Halogenated aromatics

In order to determine the reaction pathway for 1,2-dichlorobenzene, Schuth fitted kinetic data and found that the primary pathway was direct reaction to benzene with a parallel reaction of sequential dechlorination through chlorobenzene to benzene. (Figure 7) These pathways were further supported by independent determination and verification of the reaction rate constant for the second hydrodechlorination step. (Schuth and Reinhard 1998)

C6H4CI2 1,2-dichlorobenzene

C6H5CI chlorobenzene

C6H6 benzene

Figure 7. Transformation pathway of 1,2-dichlorobenzene. Reprinted from Applied Catalysis B: Environmental, Vol. 18, Schilth and Reinhard, "Hydrodechlorination and Hydrogenation of Aromatic Compounds over Palladium on Alumina in Hydrogen-Saturated Water," pp. 218, Copyright 1998, with permission from Elsevier Science.

The lack of partially chlorinated by-products in the reactions of the Araclor PCB congeners (Grittini et al. 1995) and of all the chlorophenols (Hoke et al. 1992) may indicate direct transformation of these compounds to biphenyl and phenol, respectively; these were the final observed products of the reactions. Schüth also observed the rapid dechlorination of 4-chlorobiphenyl to biphenyl. The first benzene ring of biphenyl was then gradually hydrogenated to form cyclohexylbenzene, then the second benzene ring was hydrogenated to form the final product of dicyclohexyl. The best fit of the kinetic data was obtained by including a direct reaction of 4-chlorobiphenyl to cyclohexylbenzene. (Figure 8, Schüth and Reinhard 1998)

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C12H9CI 4-chlorobiphenyl

C12H10 biphenyl

C12H16 cyclohexylbenzene

C12H22 dicyclohexyl

Figure 8. Transformation pathway of 4-chlorobiphenyl. Reprinted from Applied Catalysis B: Environmental, Vol. 18, Schüth and Reinhard, "Hydrodechlorination and Hydrogenation of Aromatic Compounds over Palladium on Alumina in Hydrogen-Saturated Water," pp. 219, Copyright 1998, with permission from Elsevier Science.

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