USE OF KINETIC ISOTOPE EFFECTS IN MECHANISM STUDIES - ISOTOPE EFFECTSAND 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 m ethoxide-promoted dehydrohalogenations of m-(ClC6H4CHClCH2Cl)-H-i (I), m-(CF3C6H4CHClCH2Cl)-H-i (II) and p-(CF3C6H4CHClCH2F)-H-i (III) has b een used to calculate the internal-return parameters, a = k(-1)/K-Elim (X) in a two-step mechanism featuring a hydrogen-bonded carbanion. Thi s carbanion partitions between returning the hydron to carbon, k(-1), and the loss of halide, k(Elim)(X). Isotope effects at 25 degrees C fo r 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 val ues for a: a(H) = 0.59, a(D) = 0.13-0.14 and a(T) = 0.07. Smaller valu es 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 tran sfer 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-(ClC6 H4CHBrCH2Br)-H-i and m-(ClC6H4CHClCH2Br)-H-i have very small amounts o f 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 tran sfer, and the element effect, (k(HBr)/k(HCl)) = 29, for m-ClC6H4CHClCH 2X 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 ef fects, k(MeOD)/k(MeOH) approximate to 2.5, are consistent with three m ethanols of solvation lost prior to the hydron-transfer step. The ener getics associated with desolvation of methoxide ion are part of the me asured reaction energetics of these systems.