Hydroxyl Radical OH

The reactions of simple aromatic hydrocarbons with OH provide a classic example of how kinetics can be used to elucidate reaction mechanisms. Figure 6.10 shows a semilogarithmic plot of the decay of OH in the presence of a great excess of toluene from 298 to 424 K at ~ 100 Torr total pressure in argon. While one would expect such plots to be linear (Chapter 5.B.1), this is only observed to be the case at temperatures below 325 K and above about 380 K; at the intermediate temperatures, the plots are clearly curved.

Even more unusual behavior is observed for the temperature dependence of the rate constant. Figure 6.11 shows these data in Arrhenius form for the reactions of toluene and 1,2,3-trimethylbenzene. At the higher temperatures, the Arrhenius plot is linear with a normal activation energy (i.e., the rate constant increases with increasing temperature). However, as the temperature is lowered, there is a sharp discontinuity in the plot and at lower temperatures the temperature dependence is reversed; i.e., the rate constants decrease with increasing temperature.

These results are now known to reflect the occurrence of two, quite different mechanisms over this temperature range. At the higher temperatures, ab straction of a hydrogen atom occurs, in the case of the substituted benzenes from the alkyl side chains, where the C-H bonds are weaker (~85 kcal mol-1 vs ~ 110 kcal mol-1 for C-H bonds in benzene):

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