AbstractZwischen den 13 C‐chemischen Verschiebungen des Carbinyl‐CU) von 28 untersuchten Estern und den pKa‐Werten der zugrunde liegenden Carbonsäuren R‐COOH (mit R: ‐CH3, ‐CMe3, ‐Ph, p‐NO, ‐C6H4 , ‐CH2Cl, ‐CHCI2 , ‐CC13) existiert eine lineare Beziehung.

The 13C chemical shifts of the 28 carboxylic esters have been determined by high-resolution NMR spectroscopy with the aid of proton decoupling. A linear relationship is shown to exist between the 13C chemical shifts of the carbinyl carbon (C-1) of the esters and the pKa values of the acids from which they are derived. This is a consequent of the polar character of the bond. Similarly, if the carboxyl group is kept constant, but the alcoholic part of the ester is varied from primary to secondary and tertiary alcohols, the esterification effect on C-1 can be correlated with the increasing stability of the +δ charge on the carbinyl carbon. The smallest esterification effect at C-1 (1.3 ppm, relative to the parent alcohol) is observed for methyl pivalate (pKa 5.03 for the parent acid), and the highest effect (17.7 ppm) for 2-methyl-2-propyl trichloroacetate (pKa 0.70). In contrast, the C-2 esterification effect has been found to be essentially constant (−3.8±0.7 ppm), which is in agreement only with a conformation of the ester group in which the carbinyl carbon is cis with respect to the CO group.