" From Dumdei et al. (1988); see also Yu et al. (1997).

" From Dumdei et al. (1988); see also Yu et al. (1997).

toluene in air in the presence of NOx. These include additional compounds from those in Table 6.18, such as benzoquinones and epoxides, the latter hypothesized to be formed from an epoxide type of structure predicted theoretically for the OH-aromatic-02 adduct in one study (Bartolotti and Edney, 1995). Kwok et al. (1997) have studied the reactions of OH with o-, m-, and /7-xylene and identified a variety of open-chain multifunctional products, including

HC(0)CH = CHCHO, CH 3 C(0)CH = CHCHO, CH3C(0)CH=CHC(0)CH3, CH 3 C(0)C(CH 3) = CHCH = CHCHO, and their isomers. It is typical of such aromatic oxidations that only ~ 40-70% of the reacted parent organic can be accounted for in measured products.

Figure 6.13 shows one postulated reaction sequence taking the adduct to methylglyoxal and butenedial. This represents just one of many possible reaction paths. OH can also add to the ring at the meta or para positions, in addition to the ortho position shown in Eq. (62). The addition of 02, shown in Fig. 6.13 to occur at the f-position relative to the OH group, could also occur at the 3- or 5-positions. Ring-cleavage products are also observed in the absence of NOx (Atkinson and Aschmann, f994; Seuwen and Warneck, 1996). Subsequent cyclization of the peroxy radical to form allylically stabilized five-membered bicyclic radicals is expected to be the most energetically favored, with the formation of nonallylically stabilized biradicals being endothermic (Andino et al., 1996).

Theoretical studies also suggest that some of the peroxy radicals formed by addition of 02 to the OH-aromatic adducts may react with NO in competition with cyclization (Andino et al., 1996), generating N02 and an aromatic peroxy radical that may subsequently form phenolic and unsaturated derivatives through reactions with 02:

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