TRANSMISSION OF SUBSTITUENT EFFECTS THROUGH THE CARBOXAMIDE AND THIOCARBOXAMIDE GROUPS

Substituted 3-fluorothiobenzanilides (1) and 4-fluorothiobenzanilides (2) were used as model compounds for evaluating the transmission of su bstituent effects through the thiocarboxamide group: in addition to F- 19 substituent chemical shifts the C-13 and H-1 shifts were also monit ored. The transmitted effect was always weaker than when transmitted t hrough the carboxamide group in analogous carboxamides (3) and (4), th e ratio of 0.76 was estimated conformably from F-19, C-13(1) and C-13( 4) substituent induced shifts. The interpretation may refer to the gr eater polarizability of the C=S 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 co rroborated by substituent effects on the thiocarboxamide group itself (on the C-13(S) and H-1(N) substituent shifts) which are greater than in the case of carboxamides. This interpretation seems to us more prob able than reference to the conformational equilibrium, observable in t he solution of thioanilides but shifted in anilides to one side.