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 as 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 a ) 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 ( k 21 ) and 1,2,4‐TCB ( k 22 ) 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 k 21 and k 22 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.