Section 24.2. Nucleophilic catalysis.

A catalyst is a substance which speeds up a chemical process without being consumed. This role is carried out by the catalyst becoming involved in the reaction in such a way that it provides a reaction path with a lower activation energy than if it were absent. Before the reaction ends, the catalyst must be regenerated, so that it appears unused. In nucleophilic catalysis, a species which has the double property of being a good nucleophile and a good leaving group is used. One such species is the iodide ion, which, because of its size and polarizability, is both a good nucleophile and a good leaving group. Iodide catalyzes a nucleophilic substitution, such as the reaction of a chloroalkane (left) with hydroxide ion, by first reacting as a nucleophile to replace the chloride ion, giving the iodoalkane (right). The iodoalkane produced now has the good iodide leaving group, and so reacts faster with the hydroxide ion than the original chloroalkane. Overall, if this two-step process (formation of and reaction of the iodoalkane intermediate) is faster than the single-step reaction of the chloroalkane, the reaction is being catalyzed by the iodide ion.