| Abstract: | The rate constant of the title reaction is determined during thermal decomposition of di-n-pentyl peroxide C5H11O( )OC5H11 in oxygen over the temperature range 463–523 K. The pyrolysis of di-n-pentyl peroxide in O2/N2 mixtures is studied at atmospheric pressure in passivated quartz vessels. The reaction products are sampled through a micro-probe, collected on a liquid-nitrogen trap and solubilized in liquid acetonitrile. Analysis of the main compound, peroxide C5H10O3, was carried out by GC/MS, GC/MS/MS [electron impact EI and NH3 chemical ionization CI conditions]. After micro-preparative GC separation of this peroxide, the structure of two cyclic isomers (3S*,6S*)3α-hydroxy-6-methyl-1,2-dioxane and (3R*,6S*)3α-hydroxy-6-methyl-1,2-dioxane was determined from 1H NMR spectra. The hydroperoxy-pentanal OHC( )(CH2)2( )CH(OOH)( )CH3 is formed in the gas phase and is in equilibrium with these two cyclic epimers, which are predominant in the liquid phase at room temperature. This peroxide is produced by successive reactions of the n-pentoxy radical: a first one generates the CH3C·H(CH2)3OH radical which reacts with O2 to form CH3CH(OO·)(CH2)3OH; this hydroxyperoxy radical isomerizes and forms the hydroperoxy HOC·H(CH2)2CH(OOH)CH3 radical. This last species leads to the pentanal-hydroperoxide (also called oxo-hydroperoxide, or carbonyl-hydroperoxide, or hydroperoxypentanal), by the reaction HOC·H(CH2)2CH(OOH)CH3+O2→O()CH(CH2)2CH(OOH)CH3+HO2. The isomerization rate constant HOCH2CH2CH2CH(OO·)CH3→HOC·HCH2CH2CH(OOH)CH3 (k3) has been determined by comparison to the competing well-known reaction RO2+NO→RO+NO2 (k7). By adding small amounts of NO (0–1.6×1015 molecules cm−3) to the di-n-pentyl peroxide/O2/N2 mixtures, the pentanal-hydroperoxide concentration was decreased, due to the consumption of RO2 radicals by reaction (7). The pentanal-hydroperoxide concentration was measured vs. NO concentration at ten temperatures (463–523 K). The isomerization rate constant involving the H atoms of the CH2( )OH group was deduced: or per H atom: The comparison of this rate constant to thermokinetics estimations leads to the conclusion that the strain energy barrier of a seven-member ring transition state is low and near that of a six-member ring. Intramolecular hydroperoxy isomerization reactions produce carbonyl-hydroperoxides which (through atmospheric decomposition) increase concentration of radicals and consequently increase atmospheric pollution, especially tropospheric ozone, during summer anticyclonic periods. Therefore, hydrocarbons used in summer should contain only short chains (4) hydrocarbons or totally branched hydrocarbons, for which isomerization reactions are unlikely. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 875–887, 1998 |
