Organic Chemistry

What you'll learn

Organic chemistry forms a substantial portion of your IGCSE Chemistry examination, focusing on carbon-based compounds and their reactions. This guide covers the homologous series you need to know—alkanes, alkenes, alcohols and carboxylic acids—alongside fractional distillation, cracking, and addition polymerisation. You'll learn to write structural formulae, name compounds systematically, and predict reaction products.

Key terms and definitions

Homologous series — a family of organic compounds with the same general formula and similar chemical properties, differing by CH₂ units

Saturated hydrocarbon — a compound containing only carbon and hydrogen with single C-C bonds only (alkanes)

Unsaturated hydrocarbon — a compound containing carbon and hydrogen with at least one C=C double bond (alkenes)

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

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

Cracking — the thermal decomposition of long-chain hydrocarbons into smaller, more useful molecules

Addition polymerisation — joining many small alkene molecules (monomers) to form a long-chain molecule (polymer)

Fermentation — the anaerobic conversion of glucose to ethanol and carbon dioxide using yeast enzymes

Core concepts

Hydrocarbons and fossil fuels

Hydrocarbons are compounds containing only carbon and hydrogen atoms. Crude oil is a complex mixture of hydrocarbons formed over millions of years from the remains of marine organisms under high temperature and pressure.

Fractional distillation separates crude oil into useful fractions based on boiling points:

• Crude oil is heated to approximately 350°C in a fractionating column
• Vapours rise up the column, which has a temperature gradient (hotter at bottom, cooler at top)
• Different fractions condense at different heights when they reach their boiling points
• Shorter chain hydrocarbons (lower boiling points) are collected near the top
• Longer chain hydrocarbons (higher boiling points) are collected near the bottom

Main fractions from top to bottom:

• Refinery gases (C₁-C₄): camping gas, LPG
• Gasoline/petrol (C₅-C₁₀): fuel for cars
• Naphtha (C₇-C₁₄): chemical feedstock
• Kerosene (C₁₀-C₁₆): aircraft fuel
• Diesel oil (C₁₅-C₂₀): fuel for diesel engines
• Fuel oil (C₂₀-C₃₀): fuel for ships, power stations
• Bitumen (>C₃₀): road surfacing

As chain length increases:

• Boiling point increases
• Viscosity increases (becomes thicker)
• Flammability decreases (harder to ignite)
• Volatility decreases (evaporates less easily)

Alkanes

Alkanes form a homologous series with the general formula CₙH₂ₙ₊₂. They are saturated hydrocarbons containing only single covalent bonds.

First four members:

• Methane: CH₄
• Ethane: C₂H₆
• Propane: C₃H₈
• Butane: C₄H₁₀

Alkanes undergo complete combustion in excess oxygen:

Hydrocarbon + oxygen → carbon dioxide + water

Example: CH₄ + 2O₂ → CO₂ + 2H₂O

With limited oxygen, incomplete combustion occurs, producing carbon monoxide (toxic) or carbon (soot):

2CH₄ + 3O₂ → 2CO + 4H₂O

Alkanes are relatively unreactive due to strong C-C and C-H bonds. They do not react with acids, alkalis, or oxidising agents under normal conditions.

Cracking and alkenes

Longer hydrocarbon chains from crude oil can be broken down into shorter, more useful molecules through cracking. This process produces:

• Shorter alkanes (for fuels)
• Alkenes (for plastics and other chemicals)

Two types of cracking:

Thermal cracking:

• High temperature (700-900°C)
• High pressure (70 atmospheres)

Catalytic cracking:

• Lower temperature (450-500°C)
• Atmospheric pressure
• Aluminium oxide or silicon dioxide catalyst

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

Alkenes have the general formula CₙH₂ₙ and contain a C=C double bond, making them unsaturated.

First four members:

• Ethene: C₂H₄
• Propene: C₃H₆
• Butene: C₄H₈
• Pentene: C₅H₁₀

Test for alkenes: Add bromine water (orange/brown). Alkenes decolourise bromine water; alkanes do not.

C₂H₄ + Br₂ → C₂H₄Br₂ (ethene + bromine → 1,2-dibromoethane)

Alkenes undergo addition reactions because the C=C double bond can break, allowing other atoms to add across it:

Hydrogenation (addition of hydrogen): C₂H₄ + H₂ → C₂H₆ Conditions: nickel catalyst, 150°C (ethene → ethane)

Halogenation (addition of halogens): C₂H₄ + Br₂ → C₂H₄Br₂

Hydration (addition of steam/water): C₂H₄ + H₂O → C₂H₅OH Conditions: phosphoric acid catalyst, 300°C, 60 atmospheres (ethene → ethanol)

Addition polymerisation

Small alkene molecules (monomers) can join together to form very long chain molecules (polymers) through addition polymerisation.

The C=C double bond breaks and monomers link together:

n(C₂H₄) → -(C₂H₄)ₙ- (ethene → poly(ethene))

Common polymers:

• Poly(ethene): plastic bags, bottles, containers
• Poly(propene): ropes, carpets, crates
• Poly(chloroethene)/PVC: window frames, pipes, insulation

Drawing polymer structures:

• Identify the monomer structure
• Change C=C to C-C single bond
• Draw repeating unit in brackets with bonds extending outside
• Add 'n' subscript outside brackets

Polymers are:

• Unreactive and chemically inert
• Non-biodegradable (cause disposal problems)
• Can be recycled or used as fuel through combustion

Alcohols

Alcohols contain the hydroxyl functional group -OH. The homologous series has the general formula CₙH₂ₙ₊₁OH.

First four members:

• Methanol: CH₃OH
• Ethanol: C₂H₅OH
• Propanol: C₃H₇OH
• Butanol: C₄H₉OH

Properties:

• Dissolve in water to form neutral solutions
• React with sodium to produce hydrogen gas
• Flammable—burn in oxygen to produce carbon dioxide and water

2C₂H₅OH + 7O₂ → 4CO₂ + 6H₂O

Production of ethanol:

Method 1: Fermentation of glucose

• Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
• Conditions: yeast enzymes, 25-35°C, anaerobic (absence of oxygen)
• Concentration: produces approximately 12-15% ethanol solution
• Source: renewable (sugar cane, corn, other crops)
• Rate: slow process (several days)

Method 2: Hydration of ethene

• Equation: C₂H₄ + H₂O → C₂H₅OH
• Conditions: phosphoric acid catalyst, 300°C, 60 atmospheres
• Concentration: produces pure ethanol (100%)
• Source: non-renewable (crude oil)
• Rate: fast, continuous process

Oxidation of alcohols:

Alcohols can be oxidised to carboxylic acids using oxidising agents (e.g., acidified potassium manganate(VII) or acidified potassium dichromate(VI)):

C₂H₅OH + 2[O] → CH₃COOH + H₂O (ethanol → ethanoic acid)

The solution turns from orange to green (with dichromate) or from purple to colourless (with manganate).

Carboxylic acids

Carboxylic acids contain the functional group -COOH. The homologous series has the general formula CₙH₂ₙ₊₁COOH.

First four members:

• Methanoic acid: HCOOH
• Ethanoic acid: CH₃COOH
• Propanoic acid: C₂H₅COOH
• Butanoic acid: C₃H₇COOH

Properties:

• Weak acids (partially ionise in water)
• pH 3-5 in dilute solution
• React with metals to produce hydrogen gas
• React with carbonates to produce carbon dioxide
• React with bases to form salts and water

Examples: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ (ethanoic acid + magnesium → magnesium ethanoate + hydrogen)

CH₃COOH + NaOH → CH₃COONa + H₂O (ethanoic acid + sodium hydroxide → sodium ethanoate + water)

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ (ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide)

Ethanoic acid is the main component of vinegar (3-5% solution in water).

Worked examples

Example 1: A hydrocarbon X contains 85.7% carbon and 14.3% hydrogen by mass. The relative molecular mass of X is 56.

(a) Calculate the empirical formula of X. [3 marks] (b) Calculate the molecular formula of X. [1 mark] (c) Draw two possible structural formulae for X. [2 marks]

Solution:

(a)

• Carbon: 85.7 ÷ 12 = 7.14
• Hydrogen: 14.3 ÷ 1 = 14.3
• Ratio: 7.14 : 14.3 = 1 : 2
• Empirical formula: CH₂ ✓

(b)

• Empirical formula mass = 12 + 2 = 14
• Molecular formula mass = 56
• n = 56 ÷ 14 = 4
• Molecular formula: C₄H₈ ✓

(c)

• But-1-ene: CH₂=CH-CH₂-CH₃ ✓
• But-2-ene: CH₃-CH=CH-CH₃ ✓ (Or methylpropene: CH₂=C(CH₃)-CH₃)

Example 2: Describe and explain how crude oil is separated into fractions by fractional distillation. [6 marks]

Solution:

• Crude oil is heated to approximately 350°C ✓
• Vapours rise up the fractionating column ✓
• The column has a temperature gradient, hotter at the bottom and cooler at the top ✓
• Different fractions condense at different heights ✓
• This occurs when vapours reach their boiling points ✓
• Smaller molecules have lower boiling points and are collected at the top; larger molecules have higher boiling points and are collected near the bottom ✓

Example 3: Ethanol can be produced from ethene or by fermentation.

(a) Write a balanced symbol equation for the production of ethanol from ethene. [2 marks] (b) State two conditions needed for this reaction. [2 marks] (c) Give one advantage and one disadvantage of producing ethanol by fermentation compared to from ethene. [2 marks]

Solution:

(a) C₂H₄ + H₂O → C₂H₅OH ✓✓

(b)

• Phosphoric acid catalyst ✓
• Temperature of 300°C / high temperature ✓ (Also accept: 60 atmospheres / high pressure)

(c)

• Advantage: uses renewable resources / plants / sugar / does not use crude oil ✓
• Disadvantage: slow process / produces dilute solution / requires purification ✓

Common mistakes and how to avoid them

• Confusing alkanes and alkenes: Remember alkanes are saturated (CₙH₂ₙ₊₂) with single bonds only; alkenes are unsaturated (CₙH₂ₙ) with a C=C double bond. Use the bromine water test to distinguish them.

• Incorrect combustion equations: Always balance combustion equations carefully. Complete combustion produces only CO₂ and H₂O; incomplete combustion produces CO or C. Don't forget to check oxygen atoms on both sides.

• Muddling fermentation conditions: Fermentation requires yeast, 25-35°C, and anaerobic conditions. Higher temperatures denature enzymes; oxygen presence causes oxidation to ethanoic acid instead.

• Drawing polymer structures incorrectly: Always show the repeating unit in brackets with bonds extending through the brackets, and include the 'n' subscript. The double bond in the monomer becomes a single bond in the polymer.

• Confusing oxidation products: When alcohols are oxidised, they form carboxylic acids (not aldehydes at IGCSE level). The oxidising agent provides oxygen atoms, shown as [O] in equations.

• Forgetting functional groups in naming: Alcohols end in '-ol', carboxylic acids end in '-oic acid'. The functional group determines the name ending and must be included correctly.

Exam technique for Organic Chemistry

• Command word awareness: 'State' requires a simple fact with no explanation (1 mark). 'Explain' requires a reason or mechanism (usually 2+ marks). 'Describe and explain' needs both what happens and why (higher mark allocation).

• Drawing structural formulae: Show all bonds clearly. For displayed formulae, show every single bond. For structural formulae, group atoms logically (e.g., CH₃CH₂OH not C₂H₆O). Examiners penalise ambiguous structures.

• Balanced equations earn marks: Symbol equations usually carry 2 marks—one for correct formulae, one for balancing. Show all state symbols if asked. For organic equations, molecular formulae (C₂H₄) are acceptable unless structural formulae are specifically requested.

• Conditions matter: When describing industrial processes (cracking, ethanol production, polymerisation), stating conditions (temperature, pressure, catalyst) often carries separate marks. Learn the specific values where required by the specification.

Quick revision summary

Crude oil is separated by fractional distillation into fractions with different boiling points. Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons; alkenes (CₙH₂ₙ) are unsaturated with a C=C double bond that decolourises bromine water. Cracking breaks long chains into shorter alkanes and alkenes. Alkenes undergo addition reactions and polymerise to form plastics. Alcohols (CₙH₂ₙ₊₁OH) are produced by fermentation or hydration of alkenes and oxidise to carboxylic acids (CₙH₂ₙ₊₁COOH). Know homologous series patterns, functional groups, and key reaction conditions for exam success.