Reactions Of Aromatic Compounds With OH Radicals

Abstract

Abstract As discussed in chapter I, the aromatic hydrocarbons are important contributors to the chemical processes in urban air masses. Aromatic hydrocarbons not containing unsaturated > C=C < bond(s) in the substituent groups react only slowly with 0 3 and with N0 3 radicals (Atkinson and Carter, 1984; Atkinson, 1991b). However, aromatic compounds react with OH radicals, with a rate coefficient that depends on the substituent groups attached to the aromatic ring (see section 11-C; Zetzsch,1982a; and Atkinson, 1991a). As a generalization, in the atmosphere aromatic compounds are more reactive than alkanes but less reactive than alkenes. The simplest aromatic hydrocarbon, benzene (C6H6) , is the least reactive of the aromatic hydrocarbons. In view of the fact that benzene has multiple double bonds involved in its structure, at first sight it is surprising that its reactivity is much lower that that of the alkenes. This can be seen in figure 11-A-1, where the reactivity of a series of four C6-ring hydrocarbons that contain an increasing number of carbon-carbon double bonds can be compared. Cyclohexene (C6H10), which contains one double bond, shows an increase of a factor of about 9 in the rate coefficient for reaction with OH radical (at 298 K) over that for the analogous saturated hydrocarbon, cyclohexane (C6H n) , which contains no double bonds.