Ii

Methyl glyoxal

FIGURE 6.13 One postulated fate of the OH-toluene adduct in air.

The simplest phenoxy radical, CftH50, does not react with 02 (k < 5 X 10 21 cm3 molecule 1 s ') but does react with NO (k = 1.9 X 10"12 cm3 molecule"1 s"1) and with N02 (k = 2.1 X 10"12 cm3 molecule"1 s"1), suggesting that reactions with NOx will be its primary fate in the troposphere (Platz et al., 1998b). However, this may not be the case for the larger, hydroxylated phenoxy radicals from the 0H-aromatic-02-N0 reaction sequence.

In short, there are a multitude of potential reaction pathways that must be considered in OH-aromatic reactions, and the details of the mechanism remain to be elucidated.

The formation of multifunctional, highly reactive "products" such as butenedial may be responsible for much of the missing carbon in these reactions. Such products will also react rapidly with OH, with 03, and, when present, with N03, as well as photolyze. For example, Fig. 6.14 shows the infrared spectrum obtained after a mixture of the cis and trans forms of butenedial were photolyzed using fluorescent lamps with wavelengths 3208 A < 480 nm and the absorptions due to the products CO, C02, HCHO, and HCOOH were subtracted out (Bierbach et al., 1994). Most of the remaining bands are due to the product 3//-furan-2-one formed from an intramolecular rearrangement of butenedial.

Reaction of butenedial with OH was also shown to give maleic anhydride as a major product (along with glyoxal). Figure 6.15 shows possible mechanisms for

6. GAS-PHASE REACTIONS IN IRRADIATED ORGANIC-NO^-AIR MIXTURES

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