Electrophilic and Nucleophilic Aromatic Substitution

In the preceding chapter the special stabihty of benzene was described, along with reactions in which an aromatic ring was present as a substituent. What about reactions that occur on the ring itself What sort of reagents react with benzene and its derivatives, what products are formed, and by what mechanisms  

The largest and most important class of such reactions involve electrophilic reagents. We already have some experience with electrophiles, particularly with respect to their reaction with alkenes. Electrophilic reagents add to alkenes. 

A different reaction occurs with arenes. Substitution is observed instead of addition. The electrophilic portion of the reagent replaces one of the hydrogens on the ring  

This reaction is known as electrophilic aromatic substitution. 

It is one of the fundamental processes of organic chemistry and the major concern of this chapter. 

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Substituted 1,2,3-triazole 1-oxides 448 have been reported to undergo electrophilic and nucleophilic aromatic substitution and are subject to debromination, proton-metal exchange, and halogen-metal exchange followed by electrophilic addition. Transmetallation and cross-coupling have not been described. 3-Substituted 1,2,3-triazole 1-oxides 448 can be proton-ated or alkylated at the O-atom and they can be deoxygenated and deal-kylated. The individual reactions are described in Section 4.2.7.1-4.2.7.14. 

Understand the mechanisms of electrophilic and nucleophilic aromatic substitutions. Predict the products of these reactions and use them in syntheses. 

Electrophilic and nucleophilic aromatic substitution ch22-ch23... 

Using electrophilic and nucleophilic aromatic substitution in five- and six-membered heterocycles. Chemo- and regioselectivity The synthesis of pyridazines... 

There is, for example, no end-of-text chapter entitled Heterocyclic Compounds. Rather, heteroatoms are defined in Chapter 1 and nonaromatic heterocyclic compounds introduced in Chapter 3 heterocyclic aromatic compounds are included in Chapter 11, and their electrophilic and nucleophilic aromatic substitution reactions described in Chapters 12 and 23, respectively. Heterocyclic compounds appear in numerous ways throughout the text and the biological role of two classes of them—the purines and pyrimidines—features prominently in the discussion of nucleic acids in Chapter 27. 

Note the differences between electrophilic and nucleophilic aromatic substitutions Electrophilic substitutions are favored by electron-donating substituents, which stabilize the carbocation intermediate, while nucleophilic substitutions are favored by electron-withdrawing substituents, which stabilize a carbanion intermediate. The electron-withdrawing group that deactivate rings for electrophilic substitution (nitro, carbonyl, cyano. and so on) activate them for nucleophilic substitution. What s more, these groups are meta directors in electrophilic substitution, but are ortho-para directors in nucleophilic substitution. 

The present knowledge of the fundamentals of the substitution of aromatic compounds by radicals is completely different from the respective situation in electrophilic and nucleophilic aromatic substitution. Numerous investigations of both tjrpes of heterolytic aromatic substitutions have been carried out since the nineteenth century using the classical methods of organic chemistry, and the mechanism of both nucleophilic and electrophilic aromatic substitution are well defined. 

Reactions of Arenes Electrophilic and Nucleophilic Aromatic Substitution... 

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