An infra-red study of the association of benzoic acid in the vapour phase, and in dilute solution in non-polar solvents

Abstract

The infra-red spectra of benzoic acid in the vapour phase, and in dilute solution in cyclohexane, carbon tetrachloride, and benzene show two bands between 1770 and 1670 cm −1 which are assigned respectively to “free” carbonyl groups, and to carbonyl groups involved in hydrogen bonding between acid molecules. The intensities of these have been used to determine equilibrium constants for the dimerisation of benzoic acid in the vapour phase and in these solvents, in each case at several temperatures. The enthalpy and entropy changes of dimerisation have been evaluated; their magnitudes vary with the medium in the order: benzene < carbon tetrachloride < cyclohexane < vapour. The variations are attributed to decreasing interaction between the monomeric acid and the medium along this series.