An addition reaction is a type of chemical reaction in which two substances combine to form a third substance, with no other substance being produced. Addition reactions occur mainly in organic chemistry, particularly in situations where a simple molecule is added across a carbon-carbon double bond in an unsaturated compound. For example, the addition reaction between chlorine (Cl2) and ethene (CH2=CH2 ) produces 1,2-dichloroethane (ClCH2(CH2)Cl).
Some plastics (such as polystyrene) are made by addition polymerization, usually initiated by the action of a free radical.
See also substitution reaction.