In the case of
esterification with
acetic anhydrides the currently accepted mechanism involves three steps. First, DMAP and
acetic anhydride react in a pre-equilibrium reaction to form an ion pair of
acetate and the acetylpyridinium ion. In the second step the
alcohol adds to the acetylpyridinium, and elimination of pyridine forms an
ester. Here the acetate acts as a base to remove the proton from the alcohol as it nucleophilically adds to the activated acylpyridinium. The bond from the acetyl group to the
catalyst gets cleaved to generate the catalyst and the ester. The described bond formation and breaking process runs synchronous concerted without the appearance of a tetrahedral intermediate. The
acetic acid formed will then protonate the DMAP. In the last step of the
catalytic cycle the auxiliary base (usually
triethylamine or
pyridine) deprotonates the protonated DMAP, reforming the catalyst. The reaction runs through the described nucleophilic reaction pathway irrespective of the anhydride used, but the mechanism changes with the pKa value of the alcohol used. For example, the reaction runs through a base-catalyzed reaction pathway in the case of a phenol. In this case, DMAP acts as a base and deprotonates the phenol, and the resulting phenolate ion adds to the anhydride. ==Safety==