Electrophilicity versus Electrofugality of Tritylium Ions in Aqueous Acetonitrile

Abstract

First-order rate constants k(w) for the reactions of a series of donor-substituted triphenylmethylium (tritylium) ions with water in aqueous acetonitrile have been determined photometrically at 20 degrees C using stopped-flow and laser-flash techniques. The rate constants follow the linear free energy relationship log k(20 degrees C)=s(N+E). The reactivities k(w) of the methyl- and methoxy-substituted tritylium ions towards water correlate linearly with the corresponding pK(R(+) ) values with a Leffler-Hammond coefficient alpha=deltaDeltaG(++)/deltaDeltaG(0) of 0.62. The amino-substituted compounds react more slowly than expected from the correlation of the less stabilized systems. Quantum chemical calculations of tritylium ions and the corresponding triarylmethanols and 1,1,1-triarylethanes have been performed at the MP2(FC)/6-31+G(2d,p)//B3LYP/6-31G(d,p) level. The calculated gas-phase hydroxide and methyl anion affinities of the tritylium ions correlate linearly with a slope of unity, indicating that the relative anion affinities do not depend on the nature of the anion. The pK(R(+)) values of the methyl- and methoxy-substituted tritylium ions correlate linearly with the calculated gas-phase hydroxide affinities, and the slope of this correlation shows that the differences in carbocation stabilities in the gas phase are attenuated to 66% in solution. Mono- and bis(dimethylamino)-substituted derivatives deviate from this correlation; their pK(R(+)) values are higher than expected from their calculated gas-phase hydroxide affinities, which is explained by the extraordinary solvation of unsymmetrically amino-substituted tritylium ions. Complete free-energy profiles for the solvolyses of substituted trityl benzoates in 90:10 (v/v) acetonitrile/water have been constructed.