Mechanisms of Elimination Reactions. 35. Deuterium Kinetic Isotope Effects and Tunneling in the Reaction of ethyl>trimethylammonium Ion with Hydroxide Ion in Mixtures of Water and Dimethyl Sulfoxide

Article abstract of DOI:10.1021/jo00334a028

The rates of reaction of <2-<(p-trifluoromethyl)phenyl>ethyl>trimethylammonium (1h) and <2-<(2-trifluoromethyl)phenyl>ethyl-2,2-d₂>trimethylammonium (1d) iodides with hydroxide ion in mixtures of dimethyl sulfoxide and water were determined as a function of temperature.A maximum in k_H/k_D (7.25 at 50 degC) is observed at 35percent 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_tH/Q_tD, of 1.6-1.8 at 50 degC.Equations for calculating the effect of error in the Arrhenius preexponential factor, A_aH/A_aD, on the tunnel correction are derived.The error in Q_tH/Q_tD (3-18percent) is less than the error in A_aH/A_aD (10-50percent) but is still enough to render uncertain the significance of observed variations in Q_tH/Q_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_H/k_D variation in these reactions is considered.