Theoretical calculations on the mechanisms of the gas phase elimination kinetics of chlorocyclohexane, 3-chlorocyclohexene and 4-chlorocyclohexene

The kinetics of the thermal decomposition of the title compounds in the gas phase have been studied at the B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW91PW91/6-31G(d,p), MPW91PW91/6-31++G(d,p), PBEPBE/6-31G(d,p), and PBEPBE/6-31++G(d,p) levels of theory. These halide substrates produce the corresponding cyclohexadiene and hydrogen chloride. The DFT calculations suggest a non-synchronous four-membered cyclic transition state type of mechanism. The elongation and subsequent polarization of the C-Cl bond, in the direction of C^δ+...Cl^δ-, is rate determining step in these elimination reactions. Differences in reactivity in these substrates are discussed in terms of the transition state structure and electron distribution.