A RELATIONSHIP BETWEEN SELECTIVITY AND SOLVENT COMPOSITION FOR NUCLEOPHILIC-ATTACK ON CARBOCATIONS IN ALCOHOL-WATER MIXTURES

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 degrees 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 it acetonitrile is also present.