17 Reactions of Aromatic Compounds With an understanding of the properties that make a compound aromatic, we now consider the reactions of aromatic compounds. A large part of this chapter is devoted to electrophilic aromatic substitution, the most impor-tant mechanism involved in the reactions of aromatic compounds. Many reactions of benzene and its derivatives are explained by minor variations of electrophilic aromat-ic substitution. We will study several of these reactions and then consider how sub-stituents on the ring influence its reactivity toward electrophilic aromatic substitution and the regiochemistry seen in the products. We will also study other reactions of aro-matic compounds, including nucleophilic aromatic substitution, addition reactions, reactions of side chains, and special reactions of phenols. Like an alkene, benzene has clouds of pi electrons above and below its sigma bond framework. Although benzene's pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabi-lized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. + E H ----? H H sigma complex The sigma complex (also called an arenium ion) is not aromatic because the sp3-hybrid carbon atom interrupts the ring of p orbitals. This loss of aromaticity con-tributes to the highly endothermic nature of this first step. The sigma complex regains aromaticity either by a reversal of the first step (returning to the reactants) or by loss of the proton on the tetrahedral carbon atom, leading to the substitution product. The overall reaction is the substitution of an electrophile (E+) for a proton (H+) on the aromatic ring: electrophilic aromatic substitution. This class of reactions includes EPM of anisole 17-1 Electrophilic Aromatic Substitution substituted E 749
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