Solvent sensitivity of ortho substituent effect on 13C NMR chemical shift of the carboxyl carbon (δco) in benzoic acid

Abstract

A reverse ortho effect is observed for the 13C NMR chemical shifts of the carboxyl carbon (δco) in benzoic acids measured in aprotic solvents of varying polarity. The ortho effect on δco is best described by a combination of the reverse field and steric accelerating effects of the substituent in an 80:20 pattern in apolar aprotic solvents and a 60:40 pattern in dipolar aprotic ones. Interestingly, no good enough correlation was found between δco and log k 1 of the acids measured in similar solvents. A critical analysis of the results clearly indicates the use of an apolar aprotic solvent and not a dipolar aprotic one as the solvent of choice for investigating intrinsic substituent effects on δc in an aromatic system. Copyright © 2011 John Wiley & Sons, Ltd.