Studies on the Reactions of Polycyclic Aromatic Hydrocarbons With Nitrogen-Dioxide in Aprotic Solvents.

There is recent evidence that free radical nitration reactions occur in polluted urban air, and that the nitrated polycyclic aromatic hydrocarbons (NO$\sb2$-PAH) produced by this reaction pathway account for a substantial amount of the total NO$\sb2$-PAH found in ambient particulate organic matter. The nitration of PAH with nitrogen dioxide in solvents of low polarity has been studied as a model for the reactions occurring in the atmosphere (gas phase). While vast amounts of data are available for the electrophilic ionic nitration and its mechanism of reaction is considered well-established (and often included in sophomore Chemistry books), the literature documents relatively few reports on the mechanism of reaction of PAH with nitrogen dioxide. Our studies indicate that: (1) a free radical mechanism operates in solvents of low dielectric constant (e.g. CCl$\sb4$) and becomes less important as the dielectric constant is increased (CHCl$\sb3$, CH$\sb2$Cl$\sb2$, THF, CH$\sb3$CN, CH$\sb3$NO$\sb2$), (2) Bronsted and Lewis acids, and lower temperatures promote the electrophilic ionic reaction pathway, (3) the free radical mechanism involves metastable adducts of the PAH and two to four nitrogen dioxide equivalents, (4) the radical reaction yields nonconventional substitution patterns of dinitro-PAH even at low conversions (e.g. 1,2- and 1,3-dinitrofluoranthenes and, 1,3- and 2,3-dinitronaphthalenes), (5) the radical reaction of anthracene with nitrogen dioxide yields cis- and trans-9,10-dinitro,9,10-dihydroanthracene, which are isolable, (6) in radical nitration, the nitro-group does not have a strong deactivating effect towards further nitration as in electrophilic ionic nitration, in fact, its effect in radical nitration is frequently that of slight activation