Mechanisms of elimination reactions. 34. Deuterium and nitrogen isotope effects and Hammett correlations in the reaction of (2-arylethyl)trimethylammonium ions with hydroxide ion in mixtures of water and dimethyl sulfoxide

Abstract

When (2-phenylethyl)trimethylammonium and (2-phenylethyl-2,2-d/sub 2/)trimethylammonium bromides are treated with sodium hydroxide in aqueous dimethyl sulfoxide at 60/sup 0/C, increasing the dimethyl sulfoxide concentration leads to an increase in the elimination rate (> 10/sup 3/ between 17.1% and 57.1% Me/sub 2/SO). There is also an increase in k/sub H/k/sub D/ from 3.22 at 17.1% Me/sub 2/SO to 5.21 at 34.3%, followed by a decrease to 3.83 at 57.1%. The nitrogen isotope effect (k/sub 14//k/sub 15/) decreases initially from ca 1.009 (estimated from data at 97/sup 0/C) in water to 1.007 in 22.8% Me/sub 2/SO but thereafter remains essentially constant around 1.006 to 1.007 up to 57.1% Me/sub 2/SO. When substituted (2-phenylethyl)trimethylammonium bromides (substituents p-Cl, H, p-Me, and p-MeO) are used, the rates fit the Hammett equation. The Hammett rho is + 3.11 in 17.1% Me/sub 2/SO, but after an initial rise it remains essentially constant (+ 3.38 +- 0.09) from 30% to 50.6% Me/sub 2/SO. Both the nitrogen isotope effects and the Hammett rho values indicate a transition state that becomes somewhat more reactant-like with increasing dimethyl sulfoxide concentration. The deuterium isotope effects are consistent with this picture but suggest a greater shift in transition-state structure.