An addition reaction is an organic reaction where two or more molecules combine to form a larger one.
Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon-carbon double bonds (alkenes), with triple bonds (alkynes) or with carbonyl (C=O) groups can undergo addition as they too have double bond character. For example,
CH2=CH2 + Cl2 → CH2Cl−CH2Cl.