Use of kinetic isotope effects in mechanism studies. Isotope effects and element effects associated with hydron-transfer steps during alkoxide-promoted dehydrohalogenations

The Arrhenius behavior of the primary kinetic isotope effect, (k(H)/k(D))(Obs) and (k(H)/k(T))(Obs), associated with the methanolic sodium methoxide-promoted dehydrohalogenations of m-(ClC6H4CHClCH2Cl)-H-i (I), m-(CF3C6H4CHClCH2Cl)-H-i (II) and p-(CF3C6H4CHClCH2F)-H-i (III) has been used to calculate the internal-return parameters, a = k(-1)/K-Elim(X) in a two-step mechanism featuring a hydrogen-bonded carbanion. This carbanion partitions between returning the hydron to carbon, k(-1), and the loss of halide, k(Elim)(X). Isotope effects at 25 degrees C for I, (k(H)/k(D))(Obs) = 3.40 and (k(H)/k(T))(Obs) = 6.20, and II, (k(H)/k(D))(Obs) = 3.49 and (k(H)/k(T))(Obs) = 6.55, result in similar values for a: a(H) = 0.59, a(D) = 0.13-0.14 and a(T) = 0.07. Smaller values of (k(H)/k(D))(Obs) = 2.19 and (k(H)/k(T))(Obs) = 3.56 for III are due to more internal return [a(H) = 1.9, a(D) = 0.50, and a(T) = 0.28] associated with the dehydrofluorination reaction. Calculation of k(1) (= k(Obs) [a + 1]) results in similar isotope effects for hydron transfer in these reactions: I, k(1)(H)/k(1)(D) = 4.74 and k(1)(H)/k(1)(T) = 9.20; II, k(1)(H)/k(1)(D) = 4.91 and k(1)(H)/k(1)(T) = 9.75; III, k(1)(H)/k(1)(D) = 4.75 and k(1)(H)/k(1)(T) = 9.17. Reactions of m-(ClC6H4CHBrCH2Br)-H-i and m-(ClC6H4CHClCH2Br)-H-i have very small amounts of internal return, a(H) = 0.05 and a(D) = 0.01, and (k(H)/k(D))(Obs) = 4.95 results in k(1)(H)/k(1)(D) = 5.11. The measured isotope effects are therefore due to differences in the amount of internal return and not in the symmetry of transition state structures for the hydron transfer, and the element effect, (k(HBr)/k(HCl)) = 29, for m-ClC6H4CHClCH2X is mainly due to the hydron-transfer step, k(1)(HBr)/k(1)(HCl) = 19, and not the breaking of the C-X bond. The kinetic solvent isotope effects, k(MeOD)/k(MeOH) approximate to 2.5, are consistent with three methanols of solvation lost prior to the hydron-transfer step. The energetics associated with desolvation of methoxide ion are part of the measured reaction energetics of these systems.