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Halogenation of Hydrocarbons

Key Concepts

Saturated hydrocarbons (alkanes)

react with halogens under UV light
undergo substitution reactions

Unsaturated hydrocarbons (alkenes and alkynes)

readily react with halogens
undergo addition reactions

Animated Tutorial

Examples

The most commonly demonstrated halogenation reactions are brominations using bromine water, Br₂, which is a red-brown colour.
As the bromination reaction proceeds, the reaction mixture de-colourises.

Alkane + Halogen : Substitution Reaction

Under standard laboratory conditions, alkanes do not react with halogens.
Alkanes can react with halogens in the presence of UV light.
During this reaction, halogen atoms substitute for (replace) hydrogen atoms, one at a time, producing a haloalkane and a hydrogen halide.

ethane + bromine water UV
→ bromoethane + hydrogen bromide

H
| H
|

H- C - C -H

|
H |
H

+ Br-Br UV
→

H
| H
|

H- C - C -Br

|
H |
H

+ H-Br

Further substitutions are possible.

Alkene + Halogen : Addition Reaction

Alkenes readily react with halogens under standard laboratory conditions.

In this addition reaction, halogens atoms are added across the double bond of the alkene.

In an alkene containing only one double bond, the double bond is broken, the halogen atoms are added, and, the only product of the reaction will be a dihaloalkane.
In a bromination reaction, if excess alkene is present, then the reaction mixture will change from a red-brown colour to colourless.

ethene + bromine water → 1,2-dibromoethane

H
| H
|

C = C

|
H |
H

+ Br-Br →

Br
| Br
|

H- C - C - H

|
H |
H