Homogeneous unimolecular gas-phase elimination kinetics of 2-chloro-2-alkylpropanes. The electronic effect of alkyl substituents on the .alpha.-carbon of tertiary chlorides

Abstract

The pyrolysis kinetics of 2-chloro-2-methylbutane and 2-chloro-2,3-dimethylbutane have been investigated, in a static system and seasoned vessel, over the pressure range of 50-280 torr and the temperature range of 260-320 °C. The reactions are homogeneous and unimolecular, follow a first-order law, and are invariable to the presence of a cyclohexene inhibitor. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 2-chloro-2-methylbutane, log kx (s_1) = (13.77 ± 0.25) -(184.1 ± 2.6) kJ-mol"1 (2.303RD-1; for 2-chloro-2,3-dimethylbutane, log kx (s'1) = (13.33 ± 0.18) -(175.3 ± 1.9) kJ-mol"1 (2.303RT)'1. The distribution of the olefin products from these reactions has been quantitatively determined and reported in details. The alkyl series ((CH3)3C, (CH3)2CH, CH3CH2, CH3, and H) in the tertiary halides, 2-chloro-2alkylpropanes, influence the rate of elimination by electronic effect. This is similar to those obtained with a-and /3-alkyl-substituted ethyl chlorides. The plot of log k/kQ vs. *( ) gives a very good straight line with p* = -4.75, r = 0.994, and intercept = 0.048 at 300 °C. The previous and present results reveal that, if a reaction center at the transition state of an organic molecule is markedly polar, the +7 inductive electron release of alkyl substituents may affect gas-phase elimination processes.