Collect. Czech. Chem. Commun. 1999, 64, 1295-1306

Substituent Effects on Conformational Equilibria: Substituted Thiobenzanilides

Karel Waissera,*, Karel Palát, Jr.a and Otto Exnerb,*

a Department of Inorganic and Organic Chemistry, Faculty of Pharmacy, Charles University, 501 65 Hradec Králové, Czech Republic
b Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Prague 6, Czech Republic


Infrared absorption spectra of 38 thiobenzanilides in tetrachloromethane solutions were recorded in the ν(N-H) region. From the integrated absorption intensity of the computer-resolved N-H bands, the abundance of the Z and E conformations was calculated. The values of pertinent log KC depend on substitution in an unexpected way which cannot be described by the Hammett equation or by any other known type of modified constants σ. The only regularity observed is that the substituent effects from the two benzene rings of thioanilide are additive with a remarkable precision. The ν(N-H) frequencies are not much sensitive to substitution and the dependence is not simple: only ν(N-H) of the E conformer depend on substitution in a regular way.

Keywords: Thiobenzanilides; Thioamides; Conformation equilibrium; Infrared spectroscopy; Hammett equation; Substituent effects; Transmission effects.