Base-Catalyzed Isotopic Exchange of Molecular Hydrogen. 5. The Potassium Anilide-Aniline System

Article abstract of DOI:10.1021/j100298a029

Isotopic exchange between molecular deuterium and aniline under catalysis by potassium anilide has been studied at 85 and 100 deg C over a range of catalyst concentrations (0.03-0.1 M).Two consecutive processes occur, D2 -> HD -> H2, and rate constants have been evaluated; k_D2 and k_HD increase with increasing .Plots of k_D2/ vs. and k_HD/ vs. are linear in accord with second-order rate dependence on .The kinetic behavior cannot be accommodated on the basis of free anilide ion and monomeric potassium anilide ion pairs as the main reactive species.Also the kinetic data serve to exclude a mechanism involving free deuteride ion as a reaction intermediate.The most probable mechanism is considered to be a preequilibrium formation of a catalyst-D2 complex followed by rate-determining attack by a second molecule of potassium anilide in association with a solvent molecule via a cyclic transition state including participation of K⁺.Less probable is reaction occurring via a dimer of the catalyst.Kinetic isotope effects, k_HD/k_D2, are 1.82 at 85 deg C and 2.12 at 100 deg C (corrected for dependence on ).The small magnitude of the KIE and its inverse temperature dependence is consistent with a primary isotope effect involving corrected transfer of hydrogen (deuterium) in a nonlinear transition state.Comparison is drawn with exchange of D2 in the RO^-/ROH and RNH^-/RNH2 (R = H or Me) systems.It is concluded that the kinetic behavior in the PhNH^-/PhNH2 system follows more closely the aqueous or alcoholic-base, than the ammonia or methylamine-base systems.