Proton transfer from carbon acids to carbanions. 1. Reactions of various carbon acids with the anions of substituted benzylmalononitriles in 90% Me2SO-10% water. determination of intrinsic barriers of identity reactions from the marcus relationship

Article abstract of DOI:10.1021/ja00065a016

A kinetic study of the reversible deprotonation of 9-cyanofluorene (2), 1,3-indandione (3), 4-nitrophenyl-acetonitrile (4), (3-nitrophenyl)nitromethane (5), and (4-nitrophenyl)nitromethane (6) by the anions of substituted benzylmalononitrile (1-X^-) in 90% Me₂SO-10% water (v/v) at 20 °C is reported. Intrinsic rate constants and intrinsic barriers of these reactions have been determined by extrapolation or interpolation of Br?nsted plots whose slopes (β) are all close to 0.5. Intrinsic barriers of the identity reactions CH + C^- ? C^- + CH (CH = 2,3,4,and phenylnitromethane) have been estimated on the basis of the Marcus equation, coupled with either a plausible value for the identity barrier of the reaction AH⁺ + A ? A + AH⁺ (A = piperidine or morpholine) ("amine method") or a plausible value for the identity barrier of the reaction 2 + 2- ? 2^- + 2 ("9-cyanofluorene method"). There are discrepancies in the identity barriers for CH + C^- ? C^- + CH (CH = 2, 3, 4, and phenylnitromethane) calculated by the two methods. Possible reasons for these discrepancies and the significance of the results in terms of the validity and scope of the Marcus equation are discussed.