Aromatic hydrocarbon nitration under tropospheric and combustion conditions. A theoretical mechanistic study

The viability of some nitration pathways is explored for benzene (B), naphthalene (N), and in part pyrene (P). In principle, functionalization can either take place by direct nitration (NO2 or N2O5 attack) or be initiated by more reactive species, as the nitrate and hydroxyl radicals. The direct attack of the NO2 radical on B and N, followed by abstraction of the H geminal to the nitro group (most likely accomplished by 3O2) could yield the final nitro-derivatives. Nevertheless, the initial step (NO2 attack) involves significant free energy barriers. N2O5 proves to be an even worst nitrating agent. These results rule out direct nitration at room temperature. Instead, NO3 and, even more easily, HO can form pi-delocalized nitroxy- or hydroxycyclohexadienyl radicals. A subsequent NO2 attack can produce several regio- and diastereoisomers of nitroxy-nitro or hydroxy-nitro cyclohexadienes. In this respect, the competition between NO2 and O2 is considered: the rate ratios are such to indicate that the NO3 and HO initiated pathways are the major source of nitroarenes. Finally, if the two substituents are 1,2-trans, either a HNO3 or a H2O concerted elimination can give the nitro-derivatives. Whereas HNO3 elimination is feasible, H2O elimination presents, by contrast, a high barrier. Under combustion conditions the NO2 direct nitration pathway is more feasible, but remains a minor channel.