In electrophilic aromatic substitution

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. 

Electrophilic aromatic substitution reactions are important for synthetic purposes and also are one of the most thoroughly studied classes of organic reactions from a mechanistic point of view. The synthetic aspects of these reactions are discussed in Chapter 11 of Part B. The discussion here will emphasize the mechanisms of several of the most completely studied reactions. These mechanistic ideas are the foundation for the structure-reactivity relationships in aromatic electrophilic substitution which will be discussed in Section 10.2... 

Iv) Although amino group Is o- and p- directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroanlllne. 

Problem 11.3 How does the absence of a primary isotope effect prove experimentally that the first step in aromatic electrophilic substitution is rate-determining ... 

Nucleophilic aromatic substitutions of H are rare. The intermediate benzenanion in aromatic nucleophilic substitution is analogous to the intermediate benzenonium ion in aromatic electrophilic substitution, except that negative charge is dispersed to the op-positions. 

Protonated polymethylbenzenes281 and the chlorohexamethylbenzenium cation,282 intermediates in aromatic electrophilic substitutions known as Wheland intermediates, have been isolated as crystalline salts, allowing investigators to obtain their X-ray crystal structure. Nitrosoarenium a complexes of various arenes were directly observed by transient absorption spectroscopy.283 Kochi presented a method combining appropriate instrumental techniques (X-ray crystallography, NMR, time-resolved UV-vis spectroscopy) for the observation, identification, and structural characterization of reactive intermediates fa and n complexes) in electrophilic aromatic substitution.284... 

In aromatic electrophilic substitutions, 60 corresponds to the Wheland complex. 

The electronic effect of groups in aromatic electrophilic substitution processes is fully discussed in all standard organic chemistry textbooks. 

Since the early days of organic chemistry, nitration has been considered to be an important reaction and has been widely used. As early as 1825 Faraday discovered benzene and recorded its reaction with nitric acid. Shortly after, the use of nitric acid sulfuric acid mixtures to effect nitration was reported and was soon quoted in a patent. Nitration figured prominently in the development of ideas of theoretical organic chemistry in the early part of the twentieth century and, as the most widely applicable and most widely used example of electrophilic substitution, it played an important role in the consideration of aromatic stability and reactivity. In 1910 the first report of orientation and deactivation in aromatic electrophilic substitution was published (10MI1). 

In aromatic electrophilic substitution, further evidence for the formation of a 7r-complex between the halogen and the aromatic system has been found57 along with critical mecha-... 

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. 

The rates of attack of radicals on aromatic rings correlate with ionisation potential, with localisation energy and with superdelocalisability (see page 130), a picture reminiscent of the situation in aromatic electrophilic substitution. As in that field, there are evidently a number of related factors affecting reactivity. Frontier orbitals provide useful explanations for a number of observations in the field. 

We cannot, then, expect this approach to understanding chemical reactivity to explain everything. We should bear in mind its limitations, particularly when dealing with subjects like ortho/para ratios in aromatic electrophilic substitution, where steric effects are well known to be important. Likewise solvent effects (which usually make themselves felt in the entropy of activation term) are also well known to be part of the explanation of the principal of hard and soft acids and bases. Some mention of all these factors will be made again in the course of this book. Arguments based on the interaction of frontier orbitals are powerful, as we shall see, but they must not be taken so far that we forget these very important limitations. 

In aromatic electrophilic substitution, " the initial interaction between an electrophile and the aromatic n system is a multicenter interaction (of n-complex nature). The lack of substrate selectivity observed in some reactions of aromatic compounds with strong electrophiles (e.g., N()2 ) indicates that the initial multicenter complex is a separate well-defined intermediate,- " " Its nature was much discussed. Schofield et al. suggested it to be a solvent cage, whereas Perrin preferred a radical ion pair. There was general agreement of an initial intermediate involving the aromatic as an entity. The subsequent step affords a trivalent benzenium ion intermediate or a complex (Scheme 6.42). 

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