Stoichiometric Solvent Effect on S_N1 Solvolytic Reactivity Accounting for Phenomenon of Maximum Rates in Methanol-Nitromethane Mixtures

Abstract

Pseudo-first order rate constants (k obs ) are reported for the following solvolyses in approximately isodielectric mixtures: 3-and 4-methoxybenzyl chloride, bromo-and chlorodiphenylmethane, and 4-chloro-, 4,4'-dichloroand 4-methyl-chlorodiphenylmethane in 0-80% v/v nitromethane-methanol mixtures; and bromo-and chlorodiphenylmethane and 4-methyl-chlorodiphenylmethane in various acetonitrile-methanol mixtures (in the range 0-50% v/v) at 25 ºC. These data, and literature data for t-butyl halides (Cl, Br, and I), and for p-methoxybenzoyl chloride, show rate maxima in solvent compositions of ca. 30% aprotic cosolvent, explained by a stoichiometric solvent effect on electrophilic solvation. Linear relationships are observed between (k obs )/ [MeOH] 2 and [AP]/[MeOH], where [AP] refers to the molar concentration of aprotic cosolvent. The results are consistent with competing third order contributions to kobs, kMM [MeOH] 2 with hydrogen-bonded methanol as electrophile, and kMAP [MeOH][AP] with hydrogen-bonding disrupted by the aprotic solvent.