TRANSMISSION OF SUBSTITUENT EFFECTS THROUGH THE CARBOXAMIDE AND THIOCARBOXAMIDE GROUPS

Abstract Substituted 3-fluorothiobenzanilides(1) and 4-fluorothiobenzanilides (2) were used as model compounds for evaluating the transmission of substituent effects through the thiocarboxamide group: in addition to 19F substituent chemical shifts the 13C and 1H shifts were also monitored. The transmitted effect was always weaker than when transmitted through the carboxamide group in analogous carboxamides (3) and (4), the ratio of 0.76 was estimated conformably from 19F, 13C(1) and 13C(4) substituent induced shifts. The interpretation may refer to the greater polarizability of the C= bond which allows the greater part of the charge to be accommodated on sulphur and a relatively smaller part is transmitted further along the chain. This simplified picture is corroborated by substituent effects on the thiocarboxamide group itself (on the 13C(S) and 1H(N) substituent shifts) which are greater than in the case of carboxamides. This interpretation seems to us more probable than reference to the conformational equilibrium, observable in the solution of thioanilides but shifted in anilides to one side.