Formation of toxic compounds in the thermal decomposition of 1,3-dichloropropene

The formation of chlorobenzenes from oxidative thermal decomposition of 1,3-dichloropropene have been investigated using a combined combustion experiments and quantum chemical calculations. Experimental and computational results elucidate the chemistry of formation of toxic compounds from combustion chlorinated pesticides as could occur in bush fires or accidental fires of such chemicals in their storage facilities. Mono- to hexa-chlorobenzenes were observed between 800 – 1150 K, and the extent of chlorination was proportional to the combustion temperature. Higher chlorinated congeners of chlorobenzene (tetra-, penta-, hexa-chlorobenzene) were only observed in trace amounts between 950 – 1050 K. DFT calculations indicated that cyclisation of chlorinated hexatrienes proceeds via open-shell, radical pathways. Oxidation of phenylvinyl radical intermediates and subsequent ring closure were the key mechanistic pathways in the formation of benzofuran and chlorobenzofuran. Quantum chemical molecular dynamics (QM/MD) at 1,500 and 3,000 K revealed that the 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 showed that (2) is the more dominant initiation pathway, in agreement with QM/MD results. These QM/MD simulations revealed new routes to the formation of major products (H₂O, CO, HCl, CO₂), which were 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 were shown to play a dominant role in the formation of CO and CO₂. QM/MD simulations demonstrated that both combustion temperature and radical concentration can influence the product yield, however not the combustion mechanism. In order to elucidate the dehydrochlorination kinetics of 1,3-dichloropropene and related compounds, the unimolecular HCl elimination reactions of 1,3-dichloropropene and other chloroaliphatic hydrocarbons were investigated using high level computational chemistry methods. Two generic pathways for the elimination of HCl was found. 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. The effect of the length of carbon chain on reaction and activation enthalpies was also investigated 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. Dehydrochlorination reactions investigated exhibited a pressure-independent behaviour even under ambient pressure and results suggest that electronic factors rather than anchimeric assistance influence dehydrochlorination reactions of substituted ethyl halides.