Substitution of alkanes with halogens

Alkanes are saturated hydrocarbons with only single C-C and C-H bonds, which makes them fairly unreactive. Their one characteristic reaction in the Form 5 Carbon Compounds chapter, besides combustion, is substitution with a halogen. Understanding it, and contrasting it with the addition reactions of alkenes, is a very common exam theme.

Why alkanes react this way

Because every bond in an alkane is a strong single bond and there is no reactive site such as a carbon-carbon double bond, alkanes do not react readily. They are unaffected by acids, alkalis and most oxidising agents at room temperature. The one reaction they do undergo with a halogen is slow and needs the energy of light, which is exactly why substitution is the signature reaction of a saturated hydrocarbon. This unreactivity is also why alkanes make stable, storable fuels.

What substitution means

In a substitution reaction, one atom (or group) in a molecule is replaced by another atom. Here a hydrogen atom of the alkane is replaced, one at a time, by a halogen atom, and the displaced hydrogen leaves combined with a halogen atom as a hydrogen halide. Because each step swaps just one hydrogen, a mixture of products can form when excess halogen is present, and the reaction can proceed all the way to the fully substituted molecule.

Balanced equations

Methane and chlorine (in ultraviolet light)

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

The product chloromethane can be substituted further if more chlorine is present:

CH₃Cl(g) + Cl₂(g) → CH₂Cl₂(g) + HCl(g)

CH₂Cl₂(g) + Cl₂(g) → CHCl₃(g) + HCl(g)

CHCl₃(g) + Cl₂(g) → CCl₄(g) + HCl(g)

Ethane and chlorine

C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g)

Methane and bromine

CH₄(g) + Br₂(g) → CH₃Br(g) + HBr(g)

In every step, one hydrogen is swapped for one halogen atom and one molecule of hydrogen halide is produced, so the equation stays balanced.

Conditions required

Ultraviolet light (sunlight) is essential. In the dark, an alkane and a halogen do not react. The light provides the energy to start the reaction. No catalyst and no strong heating are needed, light is the key condition, and this is a favourite one-mark point.

Observations

• The greenish-yellow colour of chlorine (or the reddish-brown colour of bromine) slowly fades in sunlight as the halogen is used up.
• Misty, acidic fumes of the hydrogen halide are produced; these turn moist blue litmus paper red and fume in moist air.
• The reaction is slow compared with the instant reaction of an alkene.

Alkanes versus alkenes: the key contrast

This reaction is the classic way to tell an alkane from an alkene. An alkene decolourises bromine water quickly at room temperature in the dark by addition, because it is unsaturated. An alkane only reacts with bromine (not bromine water) slowly, and only in ultraviolet light, by substitution. Examiners set this comparison year after year, so learn both the condition (light or no light) and the reaction type (substitution or addition).

Common mistakes to avoid

Do not forget to state that ultraviolet light is needed, leaving it out loses the mark. Do not write an addition product such as CH₄Cl₂; alkanes react by substitution, not addition, so a hydrogen halide is always released. Keep the equation balanced by producing one HCl for each hydrogen replaced.

How it appears in the SPM exam

In Paper 2 (4541/2) you may be asked to write the substitution equation, state the essential condition, describe the colour change, and explain how substitution differs from the addition of an alkene. In Paper 1 (4541/1), objective items test the role of light and the identity of the products. Naming the condition and the reaction type correctly secures these marks.