Non-linearity and non-additivity of substituent effects in solvolysis of 1,1-diphenylethyl p-nitrobenzoates

The solvolysis rates of 1, 1-diarylethyl p-nitrobenzoates and chlorides Y-Ar(X-Ar)CMe-L-G (L-G = OPNB, Cl) have been determined conductimetrically at 25 degreesC in 80% (v/v) aqueous acetone. A linear Yukawa-Tsuno (Y-T) correlation was found for the symmetrical subseries (X = Y), showing a precise additivity relationship for the whole substituent range with rho(sym) = -3.78 and r(sym) = 0.77. The unsymmetrical subsets (X not equal Y) gave statistically less reliable Y-T correlations, the apparent p value decreasing significantly when the fixed substituent Y becomes more electron-donating, which is in line with expectations from the Reactivity-Selectivity Relationship. In the whole dispersion pattern, both strong p-pi-donor and electron-withdrawing substituents in any fixed-Y subsets exhibit significant rate-enhancement deviations from the points of X = Y on the reference rho(sym) line, which suggests an anti-Hammond shift of the transition state. However, there was a precise Extended Bronsted Linear Relationship between the pK(R)(+) values for the hydration of 1,1-diarylethylenes and the rates of solvolysis of the p-nitrobenzoates with a slope of unity (alpha = 1.03). This is direct, convincing evidence that there is no significant shift of the transition-state coordinate over the whole range of substituent change.