Kinetics of base-catalyzed dehydrochlorination of hexachlorocyclohexanes: I. Homogeneous systems

Base-catalyzed dehydrochlorination of α-, β-, and γ-hexachlorocyclohexane (HCH) was examined at different pH and temperature conditions. No reaction was observed for β-HCH under all tested conditions likely due to the fact that all chlorines are at the equatorial positions. Highly pH- and temperature-dependent reaction rates were observed for α- and γ-HCH, and pentachlorocyclohexenes (PCCHs) and 1,2,4- and 1,2,3-trichlorobenzene (TCB) were detected ES the intermediates and final products of the transformation of both α- and γ-HCH. The pseudo-first-order rate constants of each step of the reactions were calculated at different T and pH conditions. For the first step, the rate constants were at approximately 0.0005 d^-1 in solutions below pH 7.0, and increased by about an order of magnitude per pH unit from pH 7.01 to pH 12.02. The second-order reaction rate constants (k_b) of this step were 3.57 ± 0.03 and 3.19 ± 0.19 M^-1 d^-1, respectively, and the associated activation energies (E,) at pH 9.26 were 60.4 ± 7.8 and 67.7 ± 8.7 kJ mol^-1, respectively, for α- and γ-HCH. The rate constants for the formation of 1,2,3-TCB (k₂₁) and 1,2,4-TCB (K₂₂) were 0.0032 ± 0.0001 d^-1 and 0.051 ± 0.003 d^-1 from α-HCH, and 0.0017 ± 0.0001 d^-1 and 0.0041 ± 0.0002 d^-1 from γ-HCH at pH 8.28 and 25°C. Both k₂₁ and k₂₂ also showed an increase by about one order of magnitude per pH unit from pH 8.28 to pH 12.02. The E_a values were 64.4 ± 6.8 and 88.8 ± 5.2 kJ mol^-1, respectively, for the formation of 1,2,4-TCB and 1,2,3-TCB from α-HCH, and 70.6 ± 8.7 and 92.0 ± 4.9 kJ mol^-1, respectively, for the formation of 1,2,4- and 1,2,3-TCB from γ-HCH at pH 9.26. Data provided by this study may be used for calculation of the percentages of the TCBs formed at different environmental conditions.