Reactions of Charged Substrates. 3. The Hydrolysis of (4-Methoxybenzyl)dimethylsulfonium Chloride

Abstract

At low concentrations (ca. 10 -4 M), the hydrolysis of (4-methoxybenzyl)dimethylsulfonium chloride (1) proceeds smoothly to completion in H 2 O or D 2 O at 80 o C. The rate constants measured directly by UV or NMR methods match the rate constants obtained as the ordinate intercepts of plots of k obsd vs [NaN 3 ] or [pyridine-d 5 ]. As expected, the rate constants decrease with increasing ionic strength (NaCl or NaClO 4 , μ=0.2). There is a small solvent deuterium isotope effect, k H /k D =1.1 at μ=0 and 1.7, and the α-deuterium secondary isotope effect is k H /k D =1.26 Per deuterium. At higher concentrations (10 mM), the hydrolysis does not go to completion, with the fraction of 1 remaining reaching plateau values that are stable. Plots of the fraction of 1 remaining vs the initial concentration of 1 extrapolate to zero. Running the reaction in the presence of either Hg +2 or Zn +2 drives the reaction to completion by removing SMe 2 as the Lewis complex. While these results are consistent with common leaving group suppression of hydrolysis, they are also consistent with the establishement of an equilibrium among the starting material and the product alcohol, SMe 2 , and hydronium ion. The suppression data are fitted to the equation for the equilibrium, and running the reaction in reverse gives the amount of 1 predicted by the equation. Reinterpretation of older data in light of the equilibrium, the results reported here, and results for the nucleophilic substitution reaction suggests that the mechanism of hydrolysis is S N 1 with no ion-dipole complex intermediate