Esterification acyl-oxygen fission

The acid-catalysed esterification reaction usually proceeds via an acyl-oxygen fission process. This involves the cleavage of the bond between the original carbonyl-carbon atom and an oxygen of an hydroxyl group in the intermediate (6) arising from nucleophilic attack by an alcohol molecule on the pro-tonated carboxylic acid group (5). 

The esterification reaction is mainly bimolecular, and goes via acyl-oxygen fission, where the rate-limiting step is the attack of alcohol on the protonated carboxylic acid. Ingold proposed the following schemes ... 

Praill, having discovered the efficiency of acylium perchlorates as acylating agents, decided to examine the esterification of /err-butanol as opposed to its dehydration to isobutene. Using acetic anhydride and perchloric acid mixtures, both tert-h xty acetate and isobutene were rapidly produced and in accordance with the known alkyl oxygen fission of tertiary esters, the proportion of isobutene increased with time. In these reactions where acetic anhydride was in excess, crystalline material was deposited in the mixture. Later this was shown to be 2,4,6-trimethylpyrylium perchlorate, identified by its conversion to 2,4,6-trimethyl pyridine and its picrate. When isobutene itself was acylated using acetic anhydride and perchloric... 

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