Mechanism and Rate of Thermal Decomposition of Hexachlorocyclopentadiene and Its Importance in PCDD/F Formation from the Combustion of Cyclodiene Pesticides

Abstract

Thermal decomposition of hexachlorocyclopentadiene (HCCP) has been studied in inert gas and under oxidative conditions in a silica flow reactor at a residence time of 5.0 s between 690 and 923 K and 1 atm pressure. Pyrolysis was initiated by Cl bond fission to form pentachlorocyclopentadienyl radical; two such radicals then combined to undergo a series of intramolecular rearrangements and Cl fissions, producing principally octachloronaphthalene (8ClNP) and Cl. This process has been studied by quantum chemical calculation, and a reaction potential energy surface has been developed. The rate constant of initial Cl atom fission has been calculated by canonical variational transition state theory as k = 1.45 × 10 exp(-222 ± 9 kJ mol/RT) s between 500 and 2000 K. A minimal kinetic model was developed to model the decomposition and major products. Oxidative decomposition was studied in nitrogen with O contents of 1, 6, 12, and 20 mol %. Increasing O to 6-8% increased the rate of decomposition of HCCP and decreased the yield of 8ClNP. Above 823 K, hexachlorobenzene (HCB) and CO became major products. The oxidative reaction has also been studied quantum chemically. At high O content (>∼10%), the rate of decomposition of HCCP declined as did yields of 8ClNP and HCB, but CO yields increased.

Mechanism of the Thermal Decomposition of Chlorpyrifos and Formation of the Dioxin Analog, 2,3,7,8-Tetrachloro-1,4-dioxino-dipyridine (TCDDpy)

“…At 600 K, IM3 would possess a peak thermal energy of 240 kJ mol –1 such that IM3 would not be stabilized and would possess more than sufficient energy to surmount the small barrier to form IM4. Kislov and Mabel used similar reasoning in developing a kinetic model for combination of two cyclopentadienyl radicals and we have been able to develop an analogous kinetic model for combination of two pentachlorocyclopentadienyl radicals with a similar assumption. We have allowed a small barrier of 21 kJ mol –1 for the chemically activated combination of two TCpyoxy radicals to form IM4 + Cl.…”

“…Improved energy calculations have been carried out at the M06-2X/GTLarge//M06-2 X /6-31+G­(d,p) level of theory. We have chosen to use this method because of its previous success in the computation of large polychlorinated molecules such as endosulfan, dieldrin, hexachlorocyclopentadiene and their products. Rate constants at the high pressure limit for unimolecular reactions have been calculated by the ChemRate program using thermochemical parameters obtained from the quantum chemical computations.…”

Directed regulation mechanism on chlorine substitution of PCDD/Fs isomers based on quantum chemical computation

“…Quantum chemistry technique as a method has been developed to assist in the structural modulation for multiple catalysts. For example, N K Dharmarathne et al 12 investigated the thermal decomposition of hexachlorocyclopentadiene (HCCP) in a uidized bed reactor under the inert and oxidizing conditions, respectively. It has been found that at high O-2 levels ( > similar to 10%), the decomposition rate of HCCP as well as the yields of 8ClNP and HCB decreased, which con rmed that quantum chemistry technique can be used to simulate and calculate reactions between gaseous molecules.…”