From Benzene to Muconaldehyde: Theoretical Mechanistic Investigation on Some Tropospheric Oxidation Channels

In the tropospheric oxidation of benzene and methylated benzenes, unsaturated dicarbonyls are commonly detected products. Aldehydes are known to contribute on their own 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 muconaldehyde. With sufficiently high NOx concentration, O abstraction by NO from the cis peroxyl group in the 2-hydroxy-cyclohexadienyl peroxyl radical III can play a role. In fact, it is shown to open a facile cascade of oxidation steps by first forming the 2-hydroxy-cyclohexadienyl oxyl radical VI. This intermediate is prone to ring opening via b-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 dialdehyde. The second 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 of other pathway, possibly playing a major role in yielding aldehydes in the case of low NOx concentration, indicates that only ring closure of 2-hydroxy-cyclohexadienyl peroxyl radical III to the [3.2.1] bicyclic endo-peroxy allyl radical 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 dialdehydes (as direct oxidation of muconaldehyde itself can actually do).