From benzene to muconaldehyde: Theoretical mechanistic investigation on some tropospheric oxidation channels

In the tropospheric oxidation of benzene and methylated benzenes, unsaturat ed dicarbonyls are commonly detected products. Aldehydes are known to contr ibute 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. This study investigates the likelyhood of some benzene oxidation steps and is especially focused on ring opening and generation of muconalde hyde. With sufficiently high NOx concentration, O abstraction by NO from th e cis peroxyl group in the 2-hydroxy-cyclohexadienyl peroxyl radical III ca n play a role. In fact, it is shown to open a facile cascade of oxidation s teps by first forming the 2-hydroxy-cyclohexadienyl oxyl radical VI. This i ntermediate is prone to ring opening via beta-fragmentation and generates t he open-chain delocalized 6-hydroxyhexa-2,4-dienalyl radical VII, in which one terminus is the first carbonyl group of the final dialdehyde. The secon d one can form either by simple H abstraction operated by O-2 or by O-2 add ition followed by HOO* elimination. The overall free-energy drop with respe ct to III is estimated to be 48 kcal mol(-1). Exploration of other pathways , possibly playing a major role in yielding aldehydes in the case of low NO x concentration, indicates that only ring closure of the 2-hydroxy-cyclohex adienyl peroxyl radical III to the [3.2.1] bicyclic endo-peroxy allyl radic al intermediate XIII is promising. In this case, however, the outcome of a subsequent ring opening can ultimately be the production of 1,2 and 1,4 dia ldehydes las direct oxidation of muconaldehyde itself can actually do).