Pulse Radiolysis Investigations of Solvation Effects on Arylmethyl Cation Reactivity in Supercritical Fluids

Pulse radiolysis has been used to measure the influence of supercritical fluid and cosolvent solvation on arylmethyl cation reactivity in supercritical fluoroform (CHF 3 ) and ethane (C 2 H 6 ). Ion−neutral reactivity was investigated for the reaction of benzhydryl cation (Ph 2 C + H) with tetramethylethylene (TME) in CHF 3 at 35 and 50 °C, and with triethylamine (TEA) in CHF 3 at 35 and 70 °C, and for the reaction of 4,4‘-dimethoxybenzhydryl cation ((4-MeOPh) 2 C + H)) with TEA in CHF 3 at 35 and 70 °C, and with TEA in C 2 H 6 at 35 °C. The effect of pressure on the bimolecular rate constants ( k bi ) for these nucleophilic reactions was investigated over significant fluid density ranges, along the isotherms indicated, and in each case was found to decrease in magnitude with an increase of pressure (solvent density). The reactivity observed was analyzed in terms of the influence of bulk solvent dielectric constant on k bi, the comparison of the pressure effect on k bi with that predicted from transition state theory, and the influence of temperature and solvation on k bi . Evidence for preferential cosolvent solvation was observed for the ionic reactions investigated in supercritical CHF 3 .