| Abstract
| - In the tropospheric oxidation of benzene and methylated benzenes, unsaturated dicarbonyls arecommonly detected products. Aldehydes are known to contribute on their own to some aspects of air pollution,and hexa-2,4-dien-1,6-dial (muconaldehyde) in particular is interesting because of its multiform toxicity. Thisstudy investigates the likelyhood of some benzene oxidation steps and is especially focused on ring openingand generation of muconaldehyde. With sufficiently high NOx concentration, O abstraction by NO from thecis peroxyl group in the 2-hydroxy-cyclohexadienyl peroxyl radical III can play a role. In fact, it is shown toopen a facile cascade of oxidation steps by first forming the 2-hydroxy-cyclohexadienyl oxyl radical VI. Thisintermediate is prone to ring opening via β-fragmentation and generates the open-chain delocalized 6-hydroxy-hexa-2,4-dienalyl radical VII, in which one terminus is the first carbonyl group of the final dialdehyde. Thesecond one can form either by simple H abstraction operated by O2 or by O2 addition followed by HOO•elimination. The overall free-energy drop with respect to III is estimated to be 48 kcal mol-1. Exploration ofother pathways, possibly playing a major role in yielding aldehydes in the case of low NOxconcentration,indicates that only ring closure of the 2-hydroxy-cyclohexadienyl peroxyl radical III to the [3.2.1] bicyclicendo-peroxy allyl radical intermediate XIII is promising. In this case, however, the outcome of a subsequentring opening can ultimately be the production of 1,2 and 1,4 dialdehydes (as direct oxidation of muconaldehydeitself can actually do).
|
