The epoxidation of alkenes by peroxyl radicals in the gas phase is examined, and it is demonstrated that the activation energies for 36 epoxidation reactions between 17 alkenes and 5 peroxyl radicals correlate well to the charge transfer, or the corresponding energy decrease, in forming the peroxyalkyl adduct, or the difference between the ionization energy of the alkene and the electron affinity of the peroxyl radical. These correlations have been used to estimate five previously unmeasured epoxidation rate constants relevant to propene autoxidation.
The relatively low lying first electronic excited states of peroxyl radicals are suggested to play a direct role in determining the rate of their addition to alkenes, with there being, in the vicinity of the transition state, an unavoided crossing of C s symmetry of the ground and first excited states. If there is no charge transfer between radical and alkene during the formation of the adduct, then the barrier height is approximately equal to the energy required to excite an isolated peroxyl radical to its first excited state; with charge transfer, the activation energy for the addition is lowered in proportion to the energy released by the charge transfer. It is also suggested that for the specific case of hydroperoxyl radical addition to ethene, this description is compatible with the generally accepted mechanism for the reaction of ethyl radicals with molecular oxygen whereby the resulting ethylperoxyl radical can decompose to ethene and a hydroperoxyl radical via a cyclic 2 A transition state. Electron affinities, ionisation energies, absolute electronegativities and hardness of acetylperoxyl, hydroperoxyl, methylperoxyl, ethylperoxyl, iso-propylperoxyl and tertbutylperoxyl radicals have been calculated at the G2MP2 level.
The mechanism of production of nitrogen oxides by electrical discharges has been examined. The velocity of shock fronts generated by laboratory scale discharges have been measured and are found to be too slow to raise the air temperature to the ∼3000 K necessary for nitrogen fixation by the Zel'dovich mechanism. The freeze‐out mixing ratio of NOx in air has been measured directly for low‐pressure discharges and is found to be of the order expected from the Zel'dovich mechanism for gas cooling over a timescale far longer than the duration of the shock front. Therefore it is concluded that NOx is formed in the gas in the slowly cooling hot channel region and not in the rapidly cooling shock front. Also, it is argued that NOx formation occurs by a freeze out mechanism due to a rapid drop in temperature, not density as has been suggested. NO2 production is found to be significant, with the [NO2]/[NO] ratio being strongly dependent on the water content of the air. Discrepancies between previous experimental studies of the [NO2]/[NO] ratio and the quantity of NOx formed per unit energy (P) are discussed. P is also found to vary with spark gap and ambient pressure. It is thought that these effects may be due to a significant loss of heat from the spark gap to the electrodes. The inclusion of atmospheric levels of N2O, CH4, and CO2 are found to have no measurable effect on the yields of NO or NO2.
A series of laboratory and modelling experiments on the oxidation of propene in the gas phase has been undertaken to determine conditions which give high yields of propene oxide. The conditions under which the experiments were conducted were 505-549 K and up to 4 bar pressure. It is proposed that propene oxide is formed from propene by reaction with several peroxy radicals including HOz and CH3C03. However, one of the more important radicals is hydroxypropylperoxy. Its reaction with propene, under these conditions
The autoxidation of branched alkanes, pristane (2,6,10,14-tetramethylpentadecane) and squalane (2,6,10,15,19,23hexamethyltetracosane), has been studied in the liquid phase at 170 °C with products identified by CG-MS and quantified by GC-FID and chemical mechanisms for their formation proposed. For pristine, the relative rate of radical attack on tertiary hydrogen atoms was measured to be 15 ( 1 times faster than attack on secondary hydrogen atoms, with 16 of the 23 products identified formed by the further reaction of tertiary alkoxyl radicals. Tertiary alcohols are the most significant product (29 ( 4% of reacted pristane early in the reaction), while most smaller products can be accounted for by fragmentation of tertiary alkoxyl radicals of pristane (55 ( 10% of reacted pristane early in the reaction). These fragment products include alkanes, formed by the reaction of 27 ( 3% of primary alkyl radicals abstracting hydrogen atoms instead of adding oxygen molecules.
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