Hydrogen bonding and substituent group effects in phenols. Part II. Apparent dipole moments and infrared spectra of N-(4-substituted benzylidene)-4-hydroxyanilines in benzene, carbon tetrachloride, and dioxan

Abstract

Apparent dipole moments of N-(benzylidene)-4-hydroxyaniline in benzene and dioxan have been determined, together with the moments in dioxan of 12 N-(4-substituted benzylidene)-4-hydroxyanilines 4 4′-R·C₆H₄·-CHN·C₆H₄·OH and the corresponding hydroxy-group stretching frequencies ν(OH) in carbon tetrachloride and dioxan. To explain the observed moments in dioxan, values of hydroxy-group bond moments µ(O–H) have been calculated. Both ν(OH) and µ(O–H) are linearly related to Hammett's substituent parameter, σ, and a linear relationship between µ(O–H) and the relative frequency shift in dioxan provide a further illustration of the dependence of µ(O–H) on the strength of hydrogen bonding. Reduction factors for the efficiency of transmission of substituent group effects through intervening aromatic ring and azomethine or azo-bridges have been derived; the –C₆H₄·NN– unit is the best transmission medium in two solvents. Reduction factors appear to be solvent-dependent.