Hyperconjugation: Equilibrium Secondary Isotope Effect on the Stability of the tert-Butyl Cation. Kinetics of Thermoneutral Hydride Transfer

Article abstract of DOI:10.1021/ja00260a003

The thermochemistry of the hydride transfer equilibrium (CD3)3C(1+) + (CH3)3CH ->/<- (CH3)C(1+) + (CD3)3CH was measured by pulsed high-pressure mass spectrometry.Measurements over a wide temperature range, 145-600 K, allowed an accurate determination of the values: ΔH⁰ = -0.57 +/- 0.02 kcal/mol; ΔS⁰ = -0.57 +/- 0.08 cal/(mol.K); ΔG⁰₂₉₈ = -0.40 +/- 0.07 kcal.mol; and K₂₉₈ = 1.97 +/- 0.20.The direction of the observed isotope effect is consistent with C-H bond weakening in the ion due to hyperconjugation.The kinetics of the reaction show a slow rate and a large negative temperature coefficient, with k₃₀₀ = 0.36 and k₆₀₀ = 0.00625 x 1E-10 cm³ s^-1, i.e., reaction efficiencies of about 0.03 to 0.0005.The observed negative temperature coefficient, k = AT^-5.8, is larger than those observed for more exothermic hydride transfer reactions.The approach to collison rate is abrupt.