A relationship between selectivity and solvent composition for nucleophilic attack on carbocations in alcohol–water mixtures

Abstract

Rate constants and products of solvolyses of p-methoxybenzyl chloride 1, chlorodiphenylmethane 2(Y = Z = H), chloro(4-chlorophenyl)phenylmethane 2(Y = H, Z = Cl) and chlorobis(4-chlorophenyl)methane 2(Y = Z = Cl) are reported in ethanol– and methanol–water mixtures at 25 °C. Product selectivities (S), defined by: S=[ether product][water]/[alcohol product][alcohol solvent] are related to four rate constants for reactions involving one molecule of solvent as nucleophile and another molecule of solvent as general base catalyst (e.g. k_wa involves water as nucleophile and alcohol as general base, and k_ww, k_aw and k_aa are defined similarly). A linear relationship between 1/S and molar ratios of solvent 1/S=(k_wa/k_aw)([alcohol solvent]/[water])+k_ww/k_aw is derived theoretically and validated experimentally for solvolyses of the above substrates from water up to 70% alcohol–water—in this range of solvents, the contribution from k_aa can be neglected. For solvolyses of p-methoxybenzyl chloride, S is independent of pH between pH 2 and 12, S decreases when acetone is added but increases if acetonitrile is added and for 90% ethanol–water S increases with added LiCl and LiClO₄ and increases further if acetonitrile is also present.