Evidence for a solvent-induced change in rate-limiting step in the hydrolysis of benzaldehyde dimethyl acetal

Abstract

Secondary deuterium isotope effects for the hydrolysis of the dimethyl acetal of benzaldehyde-formyl-d have been determined in mixed water-dioxane solvents varying between pure water and 70% dioxane, 30% water, by volume. Between 0 and 20% dioxane, the isotope effect (k/sup H//k/sup D/) decreases from 1.12 to 1.06. Between 20 and 70% added dioxane, the isotope effects increase regularly from 1.06 to 1.15. The decrease in the apparent isotope effect at low concentrations of dioxane may be an artifact of the biphasic kinetics observed due to hemiacetal buildup in the more aqueous media. The increases in k/sup H//k/sup D/ that are observed between 20 and 70% dioxane are interpreted in terms of a change in rate-limiting step from C-O bond cleavage in water to rate-limiting diffusion apart of the alcohol-oxocarbonium ion encounter pair in high concentrations of dioxane. On the basis of this assumption, ratios of the rate constants for internal return and diffusional separation of the complex are estimated. The steady decrease in rho and rho/sup r/ within the series water and 50 and 70% dioxane also suggests an increase in cationic character in the rate-limiting transition state. Isolated resonance structure-reactivity parameters (rho/sup r//rho/sub eq//sup r/) provide estimates of sp/supmore » 2/ character in rate-limiting transition states that are essentially identical with the estimates obtained from the secondary isotope effect studies.« less