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•
Definitions
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.