Thermodynamic studies of gas-phase proton transfer equilibria involving benzene: a reassessment of earlier data

Abstract

Temperature-variable equilibrium constant measurements have been performed for a number of proton-transfer equilibria in which benzene was a partner, using a newly built high-pressure pulsed source mass spectrometer. Entropy values obtained showed that the protons in protonated benzene are not as mobile as previously thought. Systems involving ethanol are found to give anomalous, though self-consistent results owing to the onset of thermal decomposition. In the light of this, previous data involving halotoluenes and xylenes which appear to show unusually large increases in entropy on protonation, have been reassessed. There is evidence of proton-induced isomerisation. In the reaction [4-CIC₆H₄CH₃]H⁺→[3-CIC₆H₄CH₃]H⁺ the free energy of activation is derived to be ca. 90 kJ mol^–1 from a computer model fit to the results, consistent with the energy calculated to be needed for a proton shift from the 3 to the 4 position in the precursor. The equivalent reaction for protonated 4-fluorotoluene has a barrier which is ca. 10 kJ mol^–1 higher. A kinetic scheme is presented which shows how this could account for the observed ‘thermodynamic’ behaviour, and also give rise to the ‘isokinetic effects’ previously noted. There has therefore been some readjustment of the recommended proton affinity values for some of these compounds.