Reactive intermediates carbocation, features

Experimental evidence concerning the relative rates for SnI reactions of halides is listed in Table 6.7. The differences in reactivity reflect stmctural features that stabilize the intermediate carbocation. Carbocations are stabilized by the electron-donating effect of alkyl groups, which help to disperse the positive charge. We have noted that alkyl groups have a modest electron-donating effect (see Section 4.3.3). In carbocations, this is not a simple inductive effect, but results from overlap of the a C-H (or C-C) bond into the vacant p orbital of the carbocation. This leads to a favourable delocalization of the positive charge. 

One remarkable feature of enzyme-catalyzed reactions is the ability to generate at enzyme sites reactive intermediates, such as carbanions, carbocations, and radicals, that normally would require strong reaction conditions to generate in chemical reactions and would be very unstable in aqueous solution. 

In order to assess the influence that substituents have on the reactivity of aromatic molecules, it is important to consider their effects not only on the benzene ring itself as above, but also on the carbocation intermediates resulting from electrophilic attack. These species are relatively unstable and any feature that affects their stability will influence their ease of formation and therefore the outcome of a reaction. 

---