The structures of protonated carboxylic acids and esters

In principle, a carboxylic acid or ester could be protonated on either of its two oxygen atoms. 

There is general agreement that in fact protonation occurs predominantly at the carbonyl oxygen as in (1). This is expected to give a more stable product than (2) because of the possibility of delocalisation, viz- 

Experimentally, the close correlation between the basicities of substituted benzoic acids and the corresponding acetophenones7 is most simply explained if protonation occurs on the carbonyl oxygen in each case as, of course, it must for the ketones. 

Further, the chemical shifts for the protons concerned, which correlate well with the acidity of the conjugate acids, fall in overlapping regions for protonated ketones and acids, at considerably lower field than for the corresponding protons of the conjugate acids of ethers and alcohols8. 

The protonation behaviour of esters is closely similar to that of the parent acids. Birchall and Gillespie9 reported the observation of protonated ethyl acetate in HSO.,F-SbF5 solution at — 75°C, and concluded that protonation occurs on the carbonyl oxygen atom as in (4b), since they did not observe the spin-spin coupling between the added proton and the methylene group of the ether oxygen which would be expected if both were attached to the same atom, as in (4a). 

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