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Alkanes

Organic Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable
Low
Stable6%

General formula, structural formulae and combustion reactions tested regularly.

What happens

  • In the presence of ultraviolet (UV) light, an alkane reacts with a halogen (chlorine or bromine) by a
  • A hydrogen atom on the alkane is swapped for a halogen atom; the displaced hydrogen leaves as a hydrogen halide (HCl or HBr)
  • General word equation:

alkane + halogen → halogenoalkane + hydrogen halide

  • UV light is needed to break the halogen–halogen bond and start the reaction; the reaction does not occur in the dark
Substitution reaction of methane with bromine — methane (CH₄) plus Br₂ gives bromomethane (CH₃Br) plus hydrogen bromide (HBr), one hydrogen atom on the alkane swapped for a bromine atom
Source: Halogens & Alkanes by Save My Exams
Exam tip

Complete the equation for an alkane–halogen substitution and state the essential condition

What comes up: given a partial equation such as C₂H₆ + Br₂ → ___ + ___, complete the products and/or state the essential condition. Worth 2 marks.

Write (two marks): (1) products are the halogenoalkane (e.g. C₂H₅Br) and the hydrogen halide (e.g. HBr), in either order; polysubstituted products are accepted as long as the equation balances; (2) the essential condition is ultraviolet (UV) radiation / UV light / sunlight.

Watch out: the condition mark is lost if you leave it blank or write "heat" — the mark scheme requires UV specifically. Also, this is a substitution reaction; writing "addition" is marked wrong by the mark scheme (alkanes do not have a double bond to open, so addition is impossible).

Products: halogenoalkanes

  • The organic product of an alkane–halogen substitution is called a (or haloalkane) — an alkane with one (or more) hydrogen atoms replaced by halogen atoms
  • Naming follows the same stem-plus-ending rule as the parent alkane, but the halogen is named as a prefix:
    • Methane + Cl2 → chloromethane (CH3Cl)
    • Ethane + Br2 → bromoethane (C2H5Br)
    • Propane + I2 → iodopropane (C3H7I)
  • If more than one halogen has been substituted, prefixes like di-, tri- and a position number show how many and where (for example, 1,2-dichloroethane has one Cl on each of its two carbons)
Exam tip

Name the type of reaction when an alkane reacts with a halogen

What comes up: given the equation for an alkane reacting with a halogen (or told that a hydrogen is replaced by a halogen atom), you are asked to name the reaction type. 1 mark.

Write: substitution (one atom is swapped for another; the carbon skeleton stays intact).

Watch out: the mark scheme explicitly notes that alkanes do not undergo addition reactions. "Addition" is the most common wrong answer because students confuse this with the alkene–bromine addition. The key distinction: alkanes are saturated, so there is no double bond for an addition reaction to occur across.

Why this reaction matters

  • Halogenoalkanes are useful as solvents, refrigerants, propellants and starting materials for pharmaceuticals
  • A historically important sub-family was the chlorofluorocarbons (CFCs), once used in fridges and aerosol propellants
  • CFCs were phased out under the Montreal Protocol because they migrate to the upper atmosphere and damage the ozone layer