Combustion of alkenes - Higher tier

Alkenes combust, but they are less likely than alkanes to combust completely.

Complete combustion of alkenes produces carbon dioxide and water, provided there is a plentiful supply of oxygen.

Incomplete combustion of alkenes occurs where oxygen is limited and produces water, carbon monoxide and carbon (soot). This causes a smoky flame.

Functional group

Alkenes have a functional group C=C – the part of the molecule that takes part in reactions. Alkanes, in contrast, do not have a functional group and are less reactive as a result.

Addition reactions of alkenes - Higher tier

Addition reactions occur when one molecule combines with another, forming one large molecule and no other products. The C=C functional group allows alkenes to undergo these reactions.


Ethene reacts with bromine to form 1,2-dibromoethane:

CH2=CH2 + Br2 → CH2BrCH2Br

Ethene is added to bromine. 1,2-dibromoethane is formed.

Alkanes cannot take part in addition reactions.


Propene, CH3CH=CH2, reacts with bromine, Br2. Write the formula of the product formed.