Spectroscopic studies on reactive intermediates of esterification of aromatic carboxylic acids

Abstract

Ultra-violet absorption spectra were measured with a total of nine benzoic acid derivatives under various conditions, with special reference to the behavior of the intramolecular charge-transfer (CT) bands, which are caused by the conjugation of the benzene ring with the carboxyl group and therefore are expected to be sensitive to the steric hindrance. In fact, it was observed that the CT band, which appears with high intensity at 228 mμ with the aqueous solution of benzoic acid, becomes weak for the aqueous or n-heptane solution of mesitoic acid [(CH 3) 3C 6H 2CO 2H)], evidently because of the resonance inhibition caused by the steric requirement of the o-methyl groups. On the other hand, in concentrated sulphuric acid solutions strong CT bands were observed with both benzoic acid (262 mμ) and mesitoic acid (282 mμ). Furthermore, the latter band was found to be very similar to those observed with mesitoic acid-trifluoroacetic anhydride and mesitoyl chloride-stannic chloride systems which contain typical acylating reagents. This means that the 282 mμ band observed with the concentrated sulphuric acid solution of mesitoic acid is due to the mesitoyl cation [(CH 3) 3C 6H 2CO +)], which may be regarded as the intermediate state of acid catalysed esterification of mesitoic acid. Concerning the 262 mμ band of benzoic acid in sulphuric acid, its origin was ascribed to protonated benzoic acid cation. Further, it was demonstrated from the measurement of Raman spectra that in the protonated benzoic acid cation a proton is attached to the carbonyl oxygen atom. Detailed quantitative analyses of equilibria were made for the interaction of sulphuric acid with benzoic acid and with mesitoic acid by the use of the new method devised by the present authors. It was, then, clearly shown that, in strongly acidic media, mesitoic acid ionizes in the following two steps as suggested by Schubert et al.: ▪