Mechanistic aspects of the thermal formation of halogenated organic compounds including polychlorinated dibenzo‐p‐dioxins

Abstract

The thermodynamical generation of polychlorinated dibenzo‐p‐dioxins (PCDDs), dibenzofurans (PCDFs) and related compounds has received considerable attention during the last few years. Work so far has concentrated on measurement of these types of compounds in effluents from incinerators and other high temperature processes on the one hand and on experimental pyrolyses of chloroaromatic precursors, mainly in simple laboratory systems, on the other. Some indirect evidence indicates that thermochemical formation of chloroaromatic compounds may be possible from chemically unrelated materials. Little experimental evidence is available, however, to show thermal synthesis of chloroaromatic compounds from non‐chlorinated aliphatic precursors and inorganic chlorine and, to our knowledge, no attempt has been made to treat this subject theoretically. We are now attempting to create a mechanistic basis to understand, explain and predict the likely thermochemical formation of PCDDs and related compounds, initially from experimental and theoretical evidence available in the literature. This subject will be covered in five parts. The overview of the subject matter, discussed in the present article (Part I) is given below.

In order to find possible sources of chlorine originating from inorganic chlorides, thermodynamic calculations for some hypothetical reactions have been conducted. Chloroform undergoes thermal decomposition reversibly to produce CCl₂ and HCl; whereby CCl₂ species react further to give chlorocarbons and chlorinated ethanes. The pyrolysis of chlorocarbons, CxCly (χ≥2; 1<y/χ<4), always results in the formation of three products which hold a thermodynamic equilibrium: 9C₂Cl₆?12CCl₄+C₆Cl₆.

Combustion of CH₄ in the presence of CH₂Cl₂ yields polycyclic aromatic hydrocarbons (PAHs) and monochloro PAHs. Thermolysis of optically active α‐chlorobenzyl alkyl ethers in aprotic solvents produces alkyl chlorides with ca 85% inversion of the configuration. Pyrolysis of anisole generates methylene carbene (CH₂). The thermodegradation products of monochlorobenzene (C₆H₅Cl) are varied with temperature. C₆H₅Cl produces isomerically stable o‐, m‐, and p‐chlorophenyl radicals in the temperature range of 500–690°C. The copyrolysis pentafluorobenzotrichloride with CF₂ = CFCl involves 1,3‐fluorine and 1,2‐chlorine shifts at the intermediate biradicals.