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This article in JEQ

  1. Vol. 35 No. 3, p. 880-888
    Received: Sept 19, 2005

    * Corresponding author(s): pinganp@gig.ac.cn


Kinetics of Base-Catalyzed Dehydrochlorination of Hexachlorocyclohexanes: I. Homogeneous Systems

  1. Mingzhong Rena,
  2. Ping'an Peng *a,
  3. Weilin Huangb and
  4. Xiangmei Liuc
  1. a Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China
    b Department of Environmental Sciences, Cook College, Rutgers University, New Brunswick, NJ 08901-8551
    c Department of Environmental Protection Enforcement and Inspection, State Environmental Protection Administration, Beijing 100035, PR China


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.

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