PULSE-RADIOLYSIS INVESTIGATIONS OF SOLVATION EFFECTS ON ARYLMETHYL CATION REACTIVITY IN SUPERCRITICAL FLUIDS

Pulse radiolysis has been used to measure the influence of supercritic al fluid and cosolvent solvation on arylmethyl cation reactivity in su percritical fluoroform (CHF3) and ethane (C2H6). Ion-neutral reactivit y was investigated for the reaction of benzhydryl cation (Ph(2)C(+)H) with tetramethylethylene (TME) in CHF3 at 35 and 50 degrees C, and wit h triethylamine (TEA) in CHF3 at 35 and 70 degrees C, and for the reac tion of 4,4'-dimethoxybenzhydryl cation ((4-MeOPh)(2)C+H)) with TEA in CHF3 at 35 and 70 degrees C, acid with TEA in C2H6 at 35 degrees C. T he effect of pressure on the bimolecular rate constants (k(bi)) for th ese nucleophilic reactions was investigated over significant fluid den sity ranges, along the isotherms indicated, and in each case was found to decrease in magnitude with an increase of pressure (solvent densit y), The reactivity observed was analyzed in terms of the influence of bulk solvent dielectric constant on k(bi), the comparison of the press ure 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 CHF3.