Hammond postulate electrophilic aromatic substitution

Hammond postulate has been used to explain the effect of substituents on the rate of benzilic acid rearrangements, mechanism of electrophillic aromatic substitution reactions and reactions involving highly reactive intermediates such as carbonium ions and carbon ions. 

We may ask How does Y know which side will give the more stable carbocation As in the similar case of electrophilic aromatic substitution (p. 508), we invoke the Hammond postulate and say that the lower energy carbocation is preceded by the lower energy transition state. Markovnikov s rule also applies for halogen substituents because the halogen stabilizes... 

To understand why some substituents make a benzene ring react faster than benzene itself (activators), whereas others make it react slower (deactivators), we must evaluate the rate-determining step (the first step) of the mechanism. Recall from Section 18.2 that the first step in electrophilic aromatic substitution is the addition of an electrophile (E ) to form a resonance-stabilized carbo-cation. The Hammond postulate (Section 7.15) makes it pos.sible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 

The substituent effects in aromatic electrophilic substitution are dominated by resonance effects. In other systems, stereoelectronic effects or steric effects might be more important. Whatever the nature of the substituent effects, the Hammond postulate insists diat structural discussion of transition states in terms of reactants, intermediates, or products is valid only when their structures and energies are similar. 

First, it is important to note that most aromatic electrophilic substitution reactions are under kinetic, and not thermodynamic, control. This is because most of the reactions are irreversible, and the remainder are usually stopped before equilibrium is reached. In a kinetically controlled reaction, the distribution of products (or product spread), i.e. the ratio of the various products formed, is determined not by the thermodynamic stabilities of the products, but by the activation energy barrier that controls the rate determining step. In a two-step reaction, it is a reasonable assumption that the transition state of the rate determining step is close in energy to that of the intermediate, which in this case is the Wheland intermediate and so by invoking the Hammond postulate, one may assume that they have similar geometries. 

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