Reactions of hydroxyl radicals with alkenes in low-temperature matrices

The reactions of hydroxyl radicals with a number of stable alkenes have bee n studied in low-temperature matrices. The reactions were initiated by broa d band UV-visible irradiation of matrices containing H2O2 and the alkene un der investigation. The hydroxyalkyl radical products were identified princi pally by comparison of their spectra with the spectra of corresponding stab le alcohols. Accordingly, IR spectra were recorded for the following series of alcohols isolated in argon matrices - methanol, ethanol, ethanol-d(6), propan-1-ol, propan-2-ol, butan-2-ol, 2-methylpropan-1-ol (iso-butyl alcoho l), 2-methylpropan-2-ol (tert-butyl alcohol), 2-methylbutan-2-ol (tert-amyl alcohol), 3-methylbutan-2-ol and 2,3-dimethylbutan-2-ol. The hydroxyalkyl radicals, which appear to be formed from the alkenes studied were as follow s - from ethene, 2-hydroxyethyl radical; from cis: or trans-but-2-ene, 1-me thyl-2-hydroxypropyl radical; from propene, 1-methyl-2-hydroxyethyl and 2-h ydroxypropyl radicals; from but-l-ene, 1-hydroxymethylpropyl and 2-hydroxyb utyl radicals; from 2-methylpropene (iso-butene), 1,1-dimethyl-2-hydroxyeth yl and 2-methyl-2-hydroxypropyl radicals; the radical products from buta-1, 3-diene and isoprene could not be identified. In the cases, where two radic al products were possible, i.e. when propene, but-l-ene or 2-methylpropene were the substrates, it was found that the concentration of the secondary o r tertiary radical always exceeded that of the primary radical. However, th e relative concentration of these radicals appears to be determined by subs equent photolysis to give carbonyl compounds. There seems, therefore, to be little preference for the secondary and tertiary radicals over the primary radicals in the primary addition process. Comments on the mechanism of the transformation from radical to carbonyl compound based upon identification of intermediates within the matrix and isotopic substitution experiments a re made. The characterisation of the 2-hydroxyethyl radical has been backed up by experiments utilising isotopic substitution with C-13 and D (H-2). T he other radicals have been identified with varying degrees of certainty. T hose radicals, which are observed at the highest concentration and which ar e, therefore, characterised more certainly are - 2-hydroxyethyl (1), from e thene; 1-methyl-2-hydroxypropyl (2), from cis- and trans-but-2-ene; 1-methy l-2-hydroxyethyl (3), from propene; 1-hydroxymethylpropyl (5), from but-l-e ne; and 1,1-dimethyl-2-hydroxyethyl (8), from 2-methylpropene. (C) 2000 Els evier Science B.V. All rights reserved.