Experimental Measures of Reactivity

We might ask just how reactive indole is in comparison with other aromatic rings. Mayr and his coworkers have carried out extensive studies on relative nucleophi-licity, N, by examining reactivity towards a series of benzhydryl carbocations [1]. In this comparison, indole was found to have the relative reactivity 5.55. The 1 -methyl (5.75) and 1,2-dimethyl derivatives are more reactive, while 2-methylindole is somewhat less reactive (4.42) because of its steric effects [2]. Pyrrole is less reactive with an N value of 4.63. Indole is substantially less reactive than representative enamines, such as 1-pyrrrolidinocyclohexene, for which N is 14.91 [3]. Being a logarithmic scale, this indicates that indole is some 15 times more reactive than pyrrole, but 10 ° less reactive than the cited enamine. For simpler aromatics, 1,3-dimethoxybenzene is assigned an N value of 2.48 and toluene, —4.47 Thus, indole is about 10 ° more reactive than toluene. 

The rate of reaction of indole and 3-methylindole have been measured with a series of substituted benzenediazonium ions and give Hammett correlations with p of +3.4 and +3.8, respectively. The 3-methyl derivatives are about an order of magnitude slower. The second order kinetics, are consistent with a classical electrophilic substitution mechanism [7]. 

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There is another way to test chemical stability hat can be applied to aromatic organic compounds. Aromaticity is usually taken to mean high stability and low reactivity. It can be quantified by calculating the resonance energy per electron (REPE). These values are usually calculated by simple Huckel theory, but they correlate well with experimental measures of reactivity. ... 

However, airshed model calculations are no more reliable than the chemical mechanisms upon which they are based. While the initial atmospheric reaction rates for most VOCs are reasonably well known or at least can be estimated, for most VOCs the subsequent reactions of the radicals formed are complex and have uncertainties that can significantly affect predictions of atmospheric impacts. For this reason, environmental chamber experiments and other experimental measurements of reactivity are necessary to test and... 

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