Electrophilic aromatic substitution energy profile

Energy profiles with a deactivating group. Nitrobenzene is deactivated toward electrophilic aromatic substitution at any position, but deactivation is strongest at the ortho and para positions. Reaction occurs at the meta position, but it is slower than the reaction with benzene. 

Hammond discussed electrophilic aromatic substitution in his paper. A typical energy profile of the reaction is presented in Figure 6.15, where A is a reactant, B is a 7t-complex, D is a a-complex, and C and E are transition states, through which D is formed and destroyed, respectively. The transition state of the rate-determining step (C or E) is closely related to D. 

Figure 6.15 Energy profile of an electrophilic aromatic substitution...

Fig. 9.8. Reaction energy profiles for electrophilic aromatic substitution showing variation in ratedetermining step and electrophile selectivity.

Now we can remove the "blocking group" - SO3H and next form the final product 2,4-D. To do this we treat 2-hydroxy 3,5-dichloropbenzenesulfonic acid with steam at 120°C with a trace of acid catalyst. This is essentially the reverse of sulfonation, or "desulfonation." We note that sulfonation is the only one of the common electrophilic aromatic substitutions that is reversible and the question arises why this is so. This calls for an analysis of the general mechanism of electrophilic aromatic substitution. The idealized energy profile for such a reaction (chlorination) is ... 

Figure 10.6 summarizes the general ideas presented in Section 10.2. At least four types of energy profiles can exist for individual electrophilic aromatic substitution reactions. Case A is the case of rate-determining generation of the electrophile. It is most readily identified by kinetics. A rate law independent of the concentration of the aromatic is diagnostic of this case. Case B represents rate-determining... 

This completes our preliminary survey of the most important reactions in aromatic electrophilic substitution. We shall switch our attention to the benzene ring itself now and see what effects various types of substituent have on these reactions. During this discussion we will return to each of the main reactions and discuss them in more detail. Meanwhile, we leave the introduction with an energy profile diagram in the style of Chapter 13 for a typical substitution. 

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