In an addition reaction, two or more molecules combine to make one molecule. This is different than the substitution reactions we have been looking at, where two molecules combine to make two different molecules.

The addition of an acid to an alkene is the simplest example. Here, an acid proto-nates a carbon–carbon double bond. The carbocation that is formed will be the more substituted one of the possible products that could form. The conjugate base of the acid then reacts with this carbocation.