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

Isomerization 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 RO 2 radicals. In the temperature range 373–473 K, 2‐hexoxy (C 6 H 13 O) 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: equation image Its rate constant has been determined at 453 and 483 K, and the general expression can be written as equation image Hexylperoxy C 6 H 13 O 2 radical, present in n ‐hexane oxidation by oxygen/nitrogen mixtures in the temperature range 543–573 K, gives 2‐hexanone‐4‐hydroperoxide, 3‐hexanone‐5‐hydroperoxide, and 2‐hexanone‐5‐hydroperoxide. The first two are formed through an isomerization 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 RO 2 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: equation image The role of these reactions in the formation of radicals in the troposphere is discussed. Other products arising in the reactional path, such as ketones, furans, and diketones, are identified. Identification of these ketohydroperoxides was made using gas chromatography/mass spectrometry with electron impact, and with NH 3 (or ND 3 ) chemical ionization.