Dipole moments and infrared spectra of a series of 4′-substituted 4-hydroxyazobenzenes

Abstract

Apparent dipole moments in benzene and dioxan, and i.r. spectra of a series of 4′-substituted 4-hydroxyazobenzenes have been determined. The effect of the phenylazo-group is shown by comparing the moments with those of corresponding substituted phenols. The dipole moments determined in dioxan are all greater than those in benzene, indicating hydrogen bonding between the hydrogen atom of the hydroxyl group and the oxygen atom of the dioxan molecule. Calculations have been made of the O–H bond moments necessary to explain the observed moments in dioxan and µ(O–H) is found to be linearly related to Hammett's substituent constants, σ. Similar linear relationships are observed between the i.r. OH stretching frequency, ν(O–H), and σ. If the relative frequency shift in dioxan is a measure of the enthalpy change on solute–solvent association, then the dependence of O–H bond moment upon the strength of hydrogen bonding is demonstrated.