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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
Miller, David J.,Saunders, William H.
, p. 4247 - 4252 (1981)
The rates of reaction of ethyl>trimethylammonium (1h) and 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 kH/kD (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, QtH/QtD, of 1.6-1.8 at 50 degC.Equations for calculating the effect of error in the Arrhenius preexponential factor, AaH/AaD, on the tunnel correction are derived.The error in QtH/QtD (3-18percent) is less than the error in AaH/AaD (10-50percent) but is still enough to render uncertain the significance of observed variations in QtH/QtD.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 kH/kD variation in these reactions is considered.
