An analysis of 13C nuclear magnetic resonance substituent chemical shifts in 4- and 5-substituted thiophene-2-carboxylic acids by linear free energy relationships

Abstract

The ¹³C n.m.r. chemical shifts of some 4- and 5-substituted thiophene-2-carboxylic acids and of the corresponding anions have been measured in methanol. The observed substituent chemical shifts have been compared with the calculated values and then analysed according to single and dual substituent parameter linear free energy relationships. The results have been rationalized in terms of separate resonance and inductive contributions of substituent effects. The resonance term has shown that no conjugation occurs between the ring and the carboxy group.