Photochemistry of dibenzo-p-dioxin
| dc.contributor.author | Guan, Bing | en_US |
| dc.date.accessioned | 2024-08-13T22:57:46Z | |
| dc.date.available | 2024-08-13T22:57:46Z | |
| dc.date.copyright | 1993 | en_US |
| dc.date.issued | 1993 | |
| dc.degree.department | Department of Chemistry | |
| dc.degree.level | Master of Science M.Sc. | en |
| dc.description.abstract | The photochemistry of dibenzo-p-dioxin (DBD; 34) (a diphenyl diether)--the parent compound of the notorious 2,3, 7 ,8-tetrachlorodibenzo-p-dioxin (TCDD; commonly known as dioxin)--has been studied in aqueous CH₃CN, dry CH₃CN and selected organic solvents. The aim of this investigation is to achieve a comprehensive understanding of the photochemical behaviour of cyclic diaryl ethers, which may be relevant to the photodecomposition of TCDD and related contaminants. In non-hydroxylic solvents (CH₃CN, THF or 1,4-dioxane), photolysis of DBD (34) gives rise to an observable (by UV-Vis spectrophotometry) intermediate 2,2'-biphenylquinone (73), which could be reduced by the organic solvent to 2,2'-biphenol (69) as a major product, or isomerizes to 1-hydroxydibenzofuran (75) via secondary photolysis. In hydroxylic solvents (aqueous CH₃CN, MeOH or 2-propanol), photolysis of DBD (34) gives only one major product, viz., 69; the formation of 75 is suppressed. Although the intermediate quinone 73 could not be observed by UV-Vis in these solvents, the product yield of 69 is increased dramatically in the presence of added NaBH₄, along with > 90% material balance, implicating the formation of 73 as the crucial intermediate. Quinone 73 originates from intramolecular rearrangement of singlet biradical 72, which is formed from DBD (34) via aryloxygen bond homolysis. The intramolecular ipso attack of 72 to form spiro ketone 91 (the precursor of73) is the key point of this novel mechanism, which distinguishes it from the general accepted, but incorrect mechanism of the formation of 69 from DBD (34). The generality of this mechanism is demonstrated by the photoreaction of 2,3,7,8-tetramethyldibenzo-p-dioxin (TMDBD; 76), in which 4,4',5,5'tetramethyl-2,2'-biphenylquinone (79) is observed in non-hydroxylic solvents; again the product yield of 4,4',5,5'-tetramethyl-2,2'-biphenol (77) is greatly enhanced in the presence of added NaBH₄. The pathway of this novel intramolecular isomerization of cyclic diaryl ethers reveals some interesting aspects of the photochemical behaviour of diaryl ethers previously unrealized. In addition, as photodecomposition of TCDD may involve aryl-oxygen bond cleavage, an efficient photodegradation method for dioxin related contaminants could be acquired by understanding the behaviour of DBD (34). | en |
| dc.format.extent | 108 pages | |
| dc.identifier.uri | https://hdl.handle.net/1828/17988 | |
| dc.rights | Available to the World Wide Web | en_US |
| dc.title | Photochemistry of dibenzo-p-dioxin | en_US |
| dc.type | Thesis | en_US |
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