Mechanisms of elimination reactions. 35. Deuterium kinetic isotope effects and tunneling in the reaction of [2-[p-(trifluoromethyl)phenyl]ethyl]trimethylammonium ion with hydroxide ion in mixtures of water and dimethyl sulfoxide

Abstract

The rates of reaction of (2-((p-trifluoromethyl)phenyl)ethyl)trimethylammonium (1h) and (2-((2-trifluoromethyl)phenyl)ethyl-2,2-d/sub 2/)trimethylammonium (1d) iodides with hydroxide ion in mixtures of dimethyl sulfoxide and water were determined as a function of temperature. A maximum in k/sub h//k/sub D/ (7.25 at 50/sup 0/C) is observed at 35% dimethyl sulfoxide. The Arrhenius parameters provide qualitative evidence for tunneling, and fitting the Bell theory expression for tunneling to the observed temperature dependence gives tunnel corrections, Q/sub tH//Q/sub tD/, of 1.6 to 1.8 at 50/sup 0/C. Equations for calculating the effect of error in the Arrhenius preexponential factor, A/sub aH//A/sub aD/, on the tunnel correction are derived. The error in Q/sub tH//Q/sub tD/ (3 to 18%) is less than the error in A/sub aH//A/sub aD/ (10 to 50%) but is still enough to render uncertain the significance of observed variations in Q/sub tH//Q/sub tD/. These results are compared with earlier results on the (2-phenylethyl)trimethylammonium ion, and the effect of the p-trifluoromethyl substituent on the structure of the transition state for elimination is discussed. The application of Marcus theory to k/sub H//k/sub D/ variation in these reactions is considered.