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addition reaction[ə′di·shən rē′ak·shən]
one of the main types of chemical reaction in which a single new substance is formed from two or more individual substances, for example, NH3 + HCl = NH4Cl. Addition reactions include many reactions involving the recombination of atoms and radicals (H + H = H2 and CH3 + C2H5 = C3H8), reactions involving the addition of halogens to unsaturated aromatic compounds (C6H6 + 3C12 = C6H6C16), reduction reactions (C2H4 + H2 = C2H6), and reactions for the formation of complex compounds. Addition reactions are often intermediate steps in complex chemical processes.