MYERS SCIENCE HUB · IGCSE CHEMISTRY 0620

Organic Chemistry: Alkanes, Alkenes, Alcohols & Carboxylic Acids

An interactive Cambridge IGCSE lesson covering syllabus sections 11.4–11.7, including reactions, conditions, equations, comparisons and exam practice.

15 short-answer questions 30 marks Manual marking Embedded video support

Student information

Learning objectives

Explain why alkanes are saturated and generally unreactive.
Describe cracking and the addition reactions of alkenes.
Compare fermentation with catalytic hydration for producing ethanol.
Describe reactions of ethanoic acid, oxidation and esterification.

11.4 Alkanes

Saturated hydrocarbons

Alkanes contain only single carbon–carbon covalent bonds. Each carbon atom forms four covalent bonds, so the molecules are described as saturated.

General formula: CnH2n+2
Examples: methane CH4, ethane C2H6, propane C3H8

Combustion

Complete combustion in excess oxygen forms carbon dioxide and water.

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion can form carbon monoxide and/or carbon (soot).

Substitution with chlorine

Alkanes react with chlorine in ultraviolet light. One atom or group is replaced by another. In monosubstitution, one hydrogen atom is replaced by chlorine.

CH₄ + Cl₂ → CH₃Cl + HCl   (UV light)
Why is ultraviolet light needed?
Ultraviolet light supplies the activation energy needed to start the photochemical reaction.
Why are alkanes generally unreactive?
They contain strong C–C and C–H single covalent bonds and do not contain a highly reactive functional group.
Quick check: Which condition is essential for the substitution reaction between methane and chlorine?

11.5 Alkenes

Unsaturated hydrocarbons

Alkenes contain at least one carbon–carbon double bond, C=C. They are unsaturated and usually more reactive than alkanes.

Bromination
Ethene + bromine → dibromoethane
Orange/brown bromine water becomes colourless.
Hydrogenation
Ethene + hydrogen → ethane
Nickel catalyst and heat.
Hydration
Ethene + steam ⇌ ethanol
Acid catalyst, 300°C, 6000 kPa.
C₂H₄ + Br₂ → C₂H₄Br₂
C₂H₄ + H₂ → C₂H₆
C₂H₄ + H₂O ⇌ C₂H₅OH

Cracking

Cracking breaks larger alkane molecules into smaller, more useful molecules. It uses a high temperature and a catalyst and can produce a smaller alkane plus an alkene.

C₁₀H₂₂ → C₈H₁₈ + C₂H₄
Reasons for cracking
There is greater demand for shorter-chain fuels, and alkenes are valuable feedstocks for polymers and other chemicals.
Quick check: What happens when bromine water is shaken with an alkene?

11.6 Alcohols

Ethanol

Method 1: Fermentation

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
  • Aqueous glucose
  • Yeast
  • 25–35°C
  • Absence of oxygen

Advantages: renewable raw material; relatively low temperature.

Disadvantages: slow; batch process; dilute ethanol needs purification; fermentation stops at higher alcohol concentrations.

Method 2: Catalytic hydration

C₂H₄ + H₂O ⇌ C₂H₅OH
  • Ethene and steam
  • 300°C
  • 6000 kPa / 60 atm
  • Acid catalyst

Advantages: fast; continuous process; purer product.

Disadvantages: ethene is commonly obtained from crude oil; high temperature and pressure require energy.

Combustion and uses

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Ethanol is used as a fuel and as a solvent.

Quick check: Why must oxygen be absent during fermentation?

11.7 Carboxylic acids

Ethanoic acid

Ethanoic acid, CH3COOH, is a weak acid. It has the typical reactions of acids.

With metals
acid + metal → salt + hydrogen
With bases
acid + base → salt + water
With carbonates
acid + carbonate → salt + water + carbon dioxide
2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
CH₃COOH + NaOH → CH₃COONa + H₂O
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Oxidation of ethanol

Ethanol can be oxidised to ethanoic acid using acidified aqueous potassium manganate(VII), or by bacterial oxidation during vinegar production.

C₂H₅OH + 2[O] → CH₃COOH + H₂O

Esterification

A carboxylic acid reacts with an alcohol, using an acid catalyst, to form an ester and water.

ethanoic acid + ethanol ⇌ ethyl ethanoate + water
Quick check: What two products form when ethanoic acid reacts with sodium carbonate?

Video learning station

Watch the videos and tick each box when completed. Video availability is controlled by YouTube.

Complete syllabus overview

Cracking and alkenes

Addition reactions of alkenes

Alcohols and ethanol production

Short-answer assessment — 30 marks

Answer all 15 questions. Each question is worth 2 marks and will be marked manually.

Review and submit

Your work is also saved automatically on this device while you type.