Carboxylic Acid Derivatives and Esterification

At a first glance it may appear eccentric to show interest in the basicities of carboxylic acids. However, this is not as curious as it seems, since protonated carboxylic acids and their derivatives play the role as important intermediates during acid-catalysed reactions of these compounds. As a matter of fact, Scheele showed as early as in 1782 that addition of sulphuric acid to a mixture of acetic acid and ethanol accelerated esterification [192]. In this respect it is a key point that carboxylic acids, esters and anhydrides possess two oxygens which are po- 

There appear to be few studies related to substituent effects on the properties of protonated carboxylic acid and related molecules. It has been found that the proton affinities of the saturated methyl esters increase with increasing chain length ( ) according to the equation (in units of kj mol-1) 

The proton affinities of alicyclic carboxylic acids are identical within 5 kj mol-1, with cyclohexane cyclopropane cyclopentane cyclobutane [205]. This quite surprising situation seems not to be fully explained yet. 

Although chemical ionisation spectra have been recorded for a large number of carboxylic acids and esters, there are few detailed studies of the unimolecular reactivity of protonated molecules of these classes of compounds. Early investigators [206] reported the general reaction  

It should also be noted that in the case of formic acid, two products are formed, the formyl cation plus water (as discussed above), plus and hydronium ion plus carbon monoxide [206]. The latter product seems to be the result of intracomplex proton transfer within the dissociating RCO OHJ. Likewise, the higher homologue, protonated methylformiate gives CHsOH) + CO as the only product [209,210]. 

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