Dimerization of p-substituted benzoic acids

Abstract

Infrared spectroscopic studies have been carried out to determine the type and extent of hydrogen bond formation occurring in dilute solutions of benzoic, p-toluic, p-chlorobenzoic, p-iodobenzoic and p-fluorobenzoic acids in CCl 4 solution. Association constants were calculated separately from absorbance-concentration data using (a) the free—OH band, (b) the carbonyl dimer band, (c) the carbonyl monomer band, and (d) data from both carbonyl monomer and dimer bands. Results of the different methods of calculation did not differ significantly. At concentrations below 10 −3 M, the primary process is self-association to form cyclic dimers. The individual association constants of the various acids did not differ significantly. For the process 2 monomer ⇌ dimer, the average equilibrium constant is 5.89 × 10 3 M −1 at 25°, and 2.33 × 10 3 M −1 at 45°. The enthalpy of dimerization is −9.0 kcal/mole of dimer.