The basicity of carboxylic acids and esters

The single most crucial piece of information necessary for the interpretation of the behaviour of a specific acid-catalyzed reaction is the concentration of the protonated, reactive species this depends on a knowledge of the basicity of the substrate. Until very recently this information has not been available for carboxylic esters, and consequently most work has necessarily been done using relatively weakly acidic media, where it can be assumed that the concern ration of the protonated species is small. 

The pKa s of protonated benzoic acid and its derivatives are readily measured by conventional spectrophotometric methods. Data for over forty compounds have been obtained by Stewart and Granger34 and Stewart and Yates35 and the results have been summarized by Arnett36. Substituted benzoic acids are well-behaved Hammett bases, with slopes of the logarithm of the ionization ratio plots close to 1.0. But, as explained by Arnett36, this is only true, for benzoic itself t any rate, in its region of protonation. In more dilute acid a sudden sharp change in activity coefficient occurs, which means that the observed 

The protonation of aliphatic carboxylic acids can be followed by nmr techniques (as explained above) or spectrophotometrically, using a band near 190 nm in the ultraviolet. Some values are given in Table 3. The aliphatic 

BASICITIES OF CARBOXYLIC SULPHURIC ACIDS AND ACID ESTERS 1 N  

Aliphatic esters do not behave even approximately as expected for Hammett bases. Plots of the logarithms of the ionization ratios against — H0 are linear, but have a slope of 0.62 for simple acetates37. Lane et al.3t have found a similar slope, 0.65, for ethyl acetate38. 

---