Deuterium isotope effects in the solvolysis of benzal chlorides. 5. Salt effects and solvent isotope effects

Rates and solvent isotope effects have been determined for the solvolysis of p-methoxybenzal chloride (1) and diphenyldichloromethane (2) in 75% (v/v) dioxane-25% H2O and 85% (v/v) dioxane-15% H2O in the presence of various salts. Both 1 and 2 have solvolysis rates which follow the Winstein equation for the special salt effect, i.e., k/sub obsd/ = k0/sub ext/(1 + b/sub s/[salt]). For both 1 and 2, k0/sub ext//k0/sub obsd/ is identical for the two different salts LiClO4 and Bu4NClO4 in 85% dioxane. For dissociation of intimate ion pair rate limiting in 85% dioxane for 1 and 2, the solvent isotope effect is approx. 1.3. Rate-limiting capture of ion-pair intermediates in 75% and 85% dioxane furnish solvent isotope effects of approx. 1.5. It is suggested that these differences are due predominantly to the presence or absence of some covalent bonding to water in these transition states