CHAPTER TWELVE Reactions of Arenes: Electrophilic Aromatic Substitution is not aromatic and possesses only a fraction of the resonance stabilization of benzene Once formed, it rapidly loses a proton, restoring the aromaticity of the ring and giving the product of electrophilic aromatic substitution H E Observed product of electrophilic H H E Not observed-not aromatic If the Lewis base (Y) had acted as a nucleophile and added to carbon, the prod- uct would have been a nonaromatic cyclohexadiene derivative. Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation. Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12. 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution. In order for electrophilic aromatic substitution reac- tions to overcome the high activation energy that characterizes the first step, the elec trophile must be a fairly reactive one. Many electrophilic reagents that react rapidly with alkenes do not react at all with benzene. Peroxy acids and diborane, for example, fall into this category. Others, such as bromine, react with benzene only in the presence of catalysts that increase their electrophilicity. The low level of reactivity of benzene toward FIGURE 12.1 Energy two steps of electrophile aromatic substitution E H E -H--.yo- E H E-Y E H-Y Reaction coordinate Back Forward Main MenuToc Study Guide ToC Student o MHHE Websiteis not aromatic and possesses only a fraction of the resonance stabilization of benzene. Once formed, it rapidly loses a proton, restoring the aromaticity of the ring and giving the product of electrophilic aromatic substitution. If the Lewis base (:Y) had acted as a nucleophile and added to carbon, the prod￾uct would have been a nonaromatic cyclohexadiene derivative. Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation. Substitution occurs preferentially because there is a substantial driving force favoring rearomatization. Figure 12.1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution. In order for electrophilic aromatic substitution reac￾tions to overcome the high activation energy that characterizes the first step, the elec￾trophile must be a fairly reactive one. Many electrophilic reagents that react rapidly with alkenes do not react at all with benzene. Peroxy acids and diborane, for example, fall into this category. Others, such as bromine, react with benzene only in the presence of catalysts that increase their electrophilicity. The low level of reactivity of benzene toward Y H H E Cyclohexadienyl cation fast Observed product of electrophilic aromatic substitution E H H Y H H E Y Not observed—not aromatic 446 CHAPTER TWELVE Reactions of Arenes: Electrophilic Aromatic Substitution H Energy Reaction coordinate E E E E H H E±Y H±Y Yδ Yδ Y H δ δ FIGURE 12.1 Energy changes associated with the two steps of electrophilic aromatic substitution. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website