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Unimolecular decomposition of C3Cl6: pathways for formation of cylic chlorinated compounds

Ahubelem, N., Altarawneh, M. and Dlugogorski, B.Z. (2012) Unimolecular decomposition of C3Cl6: pathways for formation of cylic chlorinated compounds. Organohalogen Compounds, 74 . pp. 640-643.

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Great deal of research has shown that catalytic and non-catalytic thermal decomposition (pyrolysis and oxidation) of chlorinated alkanes and alkenes results in the formation of heavier cyclic chlorinated pollutants. Central to these processes is the Diels-Alder addition/cyclisation of small chlorinated carbon chains and the successive replacement of hydrogen atoms in cyclic compounds by chlorine atoms at high temperatures. Polychlorinated benzenes, dibenzo-p-dioxins and dibenzofurans (PCDD/F) are formed during the heterogeneous reactions of propene on fly-ash in the presence of air and HCl between 623-673 K. Formation of PCDD/F from thermal oxidation of polychlorinated phenol has been demonstrated to proceed at a rate 100 times faster than the competing de novo pathway. High temperature pyrolysis of 1,3-hexachlorobutadiene concludes in the production of hexachlorobenzene (C6Cl6) and other highly chlorinated cyclic hydrocarbons. Along the same line of enquiry, experimental results have explained that unsaturated aliphatic hydrocarbons such as acetylene are readily converted to hexachlorobenzene, hexachlorobutadiene and other heavier perchlorinated species in the presence of cupric oxide and HCl under post combustion conditions.

Taylor et al. observed the pyrolysis of hexachloropropene to occur readily, even at temperatures as low as 700 K to yield CCl4, C2Cl4, C2Cl6 and C3Cl4 (tetrachloroallene). At higher temperatures (up to 1223 K), distinct molecular growth was observed with reaction products including C4Cl6 (1,3-hexachlorobutadiene), C6Cl6 (hexachlorobenzene), C6Cl8 (1,3,5-octachlorohexatriene), C8Cl8 (octachlorostyrene), possible other isomers of C6Cl8, C8Cl8 and four isomers of C12Cl8. Cl displacement of CCl3 radicals was observed to be the overriding origination pathway for conversion of C3Cl6 into C2Cl4, CCl4 and C2Cl6. At higher temperatures, C3Cl3 recombination accounted for about 80 % of experimental yields with C3Cl5 recombination responsible for formation of the remainder. In the present study, we report the reaction and activation enthalpies for reactions involved in the pyrolytic decomposition of C3Cl6 to synthesise C6Cl6. Our results will help in providing an insightful understanding of one of the major routes to the formation of chlorinated cyclic persistent organic pollutant (POP) species from the combustion of hydrocarbon precursors. Thermochemical and kinetic parameters presented herein will be useful in building a robust kinetic model that could satisfactorily describe formation of cyclic chlorinated compounds from the degradation of small aliphatic moieties.

Publication Type: Journal Article
Publisher: EcoInforma Press
Notes: Paper presented at the 32nd International Symposium on Halogenated Persistent organic Pollutants - dioxin, 26 - 31 August, Cairns, Qld, Australia
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