Organic Chemistry

What you'll learn

Organic chemistry forms a substantial component of CIE IGCSE Chemistry, examining carbon-based compounds and their reactions. This topic tests your understanding of hydrocarbon structures, functional groups, chemical properties, and the industrial applications of organic molecules. Exam questions typically assess nomenclature, reaction pathways, structural formulae, and the ability to identify compounds from their properties.

Key terms and definitions

Homologous series — a family of organic compounds with the same general formula, similar chemical properties, and a gradual change in physical properties.

Saturated hydrocarbon — a hydrocarbon containing only single carbon-carbon bonds (C-C), such as alkanes.

Unsaturated hydrocarbon — a hydrocarbon containing at least one carbon-carbon double bond (C=C) or triple bond, such as alkenes.

Functional group — the atom or group of atoms responsible for the characteristic chemical reactions of a compound.

Cracking — the thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules by breaking carbon-carbon bonds.

Polymerisation — the chemical reaction in which many small molecules (monomers) join together to form a very large molecule (polymer).

Isomers — compounds with the same molecular formula but different structural arrangements of atoms.

Addition reaction — a reaction in which two molecules combine to form a single product, typically involving breaking a double bond.

Core concepts

Structure and bonding in organic compounds

Carbon atoms form four covalent bonds, allowing them to create chains and rings of varying lengths. Organic molecules are represented in three ways:

• Molecular formula: shows the actual number of each type of atom (e.g., C₂H₆)
• Structural formula: shows the arrangement of atoms and bonds (e.g., CH₃CH₃)
• Displayed formula: shows all bonds and atoms explicitly

In CIE IGCSE Chemistry examinations, you must demonstrate competence in converting between these representations and recognising that different structural arrangements can produce isomers.

Alkanes: saturated hydrocarbons

Alkanes form a homologous series with the general formula CₙH₂ₙ₊₂. The first four members are:

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)
• Butane (C₄H₁₀)

Physical properties of alkanes:

• Boiling points increase with chain length due to stronger intermolecular forces
• Insoluble in water but dissolve in organic solvents
• Less dense than water
• Poor conductors of electricity

Chemical properties of alkanes:

Alkanes are relatively unreactive due to strong C-C and C-H bonds. Their main reaction is complete combustion in excess oxygen:

CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O

Example: CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion occurs with limited oxygen supply, producing carbon monoxide (toxic) or carbon (soot):

2CH₄ + 3O₂ → 2CO + 4H₂O

Alkanes also undergo substitution reactions with halogens in the presence of ultraviolet light:

CH₄ + Cl₂ → CH₃Cl + HCl (UV light required)

Alkenes: unsaturated hydrocarbons

Alkenes contain at least one C=C double bond and have the general formula CₙH₂ₙ. The first three members are:

• Ethene (C₂H₄)
• Propene (C₃H₆)
• Butene (C₄H₈)

The presence of the C=C double bond makes alkenes more reactive than alkanes. The standard test for unsaturation involves adding bromine water (orange/brown solution):

• Alkenes decolourise bromine water rapidly
• Alkanes do not react with bromine water at room temperature

Addition reactions of alkenes:

Alkenes undergo addition reactions across the double bond:

1. Hydrogenation — adding hydrogen with a nickel catalyst at 150°C: C₂H₄ + H₂ → C₂H₆ (ethene → ethane)

2. Halogenation — adding halogens: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)

3. Hydration — adding steam with phosphoric acid catalyst at 300°C and 60 atm: C₂H₄ + H₂O → C₂H₅OH (ethene → ethanol)

4. Addition of hydrogen halides: C₂H₄ + HBr → C₂H₅Br (ethene → bromoethane)

Alcohols

Alcohols contain the hydroxyl functional group -OH attached to a carbon chain. The homologous series has the general formula CₙH₂ₙ₊₁OH. The first three members are:

• Methanol (CH₃OH)
• Ethanol (C₂H₅OH)
• Propanol (C₃H₇OH)

Properties of alcohols:

• Soluble in water due to hydrogen bonding with the -OH group
• Lower members are liquids at room temperature
• Used as fuels, solvents, and in alcoholic drinks

Production of ethanol:

1. Fermentation — biological process using yeast:

   • C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
   • Temperature: 25-35°C (optimum for enzyme activity)
   • Anaerobic conditions (absence of oxygen)
   • Produces ethanol up to 15% concentration

2. Hydration of ethene (industrial method):

   • C₂H₄ + H₂O → C₂H₅OH
   • Phosphoric acid catalyst
   • 300°C and 60 atmospheres pressure
   • Continuous process, higher purity

Chemical reactions of ethanol:

Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (burns with a clean blue flame)

Oxidation: Ethanol can be oxidised to ethanoic acid using oxidising agents (potassium dichromate acidified with dilute sulfuric acid):

C₂H₅OH + [O] → CH₃COOH + H₂O

The orange dichromate solution turns green during this reaction.

Dehydration: Concentrated sulfuric acid or aluminium oxide catalyst at 170°C removes water:

C₂H₅OH → C₂H₄ + H₂O

Carboxylic acids

Carboxylic acids contain the carboxyl functional group -COOH and have the general formula CₙH₂ₙ₊₁COOH. Examples include:

• Methanoic acid (HCOOH)
• Ethanoic acid (CH₃COOH)
• Propanoic acid (C₂H₅COOH)

Properties of carboxylic acids:

• Weak acids that partially dissociate in water: CH₃COOH ⇌ CH₃COO⁻ + H⁺
• pH values typically 3-5 in dilute solution
• Show typical acid reactions: react with metals, bases, and carbonates
• Soluble in water (lower members)

Reactions of ethanoic acid:

With sodium carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ (produces carbon dioxide gas, which gives effervescence)

With sodium hydroxide: CH₃COOH + NaOH → CH₃COONa + H₂O (neutralisation producing sodium ethanoate)

With magnesium: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ (produces hydrogen gas)

Cracking of hydrocarbons

Cracking breaks long-chain alkanes from crude oil into shorter, more useful molecules. This process is essential in petroleum refining.

Conditions for thermal cracking:

• High temperature (600-900°C)
• Catalyst (aluminium oxide or silicon dioxide)
• Produces alkanes and alkenes

Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)

The products include:

• Shorter-chain alkanes for petrol
• Alkenes for polymer production
• Hydrogen gas for industrial processes

Polymers and polymerisation

Addition polymerisation occurs when many alkene monomers join together, with the double bond breaking to form long chains.

Formation of poly(ethene):

n C₂H₄ → (C₂H₄)ₙ

Many ethene molecules join to form poly(ethene), commonly known as polythene. The displayed formula shows:

• Monomer: CH₂=CH₂
• Polymer repeat unit: [-CH₂-CH₂-]ₙ

Other important polymers:

• Poly(propene): from propene (CH₃CH=CH₂)
• Poly(chloroethene) or PVC: from chloroethene (CH₂=CHCl)
• Poly(styrene): from styrene/phenylethene

Properties and uses:

• Strong, durable, chemically unreactive
• Low density
• Electrical insulators
• Uses: packaging, bottles, pipes, clothing fibres

Environmental concerns:

• Non-biodegradable (resistant to decomposition)
• Contribute to landfill waste
• Burning releases toxic fumes (especially PVC produces HCl)
• Solutions include recycling, using biodegradable polymers, and reducing consumption

Worked examples

Example 1: Identifying an unknown hydrocarbon

A hydrocarbon X has molecular formula C₄H₈. When X is added to bromine water, the solution changes from orange to colourless. When X is burned completely, it produces carbon dioxide and water.

(a) State whether X is saturated or unsaturated. [1]

(b) Name the homologous series to which X belongs. [1]

(c) Write the balanced equation for the complete combustion of X. [2]

Answer:

(a) Unsaturated [1] — the compound decolourises bromine water, indicating presence of a C=C double bond.

(b) Alkenes [1] — the general formula CₙH₂ₙ confirms this.

(c) C₄H₈ + 6O₂ → 4CO₂ + 4H₂O [2] — correct formulae [1], correct balancing [1].

Example 2: Production of ethanol

Ethanol can be manufactured by two different methods.

(a) Name the organic compound used as a starting material for fermentation. [1]

(b) State two conditions necessary for fermentation. [2]

(c) Write the equation for the hydration of ethene to produce ethanol. [1]

(d) State one advantage of producing ethanol by fermentation rather than by hydration of ethene. [1]

Answer:

(a) Glucose [1] (or any appropriate sugar: sucrose, fructose)

(b) Any two from: temperature 25-35°C [1]; presence of yeast/enzymes [1]; absence of oxygen/anaerobic conditions [1]; aqueous solution [1] (maximum 2 marks)

(c) C₂H₄ + H₂O → C₂H₅OH [1]

(d) Any one from: uses renewable resources [1]; lower temperature/less energy intensive [1]; suitable for small-scale production [1]

Example 3: Polymer structure

Poly(propene) is a polymer used to make plastic crates.

(a) Draw the displayed formula of propene. [2]

(b) Draw the structure of the repeat unit of poly(propene). [2]

Answer:

(a) Displayed formula showing CH₃-CH=CH₂ with all bonds and atoms correctly shown [2] (deduct 1 mark for minor errors)

(b) [-CH₂-CH(CH₃)-]ₙ or displayed formula showing repeat unit in brackets with bonds extending outside brackets [2] (1 mark for structure, 1 mark for correct notation with brackets and bonds)

Common mistakes and how to avoid them

• Mistake: Confusing molecular and displayed formulae. Writing C₂H₄ when a displayed formula is required. Correction: Read the question carefully. Displayed formulae must show all atoms and all bonds explicitly. Molecular formulae only show numbers of each atom type.

• Mistake: Stating that alkanes react with bromine water. Correction: Only alkenes decolourise bromine water at room temperature. Alkanes require UV light for substitution reactions with bromine.

• Mistake: Writing incorrect general formulae, such as CₙH₂ₙ for alkanes. Correction: Memorise the correct general formulae: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH.

• Mistake: Omitting catalyst conditions in organic reactions, especially for hydration of ethene or hydrogenation of alkenes. Correction: Always state specific conditions: phosphoric acid catalyst for hydration, nickel catalyst for hydrogenation, concentrated sulfuric acid for dehydration.

• Mistake: Drawing polymer repeat units without extending bonds outside brackets or omitting the 'n' subscript. Correction: Polymer notation must show: square brackets around the repeat unit, bonds extending through both sides of brackets, and subscript n outside brackets.

• Mistake: Claiming fermentation produces 100% pure ethanol or works at any temperature. Correction: Fermentation produces maximum 15% ethanol concentration and requires optimal temperature of 25-35°C for enzyme activity. Above 40°C, enzymes denature.

Exam technique for Organic Chemistry

• Command words matter: "State" requires a brief answer without explanation (1 mark). "Explain" requires a reason or mechanism (typically 2-3 marks). "Describe" requires multiple linked points about what happens. For CIE IGCSE Chemistry, precision in terminology earns marks.

• Structural formulae questions: When asked to "draw the structure", determine if displayed formula (showing all bonds) or structural formula (e.g., CH₃CH₂OH) is required from the mark allocation. Displayed formulae questions typically award 2 marks; ensure all bonds are shown.

• Reaction conditions: Exam questions frequently test whether you can recall specific conditions. Always include temperature, pressure, and catalyst where relevant. Partial answers lose marks.

• Functional group tests: Questions often present observations and ask you to identify the compound type. Learn the diagnostic tests: bromine water for alkenes, sodium carbonate for carboxylic acids (effervescence), acidified dichromate for oxidising alcohols (colour change).

Quick revision summary

Organic chemistry centres on carbon compounds organised into homologous series. Alkanes (CₙH₂ₙ₊₂) are saturated; alkenes (CₙH₂ₙ) contain C=C bonds and undergo addition reactions. Alcohols (CₙH₂ₙ₊₁OH) are produced by fermentation or ethene hydration. Carboxylic acids (CₙH₂ₙ₊₁COOH) are weak acids showing typical acid reactions. Cracking breaks long-chain hydrocarbons into useful shorter molecules. Addition polymerisation joins alkene monomers into long-chain polymers. Master functional group tests, reaction conditions, and structural representations for examination success.