Reactions of hydroxyl radicals with alkenes in low-temperature matrices.

The reactions of hydroxyl radicals with a number of stable alkenes have been studied in low-temperature matrices. The reactions were initiated by broad band UV-visible irradiation of matrices containing H2O2, and the alkene under investigation. The hydroxyalkyl radical products were identified principally by comparison of their spectra with the spectra of corresponding stable alcohols. Accordingly, IR spectra were recorded for the following series of alcohols isolated in argon matrices--methanol, ethanol, ethanol-d6, propan-1-ol, propan-2-ol, butan-2-ol, 2-methylpropan-1-ol (iso-butyl alcohol), 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 follows--from ethene, 2-hydroxyethyl radical: from cis- or trans-but-2-ene. 1-methyl-2-hydroxypropyl radical; from propene, 1-methyl-2-hydroxyethyl and 2-hydroxypropyl radicals; from but-1-ene. 1-hydroxymethylpropyl and 2-hydroxybutyl radicals; from 2-methylpropene (iso-butene), 1,1-dimethyl-2-hydroxyethyl 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 radical products were possible, i.e. when propene, but-1-ene or 2-methylpropene were the substrates, it was found that the concentration of the secondary or tertiary radical always exceeded that of the primary radical. However, the relative concentration of these radicals appears to be determined by subsequent 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 are made. The characterisation of the 2-hydroxyethyl radical has been backed up by experiments utilising isotopic substitution with 13C and D (2H). The other radicals have been identified with varying degrees of certainty. Those radicals, which are observed at the highest concentration and which are, therefore, characterised more certainly are--2-hydroxyethyl (1), from ethene: 1-methyl-2-hydroxypropyl (2), from cis- and trans-but-2-ene; 1-methyl-2-hydroxyethyl (3), from propene; 1-hydroxymethylpropyl (5), from but-1-ene; and 1,1- dimethyl-2-hydroxyethyl (8), from 2-methylpropene.

Reactions of alkenes with ozone in the gas phase: a matrix-isolation study of secondary ozonides and carbonyl-containing reaction products.

Gas phase ozonolysis reactions of the alkenes ethene, cis- and trans-but-2-ene, isoprene and the monoterpenes alpha-pinene, beta-pinene, beta-carene, limonene and beta-myrcene have been carried out and the reaction products have been trapped in O2-doped-argon matrices onto a Csl window held at 12 K. Products have been identified by IR spectroscopy. Comparison with previous matrix spectra, where secondary ozonides have been generated either in situ by annealing or in solution reactions allows a positive identification of the secondary ozonides of ethene and of cis- and trans-but-2-ene to be made. These observations are backed up by experiments utilising the isotopes 13C and 2H (D). It appears that secondary ozonides have also been formed from isoprene and the range of monoterpenes studied; this hypothesis is based upon the similarity of spectral features seen in the products of these reactions within those of the simpler alkenes. A number of other primary and secondary products are also identified from these reactions. Ethene gives formaldehyde as a primary product and acetaldehyde as a secondary product; it is found that the yield of acetaldehyde compared to formaldehyde increases as the reaction times are increased. Formaldehyde, one of the expected primary products, is formed by ozonolysis of beta-pinene, although the other expected primary product, nopinone, is not seen. A range of secondary reaction products have been identified from the ozonolysis of the monoterpenes studied.