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Cambridge IGCSE Chemistry · 0620

Chapter 11: Organic Chemistry — Part 4

Topic 11.4 · Alkanes

Bonding and Properties

  • Bonding: Alkanes contain only single covalent bonds and are therefore saturated hydrocarbons. Each carbon atom forms four covalent bonds.
  • Properties: Generally unreactive, except for combustion and substitution reactions.

Substitution Reaction with Chlorine

  • Definition: A reaction where one atom or group is replaced by another.
  • Conditions: This is a photochemical reaction, meaning ultraviolet (UV) light is required to provide the activation energy (Ea).
  • Reaction: Alkane + Chlorine ? Chloroalkane + Hydrogen chloride.
  • Example: CH4 + Cl2 ?UV CH3Cl + HCl.
Diagram showing the replacement of a hydrogen atom in ethane with a chlorine atom under UV light
Diagram 1: Diagram showing the replacement of a hydrogen atom in ethane with a chlorine atom under UV light

Exam Traps

  • Do not say alkanes decolourise bromine water at room temperature — that test detects alkenes; alkane halogenation needs UV light.
  • Do not call alkane chlorination an addition reaction — the C–C single bond stays intact; a hydrogen atom is substituted.

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