11.5 Alkenes

2026 Syllabus Objectives

  1. Core: State that the bonding in alkenes includes a double carbon–carbon covalent bond and that alkenes are unsaturated hydrocarbons
  2. Core: Describe the manufacture of alkenes and hydrogen by the cracking of larger alkane molecules using a high temperature and a catalyst
  3. Core: Describe the reasons for the cracking of larger alkane molecules
  4. Core: Describe the test to distinguish between saturated and unsaturated hydrocarbons by their reaction with aqueous bromine
  5. Supplement: State that in an addition reaction only one product is formed
  6. Supplement: Describe the properties of alkenes in terms of addition reactions with: (a) bromine or aqueous bromine (b) hydrogen in the presence of a nickel catalyst (c) steam in the presence of an acid catalyst and draw the structural or displayed formulae of the products

What are Alkenes? 🔑

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Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double covalent bond (C=C\text{C}=\text{C}). This double bond is the defining feature that distinguishes alkenes from alkanes and makes them much more reactive.

The presence of the carbon-carbon double bond in an alkene molecule makes these molecules much more reactive than alkanes (which contain only carbon-carbon single bonds).

Key characteristics:

  • Contain a C=C\text{C}=\text{C} double bond
  • Unsaturated - can add more atoms across the double bond
  • More reactive than saturated alkanes
  • General formula: CnH2n\text{C}_n\text{H}_{2n}

Catalytic Cracking - Manufacture of Alkenes 🏭

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What is Catalytic Cracking?

Catalytic cracking (often called "cat cracking") is the industrial process used to manufacture alkenes and hydrogen from larger alkane molecules.

Definition: The decomposition of long-chain alkanes into alkenes and alkanes of lower relative molecular mass; involves passing the larger alkane molecules over a catalyst heated to 500°C.

The Cracking Process

Conditions required:

  • High temperature: around 500°C
  • Catalyst: powdered minerals such as silica, alumina, and zeolites
  • Takes place in a huge reactor

Process:

  1. Particles of catalyst are mixed with the hydrocarbon fraction at high temperature
  2. Long-chain alkane molecules break down
  3. Cracked vapours containing smaller molecules are separated by distillation

Example Cracking Reaction

A typical cracking reaction of decane:

C10H22→heatC8H18+C2H4\text{C}_{10}\text{H}_{22} \xrightarrow{\text{heat}} \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4

Or in words:

decane→heatoctane+ethene\text{decane} \xrightarrow{\text{heat}} \text{octane} + \text{ethene}

This is just one of the possible reactions when decane is cracked. The molecules may not all break in the same place. The alkene fragment is not always ethene: propene and but-1-ene may also be produced.

Products of Cracking ⚡

All cracking reactions produce:

  • An alkane with a shorter chain than the original molecule
  • A short-chain alkene, OR
  • Two or more alkenes and hydrogen

Some hydrogen is also broken off from the long-chain alkanes being cracked.

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