INTRAMOLECULAR HYDROGEN-TRANSFER AS THE KEY STEP IN THE DISSOCIATION OF HYDROXYL RADICAL ADDUCTS OF (ALKYLTHIO)ETHANOL DERIVATIVES

被引:70
作者
SCHONEICH, C [1 ]
BOBROWSKI, K [1 ]
机构
[1] UNIV NOTRE DAME,RADIAT LAB,NOTRE DAME,IN 46556
关键词
D O I
10.1021/ja00068a010
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
The reaction of the hydroxyl radical with dimethyl sulfide (DMS), 2-(methylthio)ethanol (2-MTE), 2,2'-dihydroxydiethyl sulfide (2,2'-DHE), and 3,3'-dihydroxydipropyl sulfide (3,3'-DHP) has been investigated in H2O and D2O. As an initial step hydroxyl radicals add to the sulfur moiety. These hydroxyl radical adducts subsequently decay via a thioether concentration-dependent and a thioether concentration-independent pathway. The hydroxyl radical adduct of DMS dissociates into a sulfur radical cation and HO- in the thioether concentration-independent pathway (k(H)/k(D) = 2.09), whereas a rate-limiting proton transfer from water operates in the thioether concentration-dependent mechanism (k(H)/k(D) = 5.40), as deduced from the measured solvent kinetic isotope effects. In contrast the hydroxyl radical adducts of 2-MTE and 2,2'-DHE decompose via elimination of water, formed through a rapid intramolecular hydrogen transfer from the adjacent hydroxyl groups. This mechanism leads to the formation of (alkylthio)ethoxy radicals. The latter undergo alpha,beta-fragmentation into formaldehyde and alpha-thioether radicals as well as hydrogen abstraction from a delta-methylene group, analogous to a hydrogen transfer in the Barton reaction, leading to alpha-thioether radicals. The overall rate constants for these unimolecular reaction sequences were determined to be k12,H = (6.32 +/- 0.7) x 10(7) s-1 for 2-MTE and k15,H = (1.17 +/- 0.2) X 10(8) s-1 for 2,2'-DHE. Neither of them show an appreciable kinetic isotope effect, suggesting that the actual hydrogen transfer is not the rate-determining step in the overall process.
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页码:6538 / 6547
页数:10
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