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Hyperconjugation: Equilibrium Secondary Isotope Effect on the Stability of the tert-Butyl Cation. Kinetics of Thermoneutral Hydride Transfer
Meot-Ner, Michael (Mautner)
, p. 7947 - 7950 (1987)
The thermochemistry of the hydride transfer equilibrium (CD3)3C(1+) + (CH3)3CH ->/0 = -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.
