Investigations into the mechanism of the photodechlorination of pentachlorobenzene Public Deposited

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  • Several experiments were undertaken to investigate the mechanism of photodechlorination of pentachlorobenzene. The thermal methoxydechlorination reaction of pentachlorobenzene was studied. In DMSO at 50 °C sodium methoxide reacts smoothly with pentachlorobenzene to give three tetrachloroanisole isomers. A second-order kinetic plot for this reaction is linear to high precision, indicating that the reaction is second order overall. The rate of the reaction and the product regioisomer distribution are not affected by the presence of radical traps such as galvinoxyl, by reaction in the absence of oxygen or by benzyne intermediate traps such as 1,3-diphenylisobenzofuran. The rate of the reaction is reduced linearly as the starting mixture is diluted with methanol. These facts suggest that the reaction follows the classical S[subscript N]Ar mechanism. The proposed structure of the pentachlorobenzene radical anion formed by photolysis in the presence of triethylamine is analogous to the σ-complex intermediate in the S[subscript N]Ar reaction. The regiochemistry of these two types of dechlorination was compared and showed a fair correlation. The differences in regiochemistry were attributed to steric effects between the nucleophile and the aromatic ortho chlorine atoms in the S[subscript N]Ar reaction. Tetrachlorophenyl radicals were generated by thermal decomposition of 1 ,2,4,5-tetrachlorobenzeneazotriphenylmethane in the presence of CCl₄ and a hydrogen atom donor. The selectivies of chlorine versus hydrogen or deuterium atom abstraction were determined in several systems and applied to selectivity of atom abstraction observed in photochemical experiments. The results support the hypothesis that, in the photolysis of pentachlorobenzene in the presence of triethylamine, the intermediate tetrachlorophenyl radical exists as an unencumbered free radical rather than in a solvent caged pair with triethylamine. It was anticipated that the rate of photodechlorination would increase in micellar solutions if triethylamine was added. Experiments showed no such expected increase in rate, and showed regiochemistry similar to that of the same reaction in the absence of triethylamine.
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