Reaction of Alkanes

Combustion of Alkanes

You should know this from GCSE. The only difference is at A-Level half numbers can be used when balancing, as it is considered ‘incorrect’ to put a number in front of an organic molecule.

Complete Combustion producing carbon dioxide and water.
C2H6 + 3.5O2 2CO2 +3H2O

Incomplete Combustion producing carbon monoxide and water
C2H6 + 2.5O2 2CO +3H2O

Further Incomplete Combustion producing carbon and water
C2H6 + 1.5O2 2C +3H2O

· Test for CO2 Turns limewater milky
· Test for O2 Turns anhydrous Cobalt Chloride paper pink



Free Radical Substitution

Alkanes are very unreactive, however they will undergo a halogenation reaction under special conditions. This is a reaction mechanism and you should take care not to confuse this mechanism with the reaction mechanism for alkenes in the next section.

The overall reaction is
CH4 + Br2 CH3Br +HBr

However the reaction actually occurs in three steps…

1. Initiation

UV Light causes the Br – Br molecule to break homolytically to produce Br• free radicals
Br2 2Br•


Homolytic Fission
Covalent bond breaking where the electrons are shared equally between atoms to produce free radicals

Free Radicals
Reactive species with an unpaired electron

2. Propagation

This is the middle step of the mechanism and involves two reactions:

Br• + CH4 CH3• + HBr
CH3• + Br2 CH3Br + Br•


3. Termination

The reaction may be terminated if two free radicals collide e.g.
Br• + Br• Br2
CH3• + Br• CH3Br
CH3• + CH3 C2H6

Free Radical substitution is not a good method for preparation of halogenalkanes because the reaction is uncontrollable and produces a mixture of products that are then difficult to separate.