Reaction of Alkanes with Halogens — Alkanes | IGCSE 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

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