Reactions of protonated carboxylic acids and esters

The important reactions of protonated acids and esters were deduced many years ago, largely on the basis of the anomalous cryoscopic behaviour of a few compounds. Treffers and Hammett5, while extending the work of Hantzsch4 on cryoscopic measuremnets in concentrated sulphuric acid, found that 2,4,6-trimethylbenzoic acid gives an / -value of 4, twice the usual figure. The only reasonable interpretation of this result was in terms of an ionization, as proposed by Hantzsch to account for the similar behaviour of triphenylcarbinol, viz. 

Consistent with this interpretation was the observation that the acid could readily be esterified by pouring the solution in sulphuric acid into methanol, although it is highly unreactive under normal conditions 7. 

Under the same conditions, the methyl ester is also converted to the tri-methylbenzoyl cation. The i-value in this case is 5, since the molecule of methanol lost is converted to methyl sulphate18, viz. 

For steric reasons the 2,6-disubstituted benzoic acids and esters are particularly susceptible to this type of cleavage reaction, and also particularly unreactive in the usual bimolecular solvolytic processes, and they have proved very convenient substrates for the study of the AacI mechanism. The kinetic work is discussed in a later section we are concerned at this point only with the qualitative behaviour of protonated esters amt acids, and of the structures of the cationic species. 

A second type of anomalous cryoscopic behaviour is exhibited by f-butyl esters . The benzoate and acetate both give varying / -values between 3 and 4, and by extrapolating the value for f-butyl benzoate to zero time Kuhn1 obtained a value close to 3.0. This he interpreted as evidence for alkyl-oxygen cleavage, and the formation of the f-butyl cation, viz- 

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