CARBON ACIDITY .83. THE KINETIC ACIDITY OF 1,1,1-TRIPHENYLETHANE

Hydrogen isotope exchange studies were carried out on the methyl group of 1,1,1-triphenylethane, 1, with cesium cyclohexylamide (CsCHA) in cyclohexylamine (CHA). Mixtures of labeled compounds were used, Ph3C-(CH3)-C-14 (1-C-14), Ph3CCH2T (1-t), Ph3CCH2D (1-d), and Ph3CCD3 (1-d3). The second-order rate constant for tritium exchange, 3.6 x 10(-5) M-1 s-1, is 9.0 x 10(-5) the rate of benzene and 8.5 x 10(3) times the rate of exchange of cyclohexane. The primary isotope effect, k(D)/k(T) = 1.8, and corresponds to k(H)/k(D) = 3.8, a moderate value that is comparable to that of cyclohexane and indicates an unsymmetrical transition state. The alpha-secondary isotope effect, k(H)/k(D) = 1.2, indicates that C-H bonds at the carbanion center have significantly lower bending frequency at the transition state.