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. / Graduation date: 1992
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/36674 |
| Date | 31 May 1991 |
| Creators | Schmidt, Robert Dean |
| Contributors | Freeman, Peter K. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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