An analysis of 13C nuclear magnetic resonance substituent chemical shifts in 3-substituted thiophene-2-carboxylic and 2-substituted benzoic acids by linear free energy relationships

The 13C n.m.r. chemical shifts of the title compounds and of the corresponding anions have been measured in methanol. The observed substituent chemical shifts have been analysed by means of cross-correlations and of single and dual substituent parameter linear free energy relationships. Different results have been obtained for thiophene and benzene derivatives using DSP analysis: for the first ring system the results have been rationalized in terms of separate resonance and inductive contributions of substituent effects (pointing out the occurrence of an apparent reverse polar effect); in contrast the same treatment of the data for the benzene system gave poor results, confirming once again the different proximity effects in six-and five-membered ring derivatives.