Mechanisms of thermal decomposition of cyclodiene pesticides, identification and possible mitigation of their toxic products

Abstract

Abstract Mechanisms of pyrolysis of several representative cyclodiene pesticides, viz., dieldrin, aldrin and isodrin, have been studied by quantum chemical techniques and dieldrin and isodrin have been subjected to experimental pyrolysis investigation. Comparison is made with earlier studies on another cyclodiene - endosulfan. Cyclodienes which are initially produced by Diels-Alder condensation of hexachlorocyclopentadiene (HCCP) and a substrate, initially decompose by retro Diels-Alder reaction back into HCCP and the substrate, both of which undergo further reaction. In the case of dieldrin, pyrolysis is initiated at 673 K and the substrate, norbornadiene oxide, further decomposes to produce CO and benzene which are observed experimentally. Above ∼800 K hexachlorocyclopentadiene undergoes rapid Cl bond fission and Cl atoms abstract H atoms from dieldrin forming HCl and lead to four different types of dieldrin radicals, just one of which can undergo a sequence of reactions of modest energy barriers to lead to the major observed high temperature products, pentachlorostyrene, hexachlorostyrene and polychlorinated benzenes. In contrast to other cyclodienes, pyrolysis of isodrin which is initially produced by Diels-Alder condensation of hexachloronorbornadiene and cyclopentadiene does not undergo initial retro Diels-Alder decomposition, instead undergoing an intramolecular rearrangement to a “birdcage” structure which eventually decomposes to HCl and aromatic soot precursors. A simple relationship between the rate constants for retro Diels-Alder decomposition and Cl fission of HCCP is discussed to evaluate whether cyclodiene-contaminated soils might be remediated via thermal desorption.