Carboxylic acids and esters as bases

Evidence that carboxylic acids are protonated in concentrated solutions of strong acids has existed in the literature for many years. This consisted originally of the results of cryoscopic measurements. For example Hantzsch4 and Treffers and Hammett5 showed that several substituted benzoic acids, in 

Esters show similar behaviour. Hantzsch found the /-factor for ethyl acetate to be close to 2, and accurate determinations by Leisten also gave values close to 2 for ethyl and methyl benzoate and p-nitrobenzoate. In a solvent containing sufficient water, hydrolysis of the ester occurs. The reaction of methyl benzoate has a time of half-change of a few hours at 25°C in sulphuric acid containing about 0.07 M water, and Leisten actually used the increase in the / -factor, from 2 to 3, to follow the hydrolysis reaction, viz. 

Me+ H2S04 PhCO HMe+ + HSOr PhC02HMe+ + H,S04 PhCO.H,+ + MeHSCX, 

Thus it is possible to study the hydrolysis reactions of esters under conditions where the substrate is completely protonated. The properties of the protonated ester, however, are more conveniently examined using more strongly acidic media, in the absence of water, where bimolecular reactions are reduced to insignificance. At sufficiently low temperatures under these conditions the rates of exchange of the added protons are slow, and the detailed structures of protonated carboxylic acids and esters can be investigated, particularly by proton nmr techniques. 

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