract polar substances.

4. Alkanes lack lone pairs or empty orbitals, which are often needed to initiate reactions.

However, under certain conditions, alkanes can undergo combustion, halogenation and cracking.

Halogenation Halogenation is a substitution reaction where a halogen atom (F, Cl, Br, or I) replaces a hydrogen atom in an alkane.

Conditions Required: Alkanes need light, heat, or a catalyst to react with halogens.

Alkanes do not react in the cold or dark because low temperatures reduce molecular energy, making it hard to overcome the high activation energy.

Light energy breaks halogen molecules into halogen radicals, initiating the reaction.

General Reaction: R−H + X₂→ R−X + HX (Where R is the alkane and X is the halogen).

Chlorination Reaction Conditions

Chlorine (Cl₂) and methane (CH₄) do not react at room temperature in the absence of light.

Reaction begins when the mixture is heated above 100°C or exposed to intense light, releasing heat.

Products Initially forms chloromethane (CH₃Cl) and hydrogen chloride (HCl).

With more chlorine and continued reaction, additional products are formed:

Dichloromethane (CH₂Cl₂) Trichloromethane (CHCl₃, chloroform) Tetrachloromethane (CCl₄, carbon tetrachloride) Initiation Step The covalent bond in Cl₂is broken, forming chlorine free radicals (atoms with unpaired electrons).

These radicals start the chain reaction of chlorination.

Cl − Cl heat or light ⎯⎯⟶Cl. + Cl.

The chlorine radical is highly reactive and removes a hydrogen atom from methane to become stable. This reaction produces HCl and a methyl radical (⋅CH₃).

⋅ Cl + C H₄ → HCl + ⋅ C H₃ The methyl radical is also reactive enough to abstract a chlorine atom from the excess C l₂if present.

H₃C ∙ + C l−C l →H₃C −C l+ C l This is part of the propagation phase of the chlorination chain reaction, which continues until the reactants are consumed or two radicals combine to terminate the reaction.

Fluorine (F₂) reacts with alkanes but releases a lot of energy, making the reaction hard to control. Iodine (I₂) is rarely used because it needs too much energy to start, making it inefficient and difficult to manage.

Combustion of Alkanes

The combustion of alkanes is a chemical reaction in which alkanes react with oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O), along with the release of energy in the form of heat and light.

Types of Combustion

1. Complete Combustion

Occurs when there is enough oxygen available. Produces carbon dioxide and water.

General equation:

Cₙ H₂ₙ₊₂ + (3n + 1_ 2 ) O₂ → n CO₂+ (n + 1) H₂ O + heat where n = number of carbon atoms n = 1, 2, 3, 4 etc Examples C H₄(g)+ 2 O₂(g) → 2 H₂ O(g)+ CO₂(g) + heat C₂ H₆(g)+ 7 _ 2 O₂(g) → 3 H₂ O(g) + 2CO₂(g) + heat

2. Incomplete Combustion

Happens when there is insufficient oxygen. Produces carbon monoxide (CO) or soot (C) along with water.

General equation:

Cₙ H₂ₙ₊₂ +(2n + 1_ 2 ) O₂ → nCO + (n + 1) H₂ O + heat

Example

C H₄(g) + 3/2 O₂(g) → 2 H₂ O(g)+ CO(g) + heat ;

C H₄(g) + O₂(g) → 2 H₂ O(g) + C(s) + heat Carbon monoxide is poisonous because it binds strongly to haemoglobin, preventing oxygen transport to tissues. Alkanes are excellent energy sources because they release a lot of energy when burned.

Cracking of Alkanes

Cracking is a chemical process where long-chain alkanes are broken down into shorter-chain alkanes and alkenes. This is done to make more useful products, such as fuels and chemicals used in industry.

Types of Cracking

1. Thermal Cracking

Uses high temperatures (400–900°C) and high pressure.

Produces a mixture of shorter alkanes and alkenes.

Example

C₁₀ H₂₂ h eat⎯→C₅ H₁₂ + C₅ H₁₀

2. Catalytic Cracking

Uses a catalyst (e.g., zeolites) at lower temperatures (450–750°C).

Produces branched alkanes and aromatic compounds, which are useful in fuels like gasoline.

Example: Heptane is reformed into toluene and hydrogen.

C₁₂ H₂₂ c atalyst ⎯→C₈ H₁₈ + C₄ H₈ Reformation Reformation in chemistry, refers to a process where straight-chain hydrocarbons (alkanes) are converted into branched-chain hydrocarbons, cycloalkanes, or aromatic compounds. This process improves the quality of fuels, such as gasoline, by increasing their octane number.

Catalytic Reformation

Uses a catalyst such as platinum, rhenium, or zeolites.

Operates at high temperature (500–600°C) and moderate pressure.

Example reaction:

C₇ H₁₆ c atalyst, heat ⎯⎯⟶C₇ H₈ + 4 H₂ Specific terminology Electrophile An electrophile is a species (atom, ion, or molecule) that is electron-deficient and seeks to accept electrons. Electrophiles are attracted to electron-rich areas in a reaction.

Electrophile typically have a positive charge or partial positive charge (δ+) or molecule with empty orbitals on the central atom and they act as Lewis acids (electron-pair acceptors).

Examples H+ (proton), NO₂

+ (nitronium ion), CH₃ +, BCl₃.

Nucleophile A nucleophile is a species that is electron-rich and donates electrons to form a bond. Nucleophiles are attracted to electron-deficient areas in a reaction.

Features of nucleophile Have a negative charge or lone pair of electrons.

Act as Lewis bases (electron-pair donors).

Examples: OH− (hydroxide ion), NH₃(ammonia), Cl− (chloride ion) Free Radicals A free radical is a highly reactive species with an unpaired electron. They are formed by the homolytic cleavage of bonds.

Features of free radical are;

Represented with a dot (⋅) next to the element or group.

Highly unstable and reactive because of the unpaired electron.

Can initiate chain reactions in processes like halogenation of alkanes.

Examples ⋅Cl (chlorine radical) ⋅CH₃(methyl radical) Formation Free radicals are formed by heat, UV light, or peroxides, which cause bond homolysis.

Example

Cl₂ UV light ⎯⟶ ⋅ Cl + ⋅ Cl Isomerism Isomerism is a phenomenon where two or more compounds have the same molecular formula but different structures or arrangements of atoms, leading to distinct chemical and physical properties. There are two types of Isomerism:

structural Isomerism and stereoisomerism.

Isomerism in Alkanes

Alkanes, exhibit structural isomerism. This means that alkanes with the same molecular formula can have different arrangements of their carbon atoms.

Steps to determine the number of structural isomers in alkanes

1. Use the general formula for alkanes, CₙH₂ₙ₊₂, to determine the number of carbons (n) and hydrogens (H).

2. Start by drawing the straight-chain (normal) alkane structure. This is the simplest isomer, with all carbons connected in a single continuous chain.

3. Create shorter chains by branching the carbon atoms. Follow these rules:

a. Move one carbon at a time from the main chain to create a side branch.

b. Avoid repeating structures (e.g., moving the branch from the first carbon to the last produces the same structure).

c. Consider the symmetry of the molecule to avoid duplicates.

4. Assign systematic IUPAC names to the structures to ensure they are unique.

For example:

For C₅H₁₂(Pentane):

CH₃−CH₂−CH₂−CH₂−CH₃ CH₃−CH(CH₃)−CH₂−CH₃

CH₃−C(CH₃)₂−CH₃ Physical properties of alkanes Physical Properties of Alkanes (Simplified and Summarized)

1. State at Room Temperature

Alkanes are non-polar compounds that can exist as:

Gases: C₁to C₄(weak Van der Waals forces).

Liquids: C₅to C₁₇.

Solids: C₁₈and above (stronger Van der Waals forces).

2. Melting and Boiling Points

Increase with longer carbon chains due to stronger Van der Waals forces (e.g., butane: −1.0°C, pentane: 36.1°C).

Branching decreases boiling points because it reduces surface area and weakens Van der Waals forces (e.g., pentane: 36.1°C, 2-methylbutane:

27.8°C).

3. Solubility Alkanes are insoluble in water (non-polar) but dissolve in non-polar solvents like alkanes.

Water’s hydrogen bonds exclude alkanes, preventing solubility.

4. Density Alkanes have densities between 0.62 g/ml and 0.79 g/ml, which is less than water (1.0 g/ml).

Low density is due to:

Light atoms (hydrogen and carbon) and the tetrahedral molecular structure, increases volume relative to mass.

Effect of Combustion on the Environment

Complete Combustion: Produces carbon dioxide (CO₂).

Incomplete Combustion: Produces carbon monoxide (CO) in low oxygen conditions.

Impact on Global Warming: CO₂contributes to the greenhouse effect, trapping heat and leading to global warming.

Greenhouse gases absorb and re-radiate infrared radiation, increasing Earth’s temperature.

Uses of Alkanes (Simplified)

1. Gaseous alkanes (C₁to C₄):

Main component of natural gas, used for cooking, heating, and industrial processes. Found biogas from organic matter decomposition.

Propane and butane: Components of Liquefied Petroleum Gas (LPG), used as heating and cooking fuels due to their high energy yield and easy transport in liquid form.

2. Liquid alkanes (C₅to C₁):

Gasoline (Pentane to Octane): Used in cars due to high volatility and energy release.

Kerosene, diesel, and jet fuel (C₉to C₁₇):

Diesel: High energy density for heavy-duty vehicles.

Jet fuel: Designed to remain liquid at low temperatures and provide thrust efficiently.

3. Waxy solids (C₁₈and Above):

Paraffins: Used in wax coatings for fruits and vegetables to retain moisture and mould.

Vaseline: A semisolid hydrocarbon mix for ointments, cosmetics, and lubrication.

4. Specialized uses:

Motor oil: Lubricates engine components.

Mineral oil: Used as a laxative and lubricant due to its insolubility in water.

Activity 8.1 Molecular models and Alkanes Materials needed: Whiteboard, markers, molecular model kits (optional, for hands-on building of alkanes), worksheets with examples of alkanes for naming practice Steps

1. Using the formula CₙH₂ₙ₊₂calculate the molecular formulas for the first ten alkanes.

2. Write the molecular formula and draw the structures of these alkanes on a chart worksheet.

3. Guide to naming alkanes:

a. Identify the longest continuous chain of carbon atoms (parent chain).

b. Number the chain starting from the end nearest to a substituent or branch.

c. Name the substituents and place them in alphabetical order with the appropriate numbers.

4. Practice naming alkanes Name the following:

i. C H₃ C H₂ C H₂ C H₃

ii. C H₃ C H₂ C H₂ C H₂ C H₃

iii. C H₃ CH(C H₃)C H₃

iv. C H₃ C H₂ CH(C H₃)C H₂ C H₂ C H₃

v. C H₃ CHClC H₃

Activity 8.2

Materials Needed: Molecular model kits or 3D printed structures, charts or drawings illustrating electrophiles, nucleophiles, free radicals, types of reactions, bond cleavage, and isomerism, Flashcards with examples for discussions.

Steps

1. Engage in a whole class discussion and identify which of the following structures (a), (b) and (c) is saturated and unsaturated hydrocarbons.

(a) (b) (c)

2. Explain why your chosen structure is saturated or unsaturated.

3. Name compound A.

4. Identify electrophile or nucleophile or free radicals from the species below:

C₂ H₅ O−, (C H₃)₃ N : , Cl+, H₃ C .

, C H₃ − C+ = O, N+ O₂

5. Identify the type of reaction and complete reaction:

a. CH₄+ 2O₂ ⟶

b. CH₄+ Cl₂ UV light ⎯⎯⟶

c. C₁₀ H₂₂ ⟶ C₆ H₁₄+

6. Why are alkanes less reactive under normal conditions?

7. Diagram (a) and (b) illustrate homolytic and heterolytic cleavage respectively (a) (b)

a. What would methane produce in homolytic cleavage?

b. What would methane produce in heterolytic cleavage?

8. How does adding a functional group change the properties of an alkane?

9. Use model kits to construct different isomers of C₄H₁₀(butane) and C₅H₁₂ (pentane).

Identify and name the isomers.

Activity 8.3 Exploring Isomers and Naming Alkanes

Materials Needed: Molecular model kits (or paper cutouts for atoms and bonds), whiteboard and markers, handouts or flashcards of alkane examples, worksheets for drawing and naming exercises.

Steps

1. Use model kits or cut-outs to build and identify the isomers of butane and pentane.

Compare their structures and write the molecular formulas for each.

2. Draw the structural formula and write the molecular formula for names like:

a. 2-Methylpropane

b. 3-Methylpentane

Activity 8.4 Investigate Physical Properties of Alkanes

Steps

1. Review the chart below of melting points, boiling points, and densities for different alkanes.

Table 8.4: Melting Points, Boiling Points, and Densities of Alkanes Alkane Formula Melting Point (°C) Boiling Point (°C) Densities of Alkanes Methane CH₄ -182.5 -161.5 0.000656 Ethane C₂H₆ -183.3 -88.6 0.00126 Propane C₃H₈ -187.6 -42.1 0.002 Butane C₄H₁₀ -138.4 -0.5 0.00258 Pentane C₅H₁₂ -129.7 36.1 0.626 Hexane C₆H₁₄ -95.3 68.7 0.66 Heptane C₇H₁₆ -90.6 98.4 0.684 Octane C₈H₁₈ -56.8 125.6 0.703 Nonane C₉H₂₀ -51.3 150.8 0.718 Decane C₁₀H₂₂ -29.7 174.1 0.73

2. Analyse the trends in boiling points, melting points and densities and explain using Van der Waals forces.

3. Write the balanced equations for

a. Complete combustion of hexane

b. Incomplete combustion of hexane

4. Engage in a whole class discussion to:

a. Discuss the difference in energy release between complete and incomplete combustion.

b. Discuss the harmful effects of CO from incomplete combustion.

Activity 8.5 Exploring Alkanes and Their Reactions

Material needed: computer, internet, other chemistry textbooks Steps

1. Organise yourselves into groups of no more than five. In your groups:

a. Brainstorm solutions to reduce acid rain.

b. Create posters showing the impact of CFCs on the ozone layer

c. Draw flow diagrams illustrating the mechanism.

d. Research the uses of assigned fractions of petroleum.

2. Present this to your class for discussion and feedback.

Activity 8.6 Mini reports

1. Organise yourselves into groups of no more than five. In your groups, produce a mini report on the following:

2. Outline the reasons why some alkanes will undergo cracking, make sure to include examples and chemical reactions.

3. Outlines the reasons why some alkanes will under reforming, make sure to include examples and chemical reactions.

Alkenes are hydrocarbons with carbon-carbon double bonds. They are found in plant extracts like citrus fruits (limonene), steroids (cholesterol), and insect pheromones (muscalure). Industrially, alkenes are produced by cracking hydrocarbons.

Physical Properties of Alkenes

1. Boiling Points

Alkenes have low boiling points.

Boiling points increase with longer carbon chains and decrease with branching.

2. Density Alkenes have lower densities than alkanes because their double bond reduces molecular packing.

They are less dense than water.

3. Solubility Alkenes are insoluble in water (non-polar) but dissolve in organic solvents.

4. State Alkenes with fewer than five carbons are colourless gases.

Alkenes with longer chains are volatile liquids.

5. General Formula

Alkenes follow the formula CₙH₂ₙand form straight or branched homologous series.

Isomerism in Alkenes

Alkenes show structural isomerism and geometric isomerism:

1. Structural Isomerism: Compounds with the same molecular formula but different atom arrangements.

Types structural isomerism in alkenes:

Position isomerism: The double bond is in different positions.

Chain isomerism: The carbon chain has different arrangements (straight or branched).

2. Geometric isomerism: Arises from restricted rotation around the double bond, resulting in cis (same side) and trans (opposite side) forms.

Position Isomerism

Position isomerism happens when the location of the carbon-carbon double bond changes in the molecule.

Example

The double bond is between the first and second carbon atoms.

CH₂= CH−CH₂−CH₃ 1-Butene The double bond is between the second and third carbon atoms.

CH₃−CH= CH−CH₃ 2-Butene Chain Isomerism in Alkenes Chain isomerism occurs when alkenes with the same molecular formula have different arrangements of carbon atoms. The carbon chain can be straight or branched, while the number of carbon and hydrogen atoms stays the same.

Example

For C₄H₈

1. But-1-ene: A straight chain with the double bond at the first carbon.

CH₂= CH−CH₂−CH₃

2. 2-Methylpropene: A branched chain with the double bond in a different arrangement. CH₂= C(CH₃)−CH₃ IUPAC Nomenclature of Alkenes To name alkenes (up to C), follow these steps

1. Find the longest carbon chain containing the double bond.

Replace the suffix “-ane” (from alkanes) with “-ene.”

For multiple double bonds, use prefixes like di, tri, or tetra (e.g., butadiene).

2. Start numbering from the end closest to the double bond to give the lowest possible numbers to the double bond.

3. If the double bond is equally distant from both ends, start numbering from the end closest to the first branch.

4. Use the lower-numbered carbon of the double bond to specify its position in the chain (e.g.,1-butene, 2-pentene).

Synthesis of Alkenes

Alkenes are important in organic chemistry because they are reactive and useful intermediates. They can be made from:

1. Alcohols Alkenes are produced by dehydrating alcohols using a catalyst like concentrated H₂SO₄heat.

Example

C H₃ C H₂ OH H₂S O₄, heat ⎯⎯⟶C H₂ = C H₂ + H₂ O

2. Synthesis of Alkenes from Alkyl Halides Alkenes are made from alkyl halides through elimination reactions, where a hydrogen halide (HX) is removed.

A double bond forms when;

a. The halogen is removed.

b. The hydrogen is taken from the carbon with fewer hydrogen atoms (following Zaitsev’s rule).

Example

C H₃ C H₂ Br base⎯→C H₂ = C H₂ + HBr Reactions of symmetrical and unsymmetrical alkenes Reaction of Alkenes with Hydrogen Hydrogen gas (H₂) reacts with alkenes under special conditions to produce alkanes. This reaction is called hydrogenation.

Reagent: H₂gas.

Conditions:

Temperature: Above 130°C.

Catalyst: Nickel, platinum, or palladium.

Reaction:

Two hydrogen atoms are added to the carbon-carbon double bond (C= C).

The double bond breaks, forming a saturated alkane.

This reaction is highly exothermic.

For a symmetrical alkene:

C H₃ = C H₂ + H₂ h eat, catalyst ⎯⎯⟶C H₃ − C H₃ For an unsymmetrical alkene:

C H₃ − CH = C H₂ + H₂ heat, catalyst ⎯⎯⟶C H₃ − C H₂ − C H₃ This reaction is very useful in margarine production from vegetable oils which are soft and have a high proportion of cis-alkenes.

a. Reaction of Alkenes with Halogens Halogens (Cl₂, Br₂) are symmetrical reagents that react with alkenes to form dihaloalkanes. This reaction is an example of an addition reaction.

Reagent: Halogen (Cl₂, Br₂).

Conditions Room temperature.

Dark environment.

Polar solvent.

Reaction The halogen molecule adds across the double bond, breaking it and forming a saturated dihaloalkane.

Reaction Representation

For the simplest alkene (ethene): CH₂= CH₂+ Cl₂→ CH₂Cl−CH₂Cl (Ethene reacts with chlorine to form 1,2-dichloroethane) For an unsymmetrical alkene (propene) CH₃−CH= CH₂+ Br₂→ CH₃−CH(Br)−CH₂(Br)

b. Reaction of Alkenes with Steam (Hydration) The reaction of alkenes with steam is called acid-catalysed hydration, where water (H₂O) adds across the double bond to form an alcohol.

Reagent: Steam (H₂O).

Catalyst: Acid (e.g., H₂SO₄or H₃PO₄).

The acid helps dissociate water molecules, which are otherwise too weak to react.

Reaction Water adds across the double bond of the alkene, breaking the double bond and forming an alcohol.

C H₂ = C H₂ + H₂ O acid catalyst ⎯⎯⟶C H₃ − C H₂ OH For an unsymmetrical alkene (propene) C H₃ − CH = C H₂ + H₂ O acid catalyst ⎯⎯⟶C H₃ CHOH − C H₃ Propene reacts with steam to form propan-2-ol, following Markovnikov’s rule: the OH group attaches to the more substituted carbon atom.

c. Reaction of Alkenes with Hydrogen Halides Hydrogen halides (HX, where X is Cl, Br, or I) are unsymmetrical reagents that react with alkenes to form haloalkanes. In this reaction, the double bond becomes saturated by adding hydrogen and the halogen atom.

Reagent: Hydrogen halide (HX, e.g., HCl, HBr, HI).

Reaction: The alkene reacts with the hydrogen halide, breaking the double bond and forming a single-bonded haloalkane.

Markovnikov’s Rule (for unsymmetrical alkenes) The hydrogen atom from HX adds to the carbon with more hydrogen atoms, and the halogen adds to the carbon with fewer hydrogen atoms.

For a symmetrical alkene (ethene): CH₂= CH₂+ HCl → CH₃−CH₂Cl For an unsymmetrical alkene (propene):

CH₃−CH= CH₂+ HBr → CH₃−CH(Br)−CH₃ Propene reacts with hydrogen bromide to form 2-bromopropane, following Markovnikov’s rule.

d. Reaction of KMnO₄ with Ethene Aqueous KMnO₄reacts with ethene at room temperature:

Product: 1,2-ethanediol (HOCH₂CH₂OH), a diol.

Oxidation: Manganese’s oxidation state decreases from +7 (in KMnO₄) to +4.

Observation: The solution changes colour from purple (permanganate) to brown (manganese dioxide, MnO₂).

C H₂ = C H₂ + [O] KMn O₄ ⎯⟶HOC H₂ C H₂ OH Distinguishing Alkanes and Alkenes Using Bromine Water Alkanes (saturated hydrocarbons): Do not react with bromine water under normal conditions because they lack a double bond. The reddish-brown colour of bromine remains unchanged.

Alkenes (unsaturated hydrocarbons): React with bromine water, adding bromine across the double bond. The reaction forms a dibromoalkane and decolourizes the reddish-brown bromine water.

Example: CH₂= CH₂+ Br₂→ CH₂Br−CH₂Br Importance of Alkene Reactions

1. Alkenes are raw materials for making alcohols, polymers, and other organic compounds.

2. Used to create plastics and synthetic materials.

3. Alkenes like butene and propene enhance gasoline combustion and performance.

4. Serve as intermediates in making detergents.

5. Used to produce ethylene glycol, an antifreeze chemical for car radiators.

Activity 8.6 Understanding Alkenes: General, Molecular, and Structural

Formulae Steps

1. Recall and share the general formula of alkenes (Think-Pair-Share).

2. State the molecular formulae of the alkenes with n= 3,4,5 number of carbons.

3. In pairs or small groups, use molecular model kits or paper cutouts to create the structure ethene, propene and butene.

4. Share your model or drawing with the class.

Activity 8.7 Exploring Isomerism, Properties, and Reactions of Alkenes Materials needed: Molecular model kits Steps

1. Write the molecular formula of butene

2. Build models or draw the structural formulae of 1-Butene and 2-Butene.

3. Name the structures CH₃−CH= CH−CH₃and CH₃−CH₂−CH= CH− CH₂−CH₃.

4. Create a mind map of alkenes showing:

Sources of alkenes Laboratory Preparation: Dehydration of alcohols, dehalogenation.

Physical Properties: Low boiling points, insolubility in water, reactivity due to double bonds.

5. Complete and balance equations for reactions with:

a. Hydrogen:

C H₃ − C H₂ − CH = C H₂ + H₂ Ni, catalyst ⎯⎯⟶

b. Halogens:

C H₃ − C H₂ − CH = C H₂ + Br₂ →

c. Steam:

C H₃ − C H₂ − CH = C H₂ + H₂ O H₂S O₄ ⎯→

d. Hydrogen halides:

C H₃ − C H₂ − CH = C H₂ + HCl ⟶

e. KMnO₄ C H₃ − C H₂ − CH = C H₂ + [O] KMn O₄ ⎯⟶ Activity8.8 Distinguishing Between Alkanes and Alkenes Using Bromine Water Materials Needed: Test tubes and test tube rack, bromine water or bromine dissolved in tetrachloromethane (reddish-brown solution), samples of an alkane (e.g., hexane) and an alkene (e.g., hexene), dropper or pipette, gloves and safety goggles.

Steps

1. Label two test tubes: one for the alkane and one for the alkene.

2. Pour a small amount of bromine water or bromine in tetrachloromethane into each test tube.

3. Add a few drops of hexane (alkane) to one test tube and hexene (alkene) to the other.

4. Gently shake or swirl both test tubes to mix the solutions.

5. Observe and record the colour change (or lack thereof) in each test tube.

6. Engage in a class discussion to discuss why the alkene reacted whereas the alkane did not.

7. Write down the reactions that occur.

Activity 8.9 Understanding the economic importance of alkene reactions Steps

1. Identify the products you use that might involve margarine or alcohol.

2. Organise yourselves into groups of no more than five. In your groups, discuss why it is important to produce these products economically.

3. Use the internet and watch videos on:

a. Hardening vegetable oils (margarine) through hydrogenation

b. Hydration process for making alcohol (ethanol)

4. After the Video

a. Discuss what you learned about

i. How margarine is made.

ii. Why is ethanol important in our daily lives

5. Engage in a class discussion to share your insights.

Introduction to Alkynes

Alkynes are hydrocarbons with at least one carbon-carbon triple bond, making them distinct from alkanes (single bonds) and alkenes (double bonds). Their general formula is CₙH₂ₙ₋₂ (n ≥ 2), showing they are unsaturated hydrocarbons with fewer hydrogen atoms than alkanes.

Examples include:

Ethyne (C₂H₂) for n = 2 Propyne (C₃H₄) for n = 3 Butyne (C₄H₆) for n = 4 Alkynes are less common than alkenes in nature and labs. The triple bond includes one sigma and two pi bonds, making it shorter, stronger, and resulting in a linear structure with a bond angle of 180°.

Alkynes are highly reactive due to the electron density in the triple bond, which readily undergoes addition reactions. These reactions are vital in organic synthesis for producing complex molecules.

Structure of Alkynes

In alkynes, the carbon atoms in the triple bond are sp-hybridised. This involves the mixing of one s orbital and one p orbital from each carbon atom, forming two sp hybrid orbitals. These orbitals create a linear structure with a bond angle of 180°, giving alkynes a straight-line geometry. See Figure 8.1.

Figure 8.1: sp hybridisation in alkyne The triple bond in alkynes consists of one sigma bond and two pi bonds:

The sigma bond is formed by the head-on overlap of sp orbitals from each carbon atom.

The pi bonds are created by the side-by-side overlap of the unhybridised p orbitals on each carbon atom. Refer to Figure 8.2.

Figure 8.2: sigma and pi bonds formation in alkyne The triple bond in alkynes is shorter and stronger than single or double bonds, making alkynes highly reactive. Their linear structure influences their properties:

Alkynes are generally non-polar and insoluble in water, but they dissolve in organic solvents.

The triple bond increases reactivity, enabling reactions like hydrogenation and halogenation.

Nomenclature of Alkynes

Alkyne name is derived from the names of the alkanes of the same carbon chain length. The same prefixes are used (meth-, eth-, etc.), but the suffix is different (-yne for alkynes). To determine the name of an alkyne using the IUPAC Nomenclature System, use the following simple rules:

1. Count the number of carbon atoms in the longest continuous carbon chain containing the triple bond (alkynes). Name the alkane with the same number of carbon atoms. This is the parent compound.

2. Replace the -ane ending of the alkane with the ending for an -yne ending for an alkyne. F example:

CH ≡ CH ethyne , CH ≡ C − CH₃ propyne

3. Number the parent chain to give the double or triple bond the lowest number.

For example:

1-pentyne

4. Determine the name and carbon number of each group bonded to the parent alkyne, and place the name and number in front of the name of the parent compound. Remember that with alkynes the triple bond takes precedence over a halogen or alkyl group, as shown in the following examples:

Physical Properties of Alkynes

1. State and Appearance:

The first three alkynes (ethyne, propyne, butyne) are gases at room temperature.

Higher alkynes are liquids or solids.

Alkynes are colourless, and ethyne has a garlic-like odour.

2. Polarity and Solubility:

Alkynes are non-polar.

Slightly soluble in water but dissolve well in organic solvents like benzene, acetone, and chloroform.

3. Boiling and Melting Points:

Increase with molecular weight.

Slightly higher than those of alkanes and alkenes due to stronger London dispersion forces from the triple bond.

4. Density:

Less dense than water.

Density increases with molecular weight but remains lower than corresponding alkanes and alkenes.

Chemical properties of alkynes Alkynes are highly reactive due to the presence of a carbon-carbon triple bond, which makes them susceptible to addition reactions and other transformations.

Below are the key chemical properties of alkynes:

1. Addition Reactions

Alkynes undergo addition reactions where reactants add across the triple bond, reducing it to a double or single bond.

a. With Hydrogen (Hydrogenation) Alkynes react with hydrogen gas (H₂) in the presence of a catalyst (e.g., nickel, palladium, or platinum) to form alkenes or alkanes.

Example:

CH ≡ CH + H₂ catalyst ⎯⟶C H₂ = C H₂ + H₂ catalyst ⎯⟶C H₃ − C H₃

b. With Halogens Alkynes react with halogens (Cl₂, Br₂) to form dihaloalkenes or tetrahaloalkanes.

Example:

CH≡CH + Br₂→ CHBr= CHBr (dibromoethene)

c. With Hydrogen Halides Hydrogen halides (HCl, HBr ) add across the triple bond, following Markovnikov’s rule.

Example:

CH≡CH + HCl → CH₂= CHCl + HCl→CH₃−CHCl₂

d. Hydration of Alkynes Alkynes react with water (H₂O) in the presence of a catalyst, like mercuric sulphate and sulphuric acid, to form ketones or aldehydes.

This reaction follows Markovnikov’s rule.

a. Ethyne (acetylene): Forms ethanal (an aldehyde) upon hydration.

CH ≡ CH + H₂ O H₂S O₄, HgS O₄ ⎯⎯⟶ C H₃ CHO

b. Other Alkynes: Produce ketones

Example: Prop-1-yne forms propan-2-one (acetone) CH ≡ C − C H₃ + H₂ O H₂S O₄, HgS O₄ ⎯⎯⟶ C H₃ COC H₃

e. Combustion Alkynes burn in the presence of oxygen to produce carbon dioxide and water.

Example: 2C₂H₂+ 5O₂→ 4CO₂+ 2H₂O

f. Polymerization Alkynes can undergo polymerization to form polymers. For example:

Formation of Benzene: Three ethyne (C₂H₂) molecules polymerize to form benzene (C₆H₆) 3CH ≡ CH heat, catalyst ⎯⎯⟶C₆ H₆

g. Oxidation Alkynes can be oxidized with strong oxidizing agents like potassium permanganate (KMnO₄) to form carboxylic acids.

Example: CH≡CH + [O] → 2HCOOH

h. Acidic Nature of Terminal Alkynes Terminal alkynes (with a triple bond at the end of the chain) are acidic.

They react with strong bases like sodium amide (NaNH₂) to form alkynide salts.

Example: CH≡CH + NaNH₂→ HC≡CNa + NH₃ Laboratory preparation of alkynes

1. Elimination reactions of dihalides to form alkynes Alkynes can be made from dihalides through a double elimination reaction, where two molecules of hydrogen halide (HX) are removed.

Types of Dihalides

Vicinal dihalides: Halogens are on adjacent carbons.

Geminal dihalides: Halogens are on the same carbon.

Mechanism A strong base like sodium amide (NaNH₂) in liquid ammonia (NH₃) removes a proton next to a halogen.

This creates a double bond (alkene).

The base removes another proton from the alkene, forming a triple bond (alkyne).

Example: C H₂ Br − C H₂ Br NaNH₂/N H₃ ⎯⎯⟶CH ≡ CH + 2HBr

2. Dehydrohalogenation of Alkyl Dihalides

Alkynes can be prepared by removing hydrogen halides (HX) from vicinal or geminal dihalides. This process requires a strong base and high temperatures.

Mechanism The base removes a proton (H+) from a carbon adjacent to the halogen, forming a double bond (alkene).

The base removes another proton, resulting in the formation of a triple bond (alkyne).

Reactants: Vicinal or geminal dihalides.

Conditions: Strong base (e.g., NaNH₂), high temperature.

Product: Alkyne.

Example: C H₂ Br − C H₂ Br NaNH₂, heat ⎯⎯⟶CH ≡ CH + 2HBr

3. Preparation of Alkynes from Alkenes Alkynes can be prepared from alkenes through halogenation followed by dehydrohalogenation.

Halogenation: Halogens (e.g., Br₂) are added across the double bond of an alkene, forming a dihaloalkane.

Dehydrohalogenation: A strong base removes two hydrogen halides (HX) from the dihaloalkane, forming a triple bond (alkyne).

Mechanism

Step 1: CH₂= CH₂+Br₂→CH₂Br−CH₂Br

Step 2: C H₂ Br − C H₂ Br NaNH₂, heat ⎯⎯⟶CH ≡ CH + 2HBr Preparation of Alkynes from Calcium Carbide Alkynes, especially acetylene (ethyne), can be produced from calcium carbide in an industrial process.

Limestone (CaCO₃) is heated to produce calcium oxide (CaO):

CaC O₃ heat⎯→CaO + CO₂ Calcium oxide reacts with carbon (coke) at high temperatures to form calcium carbide (CaC₂):

CaO + 3C heat⎯→CaC₂ + CO Calcium carbide reacts with water to produce acetylene (C₂H₂):

CaC₂+ 2H₂O → C₂H₂+ Ca(OH)₂ Everyday Uses of Alkynes (Simplified) Alkynes are highly useful in various fields due to their reactivity and versatility.

Some common applications include:

1. Fuel and Energy

Acetylene (C₂H₂) is used as a fuel for welding and cutting metals due to its high temperature.

It is also used in portable lighting, like miner’s lamps.

2. Chemical Synthesis

Alkynes are intermediates in making pharmaceuticals, agrochemicals, and dyes, aiding in the production of complex molecules.

3. Polymer Production

Acetylene is used to make polyvinyl chloride (PVC), commonly used in pipes, cables, and clothing.

Other alkynes help in producing plastics like polyethylene.

4. Artificial Ripening

Ethyne is used to ripen fruits artificially, ensuring optimal ripening for the market.

5. Solvents and Additives

Alkynes are used to create solvents and improve the performance of fuels, paints, coatings.

6. Pharmaceuticals Alkynes are used in synthesising active ingredients for drugs, enabling the development of targeted therapies.

Activity 8.10 Understanding Alkynes

Material needed: Chemistry textbooks, diagrams, or internet access Steps

1. Organise yourselves into groups of not more than five. In your groups, brainstorm what you know about alkynes.

a. What makes alkynes unique?

b. How might alkynes be useful in everyday life?

2. Name the following structures:

a. CH≡CH

b. CH≡C-CH₂-CH₃

c. CH₃-CH₂-C≡C-CH₂-CH₃

3. Discuss the structure, acidity, or reactions of alkynes.

4. a. Write the reaction for converting ethyne into ethanol.

b. Identify isomers of C₄H₆.

5. Discuss your findings with the class.

6. Use the internet to watch video on alkynes (e.g., their reactions or real- world uses). In your groups, discuss your insights and thoughts about the video.

7. Use the thoughts and insights from question 5 to write an essay on the role of alkynes in modern technology and medicine.

Activity 8.11 Comparing Alkanes, Alkenes and Alkynes

Organise yourselves into groups of no more than five. In your groups, complete the following and discuss your findings with the class for feedback.

1. Create a Venn diagram comparing and contrasting alkynes, alkanes, and alkenes.

2. Compare the bond angles and lengths in alkynes with those in alkanes and alkenes, and alkanes and explain the differences.

3. Explain the difference is physical properties between the alkanes, alkenes and alkynes. Focus specifically on boiling/melting points and reactivity.

Introduction to Benzene

Benzene is a fundamental organic compound with unique structure and properties, making it essential in organic chemistry.

Natural Sources: Benzene occurs naturally in volcanoes, forest fires, plants, and animals.

Industrial Production: It is produced from coal and oil for commercial use.

Applications: Benzene is found in pharmaceuticals (e.g., aspirin, paracetamol), pesticides, polymers, and dyes.

Discovery of Benzene

1825: Michael Faraday discovered benzene while studying illuminating gas.

1834: Eilhardt Mitscherlich produced benzene by heating benzoic acid with lime.

1845: A.W. von Hofmann isolated benzene from coal tar.

Structure of Benzene

Benzene (C₆H₆) consists of six carbon and six hydrogen atoms arranged in a ring.

Kekulé’s Model (1865): August Kekulé proposed that benzene has a ring of six carbon atoms with alternating single and double bonds.

This structure suggested benzene would behave like alkenes, but it does not.

Refined Understanding

The double bonds are not fixed but resonate between positions, creating a stable structure. Refer to Figure 8.3. This phenomenon, called resonance, explains benzene’s unique stability.

Figure 8.3: Kekule structures of benzene Bonding in Benzene Benzene’s structure is explained by the delocalisation of electrons using modern bonding theories.

Hybridization Each carbon atom in benzene undergoes sp² hybridisation. This combines one s orbital and two p orbitals to form three sp² hybrid orbitals.

Structure The sp² hybrid orbitals form a trigonal planar arrangement around each carbon atom, with bond angles of 120°. This creates a flat, hexagonal ring structure.

Delocalization The remaining unhybridized p orbitals on each carbon overlap sideways, forming a pi (π) electron cloud above and below the ring.

This delocalization gives benzene its unique stability and aromatic properties.

Figure 8.4: sp² hybridisation in benzene Each carbon atom in benzene uses two of its sp²hybrid orbitals to form σ-bonds with two neighbouring carbon atoms, creating a continuous ring.

The third sp²hybrid orbital forms a σ-bond with a hydrogen atom.

Each carbon atom has one unhybridised p-orbital, perpendicular to the plane of the ring.

These p-orbitals overlap sideways with adjacent p-orbitals, forming a π-electron cloud above and below the ring.

The overlapping π-orbitals result in delocalised electrons, giving benzene its stability and a hexagonal ring structure.

Figure 8.5: Sideways overlap of unhybridised p orbitals to form pi bonds Delocalised π-electrons: The π-electrons in benzene are not confined to specific carbon atoms but are spread evenly across the entire ring.

This creates a continuous π-system, shared equally among all six carbon atoms.

Aromatic stabilization: The delocalisation of π-electrons gives benzene extra stability, known as aromatic stabilisation.

Representation: Benzene is often shown with a circle inside the hexagonal ring, symbolising the equal distribution of π-electrons.

It can also be represented by two resonance structures where the double bonds alternate, but the actual structure is a resonance hybrid.

Bond lengths: Delocalisation results in all carbon-carbon bonds in benzene having the same length, intermediate between single and double bonds.

Implications of Benzene’s bonding

1. Chemical reactivity Benzene’s delocalised π-electrons make it less reactive in addition reactions, as these would disrupt its aromatic stability.

Instead, benzene undergoes electrophilic aromatic substitution, where a hydrogen atom is replaced by another group without affecting the π-system.

2. Aromaticity Benzene is a classic example of an aromatic compound due to its cyclic, planar structure and delocalised π-electrons, which provide extra stability.

Nomenclature of Benzene

Naming benzene derivatives can be complex because of the variety of possible substituents. Key points include:

1. Simple Derivatives

If there is only one substituent, the compound is named by placing the substituent’s name before “benzene” (e.g., chlorobenzene, nitrobenzene).

2. Multiple Substituents

When there are two or more substituents, their positions on the ring are indicated by numbers or prefixes:

• Ortho (o-): Substituents on adjacent carbons.

• Meta (m-): Substituents separated by one carbon.

• Para (p-): Substituents opposite each other.

3. Special Names

Some derivatives have common names, such as toluene (methylbenzene), phenol (hydroxybenzene), and aniline (aminobenzene).

4. When a benzene ring is attached to a larger molecule as a substituent, it is called a “phenyl group” (C₆H₅).

Example

Phenylalanine: An amino acid that contains a phenyl group attached to its structure. See Figure 8.6.

Figure 8.6: Structure of phenylalanine Properties of Benzene Physical Properties of Benzene (Simplified)

1. Appearance Benzene is a clear, colourless liquid with a sweet odour, noticeable even at concentrations.

2. Density Its density is 0.8765 g/cm³ at 20°C, making it less dense than water, so it floats on water.

3. Melting and Boiling Points

Melting Point: 5.53°C (just above water’s freezing point).

Boiling Point: 80.1°C (low, making it easy to vaporise).

4. Solubility Benzene is insoluble in water due to its non-polar nature.

It dissolves well in organic solvents like alcohol, ether, chloroform, and acetone.

5. Flammability Benzene is highly flammable and burns with a sooty flame due to incomplete combustion, requiring careful handling.

6. Refractive Index

Its refractive index is 1.5011 at 20°C, useful for optical applications and checking purity.

Chemical Properties of Benzene

Benzene mainly undergoes electrophilic substitution reactions instead of addition reactions to preserve its stable aromatic ring. Key reactions include:

1. Halogenation Benzene reacts with halogens (e.g., Cl₂or Br₂) in the presence of a catalyst like FeBr₃or AlCl₃.

The catalyst generates the electrophile X+ (halogen atom).

C₆ H₆ + Cl₂ C atalyst Anhy. Al Cl₃ , room temperature ⎯⎯⎯⎯⎯⟶C₆ H₅ Cl + HCl

2. Friedel-Crafts Reactions

Alkylation: A hydrogen atom in benzene is replaced with an alkyl group.

Acylation: A hydrogen atom is replaced with an acyl group.

Both reactions require a catalyst like AlCl₃.

AlCl₃+ Cl₂→ AlCl₄+ Cl and

FeBr₃+ Br₂→ FeBr₄+ Br

Friedel-Crafts alkylation C₆ H₆ + CH₃ CH₂ Cl C atalyst Anhy. AlCl₃ , heat ⎯⎯⎯⎯⟶C₆ H₅ CH₂ CH₃ + HCl Friedel-Crafts acylation C₆ H₆ + CH₃ COCl C atalyst Anhy. AlCl₃ , heat ⎯⎯⎯⎯⟶C₆ H₅ CO CH₃ + HCl

3. Nitration Benzene reacts with a mixture of concentrated nitric acid (HNO₃) and sulphuric acid (H₂SO₄) at 25-60°C to form nitrobenzene.

C₆ H₆ + HNO₃/H₂ S O₄ ⟶ C₆ H₅ NO₂ + H₂ O

4. Combustion (Reaction with Oxygen)

Benzene burns in oxygen to produce carbon dioxide and water, but with a sooty flame due to its high carbon content.

2 C₆ H₆ + 15 O₂ ⟶ 12 CO₂ + 6 H₂ O

5. Halogen Addition (Hexachloride Formation)

In the presence of sunlight, benzene reacts with chlorine to form benzene hexachloride (C₆H₆).

C₆ H₆ + 3 Cl₂ UV light ⎯⎯⟶C₆ H₆ Cl₆

6. Hydrogenation Benzene reacts with hydrogen under high pressure and in the presence of a catalyst (e.g., nickel) to form cyclohexane.

C₆ H₆ + 3 H₂ Ni, heat, pressure ⎯⎯⎯⟶C₆ H₁₂ Uses of Benzene

1. As a Solvent

Industrial Use: Used in making rubber, lubricants, dyes, detergents, drugs, and pesticides due to its ability to dissolve substances.

Laboratory Use: Used as a solvent for chemical reactions and extractions in research.

2. Chemical Production

Plastics and Resins: Benzene is used to produce styrene (for polystyrene plastics) and phenol (for epoxy resins).

Synthetic Fibers: Used to make nylon and other fibres for textiles.

3. Printing Industry

Found in printing inks and cleaning agents, helping to maintain equipment and improve ink quality.

4. Fuel Additive

Added to gasoline to increase octane rating, improving engine performance and reducing knocking (its use is now regulated due to toxicity).

5. Pharmaceuticals Used as an intermediate in the production of drugs and complex molecules.

6. Other Uses

Explosives: Used in manufacturing certain explosives.

Pesticides: Serves as a starting material for making effective pest control agents.

Activity 8.12 Benzene Structure and Stability

Material needed: Computer and internet, visual aids (videos, handouts, diagrams), molecular model kits or online simulations, paper, and colour- coding supplies Steps

1. Use the internet to watch a video about the discovery of benzene and its importance in organic chemistry or use the link below:

https://www.youtube.com/watch?v= xD7Z7SHix-w

2. Organise yourselves into small groups. In your groups, undertake the following. Make notes in your:

3. Discuss the Kekulé structure of benzene (alternating single and bonds).

4. Draw the Kekulé structure of benzene showing alternating single and double bonds.

a. How many carbon-carbon single and double bonds are shown in this structure?

5. Draw two resonance structures of benzene using colour-coded double bonds.

a. Why are these two structures not “real” but rather a representation?

6. Explain why benzene is more stable than expected from Kekulé’s model.

7. Draw the molecular orbital diagram of benzene showing the arrangement of the delocalized π-electrons.

a. How does electron delocalization contribute to benzene’s stability?

b. Benzene does not undergo typical addition reactions like alkenes.

Instead, it prefers substitution reactions. Why does benzene resist addition reactions despite having π-bonds?

8. Using the concept of resonance energy, explain why benzene is more stable than cyclohexatriene. (Hint: Resonance energy is the difference between the actual energy of benzene and the theoretical energy of its Kekulé structure).

9. Compare the enthalpy of hydrogenation of benzene (208 kJ/mol) with cyclohexene (120 kJ/mol for one double bond). Why is the enthalpy of benzene not three times that of cyclohexene? What does this reveal about its stability?

10. Aromaticity is a concept that explains benzene’s structure. According to Hückel’s Rule, a molecule is aromatic if it has 4n + 2 π-electrons.

Show how benzene satisfies Hückel’s rule. Why is this important for its stability?

Activity 8.13 Exploring Benzene’s Substitution Reactions

Steps

1. Use the link below to watch video explaining substitution reactions of benzene:

https://www.youtube.com/watch?v= 9IgmnYooHOQ

2. Use the link below to observe a virtual laboratory platform to simulate benzene reactions with:

a. Halogens (e.g., chlorine, bromine)

b. Concentrated HNO₃(nitration)

c. Concentrated H₂SO₄(sulfonation)

d. Alkyl halides (Friedel-Crafts alkylation)

e. Acyl halides (Friedel-Crafts acylation).

Link to virtual laboratory platform: Electrophilic aromatic substitution Section

3. In small groups, share your findings from the virtual lab and videos.

Focus on questions like:

a. What happens to benzene during each reaction?

b. How does the reaction mechanism work?

c. What patterns do you notice?

d. What is common in all substitution reactions?

e. Why does benzene prefer substitution over addition reactions?

Activity 8.14 Addition Reactions

Materials Needed: Videos or articles explaining benzene’s addition reactions, access to virtual lab simulations, laptops.

Steps

1. Use the link https://www.youtube.com/watch?v= pnr9ylSu64c to watch video on addition reactions of benzene with:

a. Hydrogen in the presence of nickel or platinum catalyst to form cyclohexane.

b. Halogens like chlorine or bromine in sunlight to form hexahalocyclohexanes.

2. Use a virtual lab to simulate benzene’s addition reactions. Link:

Electrophilic aromatic substitution Section

3. Create concept maps to visually organise and summarise benzene’s reactions, including both addition and substitution reactions.

4. Share with your class for discussion and feedback. Refine it if needed, based on the feedback you receive. Research on the real-world uses of benzene. Exchange your findings with a peer.

Introduction and Structure of Alkanols

Alkanols are compounds with a hydroxyl group (-OH) attached to an sp³- hybridised carbon atom. They can be viewed in two ways:

1. As alkanes where one hydrogen atom is replaced by a hydroxyl group.

2. As water molecules where one hydrogen atom is replaced by an alkyl group.

Alkanols follow the general formula CₙH₂ₙ₊₁OH (n ≥ 1). Examples include:

n = 1: CH₃OH (methanol) n = 2: C₂H₅OH (ethanol).

Nomenclature of Alkanols

The naming of alkanols follows IUPAC rules for clarity and consistency.

Basic Steps for Naming Alkanols

1. Find the longest continuous carbon chain containing the hydroxyl group (-OH).

Replace the suffix “-ane” of the parent alkane with “-anol”.

2. Start numbering the chain from the end nearest to the hydroxyl group.

This gives the hydroxyl group the lowest possible number.

3. Identify any substituents (e.g., alkyl groups, halogens).

Name and number them based on their position on the chain.

List substituents in alphabetical order.

4. Combine the position number of the hydroxyl group, the parent chain name, and names/positions of substituents.

Use the suffix “-anol” to indicate the presence of the hydroxyl group.

Example

Give the IUPAC name for each compound

a. CH₃CH₂OH

b. CH₃CH(OH)CH₃

c. CH₃C(OH)(CH₃)CH₃

d. (CH₃)₂C(OH)CH₂CH₂CH(Br)CH₃

Solution

1. CH₃CH₂OH

a. Parent hydrocarbon: Ethane

b. Replace ‘-ane’ with ‘-anol’: Ethanol

c. The hydroxyl group is on the first carbon, so no number is needed.

2. CH₃CH(OH)CH₃

a. Parent hydrocarbon: Propane

b. Number the chain: The hydroxyl group is on the second carbon.

c. Replace ‘-ane’ with ‘-anol’ and indicate the position: Propan-2-ol.

3. CH₃C(OH)(CH₃)CH₃

a. Parent hydrocarbon: Propane

b. Identify and number the longest chain: The hydroxyl group is on the second carbon.

c. Name the substituent: A methyl group is attached to the second carbon.

d. Replace ‘-ane’ with ‘-anol’ and indicate the position: Propan-2-ol.

e. Combine the name of the substituent and the parent name as one name:

2-Methylpropan-2-ol.

4. (CH₃)₂C(OH)CH₂CH₂CH(Br)CH₃

a. Parent hydrocarbon: Hexane

b. Identify and number the longest chain: The hydroxyl group is on the second carbon.

c. Name the substituents: A methyl group is attached to the second carbon and a bromine on the fifth carbon.

d. Replace ‘-ane’ with ‘-anol’ and indicate the position: Hexan-2-ol.

e. Combine the names of the substituent in alphabetical order and the parent name -bromo-2-methylhexan-2-ol Naming of Polyhydroxy Alkanols For alkanols with more than one hydroxyl group:

Use suffixes like “-diol” (2 OH groups) or “-triol” (3 OH groups) to show the number of hydroxyl groups.

Indicate the positions of the hydroxyl groups using numbers.

Example

Ethane-1,2-diol → Two OH groups on carbons 1 and 2.

Propane-1,2,3-triol → Three OH groups on carbons 1, 2, and 3.

Example

1. 1,2-ethanediol HOCH₂CH₂OH Parent hydrocarbon: Ethane Two hydroxyl groups: ‘-diol’ Indicate positions: 1,2-ethanediol.

2. Propan-1,2,3-triol CH₂(OH)CH(OH)CH₂(OH)

a. Parent hydrocarbon: Propane

b. Three hydroxyl groups: ‘-triol’

c. Indicate positions: 1,2,3-propantriol.

Classification of Alkanols

Alkanols are classified based on the number of carbon atoms attached to the carbon bearing the hydroxyl group (-OH). This affects their reactivity and behaviour, especially in oxidation reactions.

1. Primary (1°) Alkanols

The carbon with the -OH group is attached to one other carbon or none (e.g., methanol).

The -OH group is at the end of the carbon chain.

They are the most reactive in oxidation reactions.

Examples:

Methanol (CH₃OH): No carbon attached to the -OH carbon.

Ethanol (CH₃CH₂OH): The -OH carbon is bonded to one other carbon.

2. Secondary (2°) Alkanols

The carbon with the -OH group is attached to two other carbons.

Less reactive in oxidation than primary alkanols.

Examples:

Propan-2-ol (CH₃CH(OH)CH₃): -OH group on the second carbon in a 3-carbon chain.

Butan-2-ol (CH₃CH₂CH(OH)CH₃): -OH group on the second carbon of a 4-carbon chain.

3. Tertiary (3°) Alkanols

The carbon with the -OH group is attached to three other carbons.

They are the least reactive in oxidation because there is no hydrogen on the

-OH carbon.

Examples:

Tert-butanol ((CH₃)₃COH): -OH carbon bonded to three methyl groups.

2-Methylpropan-2-ol ((CH₃)₂C(OH)CH₃): -OH group on a carbon attached to two groups and one ethyl group Properties of alkanols Physical Properties of Alkanols The hydroxyl group (-OH) in alkanols influences their physical properties, including boiling points, solubility, viscosity, and odour.

1. Boiling Points

Alkanols have higher boiling points than alkanes or ethers of similar molecular mass due to hydrogen bonding between hydroxyl groups.

Figure 8.7: hydrogen bonding in methanol

Example: Methanol (CH₃OH) boils at 65°C, while methane (CH₄), with a similar molecular mass, boils at -164°C.

2. Solubility Short-chain alkanols (e.g., methanol, ethanol) are highly soluble in water because they form hydrogen bonds with water molecules.

Solubility decreases as the carbon chain length increases because of the hydrophobic nature of the alkyl group.

Examples:

Methanol, Ethanol, Propanol → Completely soluble in water.

Octanol (C₈H₁₇OH) → Limited solubility due to a long hydrophobic chain.

3. Viscosity Viscosity (resistance to flow) increases as the carbon chain length and molecular weight increase.

Reasons

a. Molecular Weight: Larger molecules have greater surface areas, leading to intermolecular interactions.

b. Intermolecular Forces: Hydrogen bonding between hydroxyl groups increases viscosity.

Trend

a. Shorter alkanols → Lower viscosity.

b. Longer alkanols → Higher viscosity.

4. Odour Alkanols have distinct odours depending on their molecular structure:

Methanol → Sweet, pungent smell.

Ethanol → Alcoholic smell (found in beverages).

Isopropanol → Sharp, medicinal odour.

Higher alkanols → Fruity, heavier odours (e.g., butanol, pentanol).

a. The hydroxyl group influences volatility and sensory perception.

b. Larger molecules are less volatile and produce heavier odours.

Chemical Properties of Alkanols

Alkanols react due to the presence of the hydroxyl group (-OH), which influences their chemical behaviour.

Acidity of Alkanols

Alkanols are weakly acidic because the hydroxyl group (-OH) can donate a proton (H+) to form an alkoxide ion (RO–).

Factors Affecting Acidity

1. Polarity of the O-H Bond

The O-H bond is polar because oxygen is more electronegative than hydrogen. This polarity helps release the proton.

2. Inductive Effects

Electron-withdrawing groups (e.g., halogens) increase acidity by stabilizing the negative charge on the alkoxide ion.

Electron-donating groups (e.g., alkyl groups) reduce acidity by increasing electron density on oxygen, making proton release harder.

3. Resonance Stabilization

Alkanols like phenol (aromatic alcohols) are more acidic because the conjugate base is stabilized by resonance.

Example: Phenol (pKa ~10) is much more acidic than aliphatic alkanols.

The order of acidity for alkanols generally follows:

Order of Acidity

Primary alkanols > Secondary alkanols > Tertiary alkanols Fewer electron-donating alkyl groups in primary alkanols increase acidity.

As the carbon chain length increases, acidity decreases due to the stronger electron-donating effect of alkyl groups.

Example:

Methanol (CH₃OH) > Ethanol (CH₃CH₂OH) > Propanol (CH₃CH₂CH₂OH) Amphoteric Nature of Alkanols Alkanols can show both acidic and basic behaviour in specific reactions.

1. Reaction with Bases

Alkanols react with strong bases like sodium hydroxide (NaOH) under specific conditions to form alkoxides and water.

This occurs when the hydroxyl group (-OH) donates a proton (H+).

Example: R−OH + NaOH → R−O−Na+ + H₂O

2. Reaction with Carbonates and Bicarbonates

Alkanols are much weaker acids than carboxylic acids and typically do not react with carbonates (e.g., Na₂CO₃) or bicarbonates (e.g., NaHCO₃) under normal conditions.

However, carboxylic acids do react with these compounds to produce:

Carbon dioxide (CO₂), Water (H₂O), and salts (alkanoates)

Example: 2R−COOH + Na₂CO₃→ 2R−COONa + H₂O + CO₂ Reaction with Sodium Metal Alkanols react with sodium metal to form alkoxide ions and release hydrogen gas. This reaction demonstrates the weak acidity of alkanols.

Reaction: 2ROH + 2Na → 2RONa + H₂ The hydroxyl group (-OH) donates a proton (H+) to sodium, forming an alkoxide (RONa) and hydrogen gas (H₂).

Oxidation Reactions of Alkanols

The oxidation of alkanols depends on their type (primary, secondary, or tertiary) and is carried out using oxidizing agents like potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) in the presence of an acid (e.g., H2SO4).

Primary alkanols → Oxidize to aldehydes → Further oxidize to carboxylic acids.

Secondary alkanols → Oxidize to ketones.

Tertiary alkanols → Do not undergo oxidation because there is no hydrogen atom on the carbon bearing the -OH group.

Oxidation is an important property used to identify and distinguish between different types of alkanols.

Combustion Reactions of Alkanols

Alkanols burn in oxygen to produce carbon dioxide (CO2), water (H2O), and release heat. This exothermic reaction makes alkanols useful as fuels.

Complete Combustion

Occurs when there is sufficient oxygen.

General equation: Cₙ H₂ₙ₊₁OH + (n + 1/2) O₂→ n CO₂+ (n + 1) H₂ O

Example: Ethanol burns with a blue flame, releasing a large amount of heat.

Incomplete Combustion

Happens when there is insufficient oxygen.

Produces carbon monoxide (CO), carbon (soot), and water instead of CO₂.

It is less efficient and releases less energy.

Reaction with Alkanoic Acids

Alkanols react with alkanoic acids to form esters (alkyl alkanoates) and water.

This process is called esterification.

General Reaction: Alkanol + Alkanoic Acid → Ester + Water Catalyst: The reaction requires concentrated sulphuric acid (H₂SO₄) as a catalyst.

The acid speeds up the reaction.

It also helps remove water, which pushes the reaction forward to form more ester.

Importance: Esterification is used to produce esters, which are important in:

• Fragrances

• Flavourings

• Solvents Preparation of Alkanols in the Laboratory

1. Hydrolysis of Halogenoalkanes:

Halogenoalkanes (alkyl halides) are alkanes with one or more halogen atoms.

Reaction: Heated with aqueous alkali (e.g., NaOH or KOH) under reflux.

Process:

Hydroxide ion (OH–) replaces the halogen atom (nucleophilic substitution).

Reflux ensures the reaction completes.

The alkanol (alcohol) is then distilled.

Example: R−X + NaOH → R−OH + NaX

2. Hydration of Alkenes

Alkenes react with water to form alkanols.

Conditions: Water is added in the presence of an acid catalyst (e.g., H₃PO₄, phosphoric acid).

Process A carbocation intermediate form.

Water is added to produce the alkanol.

Example:C₂H₄+ H₂O → C₂H₅OH Ethene → Ethanol

3. Fermentation of Sugars

Sugars are converted into ethanol and carbon dioxide using yeast.

Process:

Enzyme (zymase) in yeast breaks down sugars.

Reaction: C₆H₁₂O₆→ 2C₂H₅OH + 2CO₂ Ethanol is separated from the mixture by distillation.

Test for Alkanols (Alcohols)

1. Lucas Test

The Lucas test is used to differentiate primary, secondary, and tertiary alcohols based on their reactivity with Lucas reagent (a mixture of ZnCl₂ and concentrated HCl). The reaction produces alkyl chlorides, which cause turbidity (cloudiness).

Procedure Add 2-3 mL of Lucas reagent to a test tube.

Add 1-2 drops of the alcohol to be tested.

Shake the test tube and observe how quickly turbidity forms.

Observations Primary Alcohols:

React very slowly or not at all at room temperature.

No turbidity is seen immediately.

R−CH₂OH + HCl → R−CH₂Cl + H₂O Secondary Alcohols React in 5-10 minutes.

Turbidity forms after some time.

R₂CH−OH + HCl → R₂CH−Cl + H₂O Tertiary Alcohols React immediately.

Turbidity forms instantly.

R₃C−OH + HCl → R₃C−Cl + H₂O

2. Iodoform Test for Alkanols

The Iodoform test is used to identify alcohols with the structure R-CH(OH)- CH₃. This includes ethanol and secondary alcohols with a methyl group next to the hydroxyl group.

Reagents: Iodine (I₂) and sodium hydroxide (NaOH) Procedure

1. Add a few drops of the alcohol to a test tube.

2. Add a few drops of iodine solution.

3. Add sodium hydroxide dropwise until the iodine colour disappears.

4. Warm the mixture gently if needed.

Observation A yellow precipitate of iodoform (CHI₃) forms, with a distinctive antiseptic smell.

Reaction R−CH(OH)−CH₃+ 3I₂+ 4NaOH → CHI₃+ R−COONa + 3NaI + 3H₂O

3. Ester Test

To confirm the presence of an alcoholic group in a compound.

Procedure Add 1 ml of the organic liquid to a clean, dry test tube.

Add 1 ml of glacial acetic acid and 2-3 drops of concentrated sulphuric acid.

Heat the mixture in a water bath for 10 minutes.

Pour the hot mixture into a beaker containing cold water.

Smell the water in the beaker.

Observation A fruity smell confirms the presence of an alcoholic group due to the formation of an ester.

Everyday Uses of Alkanols

1. Household Uses

a. Cleaning Agents: Ethanol and isopropanol dissolve grease and stains, disinfect surfaces, and are used in products like window cleaners and multi-surface cleaners.

b. Personal Care Products: Ethanol is found in hand sanitizers, perfumes, and hairsprays due to its antimicrobial properties, ensuring hygiene and freshness.

2. Medical and Pharmaceutical Uses

a. Antiseptics and Disinfectants: Ethanol and isopropanol are used to clean wounds, disinfect skin, and sterilize instruments.

b. Pharmaceutical Solvents: Alkanols dissolve and extract active ingredients, aiding in the formulation of medications like pills and tinctures.

c. Preservatives: Ethanol ensures the stability and longevity of pharmaceutical products.

3. Industrial Uses

a. Solvents: Used in paints, varnishes, and coatings for smooth application.

b. Fuel Additives: Ethanol improves combustion efficiency and reduces emissions in gasoline. It’s also used as bioethanol, a renewable fuel source.

c. Chemical Synthesis: Alkanols are intermediates in producing esters, ethers, and aldehydes.

4. Food and Beverage Industry

a. Alcoholic Beverages: Ethanol is the main alcohol in beer, wine, and spirits, produced by fermenting sugars with yeast.

b. Flavouring Extracts: Alkanols extract flavours from natural sources for food and beverages.

5. Laboratory and Research Uses

a. Solvents: Useful for dissolving organic compounds in chemical reactions.

b. Cleaning and Sterilization: Ethanol cleans and sterilizes equipment, killing microorganisms and evaporating quickly.

c. Sample Preparation: Helps prepare samples for analysis by dissolving organic compounds.

d. Biomolecule Precipitation: Ethanol precipitates DNA and RNA, aiding in biochemical research.

6. Automotive Uses

a. Antifreeze: Methanol and ethanol lower the freezing point of radiator coolant.

b. Fuel: Ethanol serves as a high-energy, clean-burning fuel for racing cars and rockets.

Activity 8.15 Exploring Alkanols: Formula, Classification, Isomerism, and

Naming.

Steps

1. a. Write down the general formula for alkanols.

b. Write down the formulae for methanol, ethanol, propanol, butanol, pentanol, and hexanol.

2. In pairs compare and discuss your answers.

3. Classify the following alkanols as primary, secondary, or tertiary, based on the carbon atom bonded to the hydroxyl group:

CH₃OH, CH₃CH₂OH, CH₃CH₂CH₂OH, CH₃CH(OH)CH₃,

CH₃CH₂CH(OH)CH₃, (CH₃)₃COH, (CH₃)₂C(OH)CH₃

4. In pairs compare and discuss your examples of primary, secondary, and tertiary alkanols.

5. Draw structural isomers of alkanols for each class.

6. Explain the differences between primary, secondary, and tertiary alkanols, showcasing structural isomers.

7. Review the IUPAC naming rules for alkanols, focusing on:

a. Identifying the longest carbon chain.

b. Numbering the chain to give the hydroxyl group the lowest number.

8. In pairs, practice naming alkanols: CH₃CH₂CH₂OH, CH₃-CH₂-CH₂- CH₂OH, CH₃-CH(OH)- CH₂-CH₃, (CH₃)₂CH-CH₂OH.

9. Draw out the structural formula of each of the alcohols in question 8.

10. In pairs compare and discuss your answers.

11. Write a short essay on the differences between primary, secondary, and tertiary alkanols, including examples and their structural differences.

Activity 8.16 Exploring the Preparation and reactions of Alkanols Materials needed: laptop and internet Steps

1. Use the internet and watch a video on the preparation of ethanol or use the link: https://www.youtube.com/ watch?v= vWBf9s56H7E

2. In small groups, discuss ethanol preparation using both fermentation and hydration methods.

3. Design an experiment to demonstrate the esterification reaction of an alkanol with a carboxylic acid. Conduct the experiment, record your observations, and present your findings to the class.

4. Write a report on the oxidation reactions of primary, secondary, and tertiary alkanols. Include balanced chemical equations and explain the products formed.

5. In your groups, compare and contrast the fermentation and hydration methods, considering:

a. Reactants and conditions.

b. Yield and efficiency.

c. Environmental and industrial impacts.

6. In your groups, explore the physical properties of alkanols, such as:

a. Boiling points (increasing with chain length).

b. Solubility (in water, decreasing with chain length).

c. Density

Activity 8.17 Alcohol Breath Analysers and Biofuels

Material needed: laptop and internet Steps

1. List at least three common uses of ethanol in everyday life. Include pictures or diagrams to illustrate each use.

2. Go online and watch video on breath analysers or use the link:

https://www.youtube.com/watch?v= 3z5nMtqvGks

3. Organise yourselves into groups of no more than five. In your groups, research the mechanism of alcohol breath analysers, focusing on;

a. The redox reaction involving potassium dichromate and ethanol.

b. The chemical changes and colour shift in the reaction.

4. Brainstorm and sketch your own alcohol breath analyser using potassium dichromate.

5. Research the advantages and disadvantages of biofuels, such as ethanol and biodiesel.

Focus on;

a. Energy efficiency compared to fossil fuels.

b. Environmental impact and sustainability.

6. Share your group’s findings and insights with the class in a discussion.

Introduction to Alkanoic Acids

Alkanoic acids, also called carboxylic acids, are organic compounds with a carboxyl group (R-COOH). The carboxyl group contains:

1. A carbonyl group (C= O).

2. A hydroxyl group (-OH) attached to the same carbon atom.

General Formula

The homologous series of alkanoic acids follows the formula: CnH₂ₙ₊₁COOH, where n≥0.

Examples n= 0: HCOOH (methanoic acid) n= 1: CH₃COOH (ethanoic acid) n= 2: C₂H₅COOH (propanoic acid) n= 3: C₃H₇COOH (butanoic acid) Properties Weak Acids Alkanoic acids partially dissociate in water, meaning they do not fully release hydrogen ions (H+).

This explains their moderate conductivity in solution and reactivity with bases to form salts and water.

Acidity Factors

The presence of electron-withdrawing groups (e.g., halogens) increases acidity by stabilising the conjugate base.

Nomenclature of Alkanoic Acids

The naming of alkanoic acids follows IUPAC guidelines.

Rule

1. Start with the name of the corresponding alkane.

Replace the suffix “-e” with “-oic acid”.

2. Priority of the Carboxyl Group:

The carboxyl group (-COOH) is always assigned the highest priority.

It is always located at the end of the carbon chain and designated as carbon number 1.

Examples Methane → Methanoic acid (HCOOH) Ethane → Ethanoic acid (CH₃COOH) Propane → Propanoic acid (C₂H₅COOH) Common Names of Alkanoic Acids The common names of alkanoic acids often come from their natural sources:

Formic acid: Named after “formica” (Latin for ant), as it is found in ant venom.

formic acid Acetic acid: Comes from “acetum” (Latin for vinegar), as it is a key component of vinegar.

These common names are widely used in both industry and daily life.

acetic acid Naming Alkanoic Acids with Multiple Carboxyl Groups For alkanoic acids with two carboxyl groups, the suffix “-dioic acid” is used.

Example: A compound with two carboxyl groups on an ethane chain is called ethanedioic acid.

Naming Alkanoic Acids Attached to Rings

When an alkanoic acid group is attached to a ring, the suffix “-alkanoic acid” is used. The carbon with the carboxyl group is always numbered as carbon 1.

Structure of Alkanoic Acids

1. Carboxyl Group (-COOH)

Contains a carbon atom Double-bonded to an oxygen atom (C= O).

Single-bonded to a hydroxyl group (-OH).

General formula: R-COOH, where R can be an alkyl group, hydrogen, or a phenyl group.

2. Resonance and Stability

The carboxyl group is planar and exhibits resonance.

Resonance involves electron delocalization between the carbonyl and hydroxyl groups, stabilizing the molecule.

This stability enhances the acidity of alkanoic acids by stabilizing the conjugate base after proton loss.

3. Molecular Geometry

The carbon in the carboxyl group is sp² hybridized, creating a trigonal planar structure.

Bond angles around the carbonyl carbon are approximately 120°.

4. Hydrogen Bonding

The hydroxyl group (-OH) forms hydrogen bonds, which affect:

Boiling points: Higher due to strong intermolecular forces.

Solubility: High solubility in water for smaller alkanoic acids.

Properties of Alkanoic Acids

The physical properties of alkanoic acids depend on the length of their carbon chain and the presence of the carboxyl group.

1. Boiling and Melting Points

Alkanoic acids have higher boiling and melting points compared to other organic compounds of similar molecular weight due to strong hydrogen bonding.

Hydrogen Bonding:

a. Alkanoic acids form dimers through two hydrogen bonds between:

The hydrogen atom of the hydroxyl group (-OH).

The oxygen atom of the carbonyl group (C= O) of another molecule.

This dual bonding leads to higher boiling and melting points compared to alkanols of similar molecular weight.

b. Chain Length Effect Boiling and melting points increase as the carbon chain lengthens due to stronger Van Waals forces.

Example

Ethanoic acid (118°C) has a significantly higher boiling point than ethanol (78°C) despite their similar molecular weights.

2. Solubility Lower alkanoic acids, like methanoic acid and ethanoic acid, are highly soluble in water due to hydrogen bonding with water molecules.

As the carbon chain length increases, the hydrophobic alkyl group reduces solubility.

Example: Butanoic acid is less soluble than ethanoic acid.

3. Density Alkanoic acids are generally denser than water.

Density decreases as the carbon chain length increases.

Example: Ethanoic acid has a density of ~1.049 g/cm³, while hexanoic acid is ~0.927 g/cm³.

4. Physical State

Lower members (e.g., methanoic acid, ethanoic acid) are liquids at room temperature.

Higher members (e.g., octanoic acid, decanoic acid) are waxy solids due to stronger Van der Waals forces.

5. Odour Alkanoic acids have strong, pungent odours.

Example

Ethanoic acid: Vinegar-like smell.

Butanoic acid: Rancid butter smell.

Odour becomes less intense as molecular weight increases.

Chemical Properties of Alkanoic Acids

Alkanoic acids are highly reactive due to their carboxyl group (-COOH), which can:

Donate a proton (acidic behaviour).

Undergo nucleophilic attack at the carbonyl carbon.

1. Acid-Base Reactions

Alkanoic acids partially dissociate in water, forming hydronium ions (H₃O+) and carboxylate ions (R-COO–).

React with strong bases (e.g., NaOH) to produce salts and water in neutralization reactions.

Example: CH₃COOH + NaOH→ CH₃COONa + H₂O Importance: Used in soap production, where long-chain alkanoic acids react with bases to form soap molecules.

2. Esterification Reactions

Alkanoic acids react with alcohols in the presence of an acid catalyst to form esters and water.

Example: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅+ H₂O Widely used in producing fragrances, flavourings, and plastics.

Equilibrium Reaction: Yield can be increased by removing water or using an excess of one reactant.

3. Decarboxylation Reactions

Alkanoic acids lose a carbon dioxide molecule (CO₂) to form hydrocarbons when heated, often with a catalyst.

Example: CH₃COOH → CH₄+ CO₂ Significance: Crucial in metabolic pathways in biochemistry.

4. Formation of Amides (Amidation)

Alkanoic acids react with amines to form amides, often requiring heat and a dehydrating agent.

Example: CH₃COOH + NH₃→ CH₃CONH₂+ H₂O Uses: Important in synthesising pharmaceuticals and polymers.

Tests for Alkanoic Acids

1. Litmus Test

Procedure: Dip blue litmus paper into the sample solution.

Observation: Blue litmus turns red, indicating the presence of an acid.

2. Reaction with Sodium Bicarbonate

Procedure: Add sodium bicarbonate (NaHCO₃) to the sample solution.

Observation: Effervescence (bubbling) indicates carbon dioxide (CO₂) release, confirming a carboxylic acid.

Reaction:RCOOH + NaHCO₃→ RCOONa + H₂O + CO₂

3. Esterification Test

Procedure: Mix the sample with ethanol and a few drops of concentrated sulphuric acid (H₂SO₄), then gently heat.

Observation: A sweet, fruity smell confirms the formation of an ester, indicating a carboxylic acid.

Reaction:RCOOH + C₂H₅OH → RCOOC₂H₅+ H₂O

4. Reaction with Magnesium

Procedure: Add a small piece of magnesium ribbon to the sample.

Observation: Effervescence indicates hydrogen gas (H₂) release, confirming a carboxylic acid.

Reaction:2RCOOH + Mg → (RCOO)₂Mg + H₂

5. Formation of a Salt

Procedure: Add the sample to sodium hydroxide (NaOH).

Observation: Formation of a salt and water confirms a carboxylic acid.

Reaction:RCOOH + NaOH → RCOONa + H₂O Preparations of Alkanoic Acids

1. Oxidation of Primary Alcohols

Reaction: Primary alcohols are oxidised to carboxylic acids using strong oxidising agents like KMnO₄or K₂Cr₂O₇in an acidic medium.

Equation: R−CH₂OH + 2[O] → R−COOH + H₂O

Example: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O Conditions: Acidic medium (e.g., dilute H₂SO₄) and heating under reflux.

2. Oxidation of Aldehydes

Reaction: Aldehydes are oxidised to carboxylic acids using similar oxidising agents.

Equation: R−CHO + [O] → R−COOH

Example: CH₃CHO + [O] → CH₃COOH Conditions: Acidic medium and heating under reflux.

3. Hydrolysis of Nitriles

Reaction: Nitriles are hydrolysed to carboxylic acids in the presence of an acid or base.

Equation: R−CN + 2H₂O → R−COOH + NH₃

Example: CH₃CN + 2H₂O → CH₃COOH + NH₃ Conditions: Acidic or basic medium and heating.

4. Oxidation of Alkylbenzenes

Reaction: Alkylbenzenes are oxidised to aromatic alkanoic acids using strong oxidising agents like KMnO4.

5. Hydrolysis of Esters

Reaction: Esters are hydrolysed to carboxylic acids in the presence of an acid or base.

Equation: R−COOR¹+ H₂O → R−COOH + R¹−OH

Example: CH₃COOCH₃+H₂O → CH₃COOH + CH₃OHCH₃ Conditions: Acidic or basic medium and heating.

Uses of Alkanoic Acids

1. Preservatives: Benzoic acid is used in pharmaceuticals to prevent microbial growth.

2. Flavouring Agents: Citric acid and lactic acid enhance tartness and flavour in foods and drinks.

3. Soaps and Detergents: Fatty acids like stearic acid are used to make soaps and detergents.

4. Medicinal Compounds: Salicylic acid is used to produce aspirin and other anti-inflammatory medications.

5. Fragrances: Alkanoic acids are used to synthesize esters, key components in perfumes.

6. Animal Feed: Propionic acid acts as a preservative to prevent mould in animal feed.

Activity 8.18 Exploring Alkanoic Acids

Steps

1. Organise yourselves into small groups. In your groups, research on preparation, properties and uses alkanoic acid.

2. Create a visual map illustrating the production methods of alkanoic acids (e.g., oxidation, hydrolysis).

3. Compare the physical properties (e.g., boiling points, solubility) of different alkanoic acids.

4. Present your work to the class for discussion and feedback

Activity 8.19 Investigating Reactions of Alkanoic Acids

Materials needed: Test tubes, alkanoic acids (e.g., ethanoic acid, propanoic acid), NaOH, NaHCO₃, Na₂CO₃, NH₃, ROH, LiAlH₄ Steps

1. In small groups, research the expected reaction between alkanoic acid and the following substances: NaOH, NaHCO₃, ROH.

2. Design a simple test tube experiment observing and recording results (e.g., colour changes, gas evolution, or formation of precipitates).

3. Analyse your results and determine whether they match the expected reaction.

4. Draw conclusions about the chemical behaviour of alkanoic acids.

5. Present your research, experimental setup, observations, and conclusions to the class for discussion and feedback

Activity 8.20 Testing for Alkanoic Acids

Materials needed: Test tubes, ethanoic acid (or other alkanoic acid), ethanol (alkanol), concentrated sulphuric acid (catalyst), water bath Steps Test for alkanoic acids by reacting them with alkanols to produce esters.

Procedure

a. Add a small amount of ethanoic acid and ethanol to a test tube.

b. Add 2-3 drops of concentrated sulphuric acid.

c. Heat the mixture gently in a water bath for 5-10 minutes.

d. Observe the reaction and note any changes (e.g., smell of a fruity ester).

e. Record observations in a provided template, including:

i. Initial setup details.

ii. Observations during heating.

iii. Final product (e.g., ester smell).

2. Discussion: Other Tests for Alkanoic Acids

a. Litmus Test

b. Reaction with Sodium Bicarbonate

c. Reaction with Magnesium

Activity 8.21 Exploring Alkanoic Acids: Discussion, Reflection, and

Differentiated Learning

Steps

1. Organise yourselves into groups. In your groups, share what you already know about alkanoic acids.

2. Using visual aids (e.g., reaction diagrams, flowcharts), describe how alkanoic acids are made (e.g., oxidation of alcohols, hydrolysis of nitriles).

3. Using real-life examples and multimedia resources (e.g., videos, pictures), highlight everyday applications of alkanoic acids. (e.g., preservatives, soaps, medicines).

Review Questions 8.1

1. What is the general formula for alkanes?

2. Name one use of methane.

3. Write the molecular formula for 2-methylpropane.

4. Why do alkanes with longer carbon chains have higher boiling points?

5. Which alkane fractions from crude oil are used as fuel for vehicles?

6. Explain how the structure of branched alkanes affects their boiling points compared straight-chain alkanes.

7. Explain why alkanes are insoluble in water but soluble in non-polar solvents.

8. Explain why propane and butane are suitable for use in Liquefied Petroleum Gas (LPG).

9. Analyse the trends in melting and boiling points of alkanes from C1 to C10 and relate them to their molecular structure.

10. Evaluate the environmental impact of using alkanes as fuels and suggest alternatives to minimise these effects.

Review Questions 8.2

1. What type of bond is present in alkenes and alkynes?

2. Are alkenes and alkynes soluble in water?

3. Draw the structure of 2-pentyne.

4. Why do alkenes and alkynes have low boiling points compared to alcohols?

5. Explain the geometry around the double bond in alkenes and the triple bond in alkynes.

6. Compare the reactivity of alkanes, alkenes, and alkynes.

7. Compare the bond lengths and strengths of C−C, C= C, and C≡C.

8. Analyse how branching affects the boiling points of alkenes and alkynes.

Review Questions 8.3

1. a. What is the molecular formula of benzene?

b. Name the type of bonding found in benzene.

2. a. What is the functional group of alkanols?

b. Name the simplest alkanol.

3. Explain why benzene is more stable than expected based on its structure.

4. Compare the boiling points of benzene and water.

5. Write the IUPAC name for the compound CH₃CH₂CH(OH)CH₃.

6. Compare the solubility of ethanol and hexanol in water.

7. Predict how benzene’s structure affects its reactivity in electrophilic substitution reactions.

8. Analyse the environmental impact of benzene’s use in industrial applications.

9. Explain how the Lucas test distinguishes between primary, secondary, and tertiary alkanols.

10. Analyse the environmental and economic benefits of using bioethanol as a fuel.

11. Design an experiment to differentiate benzene from alkanes in terms of reactivity.

12. Design an experiment to compare the boiling points of methanol, ethanol, and propanol.

13. Evaluate the effectiveness of ethanol as a solvent in pharmaceutical preparations compared to water.

Review Questions 8.4

1. What functional group is present in alkanoic acids?

2. Name one method used to prepare alkanoic acids.

3. Are alkanoic acids strong or weak acids?

4. What gas is produced when alkanoic acids react with sodium bicarbonate?

5. Explain the structure of the carboxyl group in alkanoic acids.

6. Write the reaction for the test of ethanoic acid with sodium bicarbonate.

7. Why does the carboxyl group exhibit resonance?

8. Compare the hydrolysis of nitriles and the oxidation of aldehydes for preparing alkanoic acids.

9. How does chain length affect the solubility of alkanoic acids in water?

10. Compare the use of benzoic acid and citric acid in the food industry.

11. Explain why effervescence is observed when alkanoic acids react with sodium bicarbonate.

12. Predict how the resonance in the carboxyl group affects the acidity of alkanoic acids.

13. Analyse the advantages and disadvantages of using oxidation versus hydrolysis in industrial alkanoic acid preparation.

14. Predict the effect of adding an electron-withdrawing group (e.g., chlorine) to a carboxylic acid.

15. Propose a method to confirm the presence of CO₂in the reaction with sodium bicarbonate.