ACETYLATION OF ALCOHOLS BY ACETIC-ANHYDRIDE IN THE PRESENCE OF N,N-DIMETHYL-4-AMINOPYRIDINE - INFLUENCE OF ACID-BASE EQUILIBRIA ON REACTION-MECHANISM

4-Dimethylamino pyridine (DMAP) is used as an efficient catalyst in organic reactions and particularly in alcohols acetylation by acetic anhydride (compared to pyridine, it is roughly 10(4) more active). To catalyse the reaction, DMAP is normally used with a strong base such as TEA; it is supposed to prevent protonation of the catalytic amount of DMAP by acid formed in the reaction. We have observed that 4-dimethylamino pyridinium acetate is also a catalyst in acetylation reaction, even without addition of any base. In that case, it was necessary to conceive a new catalytic process for this species in which the intracyclic nitrogen is not available. In this paper, we report the kinetic study we have accomplished on the action mechanism of DMAPH+ salts. The results obtained led us to analyse acido-basic equilibra in organic solvents of low dielectric constant used in this reaction. It appears that they are strongly shifted in such solvents. This behaviour explains the catalytic effect observed for 4-dimethylamino pyridinium acetate and the needlessness of a strong base in acetylation reaction.