Formation of chlorobenzenes by oxidative thermal decomposition of 1,3-dichloropropene

We combine combustion experiments and density functional theory (DFT) calculations to investigate the formation of chlorobenzenes from oxidative thermal decomposition of 1,3-dichloropropene. Mono- to hexa-chlorobenzenes are observed between 800 and 1150. K, and the extent of chlorination was proportional to the combustion temperature. Higher chlorinated congeners of chlorobenzene (tetra-, penta-, hexa-chlorobenzene) are only observed in trace amounts between 950 and 1050. K. DFT calculations indicate that cyclisation of chlorinated hexatrienes proceeds via open-shell radical pathways. These species represent key components in the formation mechanism of chlorinated polyaromatic hydrocarbons. Results presented herein should provide better understanding of the evolution of soot from combustion/pyrolysis of short chlorinated alkenes

Dehydrohalogenation of ethyl halide

Unimolecular decomposition kinetics of selected ethyl halides, phenethyl halides and methoxyphenethyl halides have been investigated using high level computational chemistry methods. The phenethyl halides decompose faster than the ethyl halides due to a more electronegative chlorine atom, induced by the chloroethyl functionality as an electron-withdrawing group. 1-Chloro-2-(methylthio)ethane exhibits faster dehydrochlorination than that of chloroethane/1-chloro-2-methoxyethane, owing to more polarisable C⋯H and C⋯Cl bonds in the transition structures. Calculations suggest that electronic factors rather than anchimeric assistance influence the dehydrochlorination reactions

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

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

Kinetic and Mechanistic Study into Emission of HCl in Fires of PVC

PVC pyrolyses in fires eliminating HCl, which can subsequently participate in the formation of chloroaromatic pollutants. In this study, the Density Functional Theory (DFT) has been deployed to simulate the mechanisms of HCl elimination from pyrolysing PVC. Although PVC consists mainly of polymerised chloroethene, it also contains other structural entities as impurities or defect compounds, which significantly enhance its decomposition. For this reason, we have studied elimination of HCl from seven compounds that represent the defects in PVC. We have found two generic pathways for the elimination of HCl. The first involves a C-Cl fission at an allylic site and a C-H cleavage at a vinylic site, whereas the second entails scissions of allylic Cl and methylenic H. The latter pathway appears more favourable from thermodynamic and kinetic standpoints. We have investigated the effect of the length of carbon chain on reaction and activation enthalpies by considering analogous dehydrochlorination pathways for short chlorinated aliphatics (i.e., C3, C4), discovering the reaction and activation enthalpies required for HCl elimination to be independent of the length of the carbon chain. We then explored the effects of temperature and pressure on rate constants for all possible dehydrochlorination pathways within the formalism of the unimolecular reaction rate theory of RRKM. Pressure fall-off regions extend generally between 0.001 and 1.0 atm, and the dehydrochlorination reactions exhibit pressure-independent behaviour even under ambient pressure. Kinetic parameters presented herein should be useful to model the decomposition of PVC in fires

Quantum chemical molecular dynamics simulations of 1,3-dichloropropene combustion

Oxidative decomposition of 1,3-dichloropropene was investigated using quantum chemical molecular dynamics (QM/MD) at 1500 and 3000 K. Thermal oxidation of 1,3-dichloropropene was initiated by (1) abstraction of allylic H/Cl by O2 and (2) intra-annular C-Cl bond scission and elimination of allylic Cl. A kinetic analysis shows that (2) is the more dominant initiation pathway, in agreement with QM/MD results. These QM/MD simulations reveal new routes to the formation of major products (H2O, CO, HCl, CO2), which are propagated primarily by the chloroperoxy (ClO2), OH, and 1,3-dichloropropene derived radicals. In particular, intra-annular C-C/C-H bond dissociation reactions of intermediate aldehydes/ketones are shown to play a dominant role in the formation of CO and CO2. Our simulations demonstrate that both combustion temperature and radical concentration can influence the product yield, however not the combustion mechanism. (Figure Presented)