Kinetic isotope effects for proton abstraction from methanol by polyhalogenomethyl carbanions. Cleavage of Me3SiCHX2 and Me3SiCX3 by base in methanol

Abstract

The carbanions X_xH_{(3 –x)}C^–(X = Cl or Br; x= 2 or 3) generated by base cleavage of Me₃SiCH_{(3 – x)}X_x(or some related compounds) in MeOH, show a kinetic isotope k_H/k_D of ca. 1.1 in proton abstraction from methanol, as given by the product ratio X_xH_{(3 –x)}CH/X_xH_{(3 –x)}CD observed for reaction in MeOH–MeOD (1:1) at ca. 21 °C. The low value of the isotope effect is attributed to the fact that the free electron pair in the carbanion is localized on the carbon centre; carbanions derived from acids of acidities comparable with those of X₃CH and X₂CH₂ but in which the electron pair is conjugatively delocalized, show much larger isotope effects