Dimerization of Carboxylic Acids in Solution up to High Pressures and Temperatures. 2. Benzoic Acid

Abstract

Benzoic acid in dilute solution of n-heptane and of CCl4 is studied via high-pressure hightemperature IR spectroscopy on the C = O and O - H stretching fundamentals. Lambert-Beer’s law is shown to be valid for the C = O modes of the acid monomer and of the hydrogen-bonded cyclic dimer, which enables the quantitative measurement of the dimerization equilibrium to a maximum pressure of 2 kbar and up to 175 °C. At identical pressure and temperature the dimerization equilibrium constant is larger in n-heptane than in CCl4. From the monomer-cyclic dimer equilibrium constants the pressure dependence of the dimerization enthalpy is determined and compared with direct information on both species as derived from their O - H fundamental mode absorption. Toward lower temperature the dimerization volume is decreasingly negative and, in CCl4 solution, ΔV even changes sign around 40 °C. Increasing pressure thus favours the dissociation of the hydrogen-bonded dimer into benzoic acid monomer molecules at ambient temperature.