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.