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

Article abstract of DOI:10.1021/jo00334a027

When (2-phenylethyl)trimethylammonium and (2-phenylethyl-2,2-d₂)trimethylammonium bromides are treated with sodium hydroxide in aqueous dimethyl sulfoxide at 60 degC, increasing the dimethyl sulfoxide concentration leads to an increase in the elimination rate (>10³ between 17.1percent and 57.1percent Me2SO).There is also an increase in k_H/k_D from 3.22 at 17.1percent Me2SO to 5.21 at 34.3percent, followed by a decrease to 3.83 at 57.1percent.The nitrogen isotope effect (k₁₄/k₁₅) decreases initially from ca. 1.009 (estimated from data at 97 degC) in water to 1.007 in 22.8percent Me2SO but thereafter remains essentially constant around 1.006-1.007 up to 57.1percent Me2SO.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 ρ is +3.11 in 17.1percent Me2SO, but after an initial rise it remains essentially constant (+3.38 +/- 0.09) from 30percent to 50.6percent Me2SO.Both the nitrogen isotope effects and the Hammett ρ 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.