A chemical reaction in which one molecule adds to another. Addition reactions occur with unsaturated compounds containing double or triple bonds, and may be electrophilic or nucleophilic. An example of electrophilic addition is the reaction of hydrogen chloride with an alkene, e.g.HCl+CH2:CH2 → CH3CH2Cl An example of nucleophilic addition is the addition of hydrogen cyanide across the carbonyl bond in aldehydes to form cyanohydrins. Addition-elimination reactions are ones in which the addition is followed by elimination of another molecule (see condensation reaction).
HCl+CH2:CH2 → CH3CH2Cl