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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
 

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Related AUS-e-TUTE Topics

Summary of Nomenclature

Naming Straight Chain Alkanes

Naming Branched Chain Alkanes

Naming Simple Alkenes

Naming Simple Alkynes

Naming Structural Isomers of Alkanes

Naming Structural Isomers of Alkenes

Naming Alkanols

Naming Haloalkanes

Functional Groups

Combustion of Hydrocarbons

Hydrohalogenation of Unsaturated Hydrocarbons

Hydration of Alkenes to Alkanols

Oxidation of Alkanols

Dehydration of Alkanols (alcohols)

Preparation and Naming of Esters (Esterification)

Substitution Reactions of Haloalkanes (alkyl halides)

Preparation and Naming of Polymers (Polymerization)

Properties and Uses of Alkanes

Properties and Uses of Ethene (ethylene)

Properties and Uses of Polythene (polyethylene)

 
 

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