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Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals

✍ Scribed by Xiangyang Zhang; Werner M. Nau

Publisher: John Wiley and Sons
Year: 2000
Tongue: English
Weight: 79 KB
Volume: 13
Category: Article
ISSN: 0894-3230
DOI: 10.1002/1099-1395(200010)13:10<634::aid-poc263>3.0.co;2-#

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✦ Synopsis

Five aryl-substituted phenacetyl radicals (X = p-MeO, p-Me, H, p-Cl, p-CF 3 ) were generated by laser photolysis of the corresponding dibenzyl ketones in n-hexane and acetonitrile. The decarbonylation reaction was monitored through the rise in time-resolved absorption of the benzyl radical chromophore at 317 nm. The decarbonylation rate constants were obtained by a numerical integration procedure, where second-order radical reactions were explicitly taken into account. Values of (2-3) Â 10 6 s À1 in acetonitrile and (6-10) Â 10 6 s À1 in nhexane revealed a large solvent effect for all derivatives (by a factor of $3). The electronic substituent effect indicates that both electron-withdrawing and electron-donating para substituents accelerate the decarbonylation slightly. The rate constants followed the order MeO b Me, Cl, CF 3 b H. The substituent effects are interpreted in terms of the ability of the para substituent to stabilize the benzyl radical resulting from decarbonylation. Evidence for a polar effect was not obtained. The underlying reasons for the observed solvent effect are discussed.

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