Isomeric hexyl-ketohydroperoxides formed by reactions of hexoxy and hexylperoxy radicals in oxygen

lsomerization reactions of peroxy radicals during oxidation of long-chain hydrocarbons yield hydroperoxides, and therefore play an important role in combustion and atmospheric chemistry, because of their action as branching agents in these chain reaction processes. Different formation mechanisms and structures are involved. Three isomeric hexyl-ketohydroperoxides are formed via isomerization reactions in oxygen of either hexoxy RO or hexylperoxy RO2 radicals. In the temperature range 373-473 K, 2-hexoxy (C6H13O) radical in O-2/N-2 mixtures gives 2-hexanone-5-hydroperoxide via two consecutive isomerizations. The second one is a H transfer from a HC(OH) group occurring via a seven-membered ring intermediate: CH3-CH(OH)-CH2-CH2-CH(OO.)-CH3 -->CH3 --> C-.(OH)-CH2-CH2-CH(OOH)-CH3 Its rate constant has been determined at 453 and 483 K, and the general expression can be written as Hexylperoxy C6H13O2 radical, present in n-hexane oxidation by oxygen/nitrogen mixtures in the temperature range 54isomerization reaction via a six-membered ring intermediate, and the last through an isomerization reaction via a seven-membered ring intermediate, The ratio of the rate constant of the isomerization reactions of RO2 radicals via a seven-membered ring intermediate to that via a six-membered ring is found to be 0.795, and the rate constant expression via a seven-membered ring intermediate is proposed:3-573 K, gives 2-hexanone-4-hydroperoxide, 3-hexanone-5-hydroperoxide, and 2-hexanone-5-hydroperoxide. The first two are formed through an CH3-CH(OO.)-CH2-CH2-CH2-CH3 --> CH3-CH(OOH)-CH2-CH2-(CH)-H-.-CH3 k = 7.9 x 10(11) x exp[-(17650 +/- 100)/RT]s(-1) (E-a in cal mol(-1)) The role of these reactions in the formation of radicals in the troposphere is discussed. Other products arising in the reactionat path, such as ketones, furans, and cliketones, are identified. Identification of these ketohydroperoxides was made using gas chromatography/mass spectrometry with electron impact, and with NH3 (or ND3) chemical ionization. (C) 2003 Wiley Periodicals, Inc.