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

The effect of LiClO/sub 4/, Bu/sub 4/NClO/sub 4/, LiCl, Bu/sub 4/NCl, and mixtures of these salts on the rate and ..cap alpha..-deuterium isotope effects are reported for the solvolysis of p-methoxybenzal chloride in 75% and 85% (v/v) dioxane-water mixtures at 25/sup 0/C. In the less polar of these solvents (85D), k/sub ext//sup 0/ values from the Winstein equation for the two salts LiClO/sub 4/ and Bu/sub 4/NClO/sub 4/ are the same, and very little common-ion rate depression is observed although induced common-ion rate depression is observed. In the more polar solvent (75D), extensive common-ion rate depression is observed for both LiCl and Bu/sub 4/NCl. k/sub ext//sup 0/ values for Bu/sub 4/NClO/sub 4/ and LiClO/sub 4/ are not the same in this more polar solvent. These results are interpreted by using Winstein's ion-pair mechanism for this solvolysis.