Theoretical calculations support the expectation that the preferred site of initial OH attack is ortho to the methyl group (Andino et al., 1996), but addition to the other positions also occurs. If the OH-aromatic adduct, which contains ~ 18 kcal mol-1 excess energy, is not stabilized, it decomposes back to reactants, reaction (— 62). The existence of the adduct in the case of the OH-benzene reaction has been observed spectroscopi-cally (Fritz et al., 1985; Knispel et al., 1990; Markert and Pagsberg, 1993; Bjergbakke et al., 1996). As expected for such a mechanism, the rate constants at temperatures below ~ 300 K exhibit a pressure dependence at lower pressures. At higher temperatures, the rate of decomposition of the excited adduct back to reactants is higher, so the net contribution of adduct formation to the overall reaction is small compared to H-abstraction.

Conversely, at the lower temperatures, the rate constant for H-abstraction is small while, at the same time, the rate of adduct decomposition is lowered. As a result, at the lower temperatures (right side of Fig. 6.11), adduct formation predominates and a "negative" temperature dependence, as well as a dependence on pressure is observed for the overall rate constant. In the intermediate region, both addition and abstraction are occurring at significant rates, leading to the curved OH decay plots in Fig. 6.10 and the discontinuities in the Arrhenius plots of Fig. 6.11.

Table 6.16 shows the room temperature rate constants for the reactions of OH with some simple aro-matics as well as the branching ratio for abstraction, i.e., the ratio kM/(k(A + kh2). Abstraction accounts for less than about 10% of the reaction at room temperature for those alkylbenzenes studied to date. It is noteworthy that the reactions are all quite fast, even that for benzene being within approximately two orders of magnitude of diffusion controlled.

The products of the abstraction path are easily predictable, based on our understanding of the fates of alkyl radicals produced in alkane reactions (see Sections C and D). For example, in the case of toluene, the

TABLE 6.16 Rate Constants at Room Temperature for OH - Aromatic Hydrocarbon Reactions and Branching Ratios for the Abstraction Reaction"


k (10 12 cm' molecule s ~ ')

Branching ratio for abstraction


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