12.3 Nitration of benzene electrophiles stems from the substantial loss of resonance stabilization that accompanies transfer of a pair of its six T electrons to an electrophile. with this as background, let us now examine each of the electrophilic aromatic substitution reactions presented in Table 12.1 in more detail, especially with respect to the electrophile that attacks benzene. 12.3 NITRATION OF BENZENE Now that we've outlined the general mechanism for electrophilic aromatic substitution, we need only identify the specific electrophile in the nitration of benzene(see Table 12.1) to have a fairly clear idea of how the reaction occurs. Figure 12.2 shows the application of those general principles to the reaction +HONO,- Ha Benzene Nitric acid Nitrobenzene(95%) Wa The electrophile(e) that reacts with benzene is nitronium ion (NO2). The concentra- The role of nitronium ion in tion of nitronium ion in nitric acid alone is too low to nitrate benzene at a convenient the nitration of benzene was rate, but can be increased by adding sulfuric acid. demonstrated by Sir Christ her ingold-the same persol who suggested the Sn1 and HO- 2HOSO,OH n→-N- H3o 2HOSO,O collaborated with Cahn and elog on the R and Snot- tional system Nitric acid Sulfuric acid Nitronium i Hydronium Hyd Step 1: Attack of nitronium cation on the Tt system of the aromatic ring FIGURE 12.2 The me- chanism of benzene. An electrostatic po- tential map of nitronium ion can be viewed on Learning H Benzene and nitronium ion ation intermediate Step 2: Loss of a proton from the cyclohexadienyl cation HE +H-0 Cyclohexadienyl Water Nitrobenzene Back Forward Main MenuToc Study Guide ToC Student o MHHE Websiteelectrophiles stems from the substantial loss of resonance stabilization that accompanies transfer of a pair of its six electrons to an electrophile. With this as background, let us now examine each of the electrophilic aromatic substitution reactions presented in Table 12.1 in more detail, especially with respect to the electrophile that attacks benzene. 12.3 NITRATION OF BENZENE Now that we’ve outlined the general mechanism for electrophilic aromatic substitution, we need only identify the specific electrophile in the nitration of benzene (see Table 12.1) to have a fairly clear idea of how the reaction occurs. Figure 12.2 shows the application of those general principles to the reaction: The electrophile (E) that reacts with benzene is nitronium ion ( NO2). The concentra￾tion of nitronium ion in nitric acid alone is too low to nitrate benzene at a convenient rate, but can be increased by adding sulfuric acid. HO N O O  Nitric acid 2HOSO2OH Sulfuric acid O N O Nitronium ion H3O Hydronium ion 2HOSO2O Hydrogen sulfate ion H Benzene HONO2 Nitric acid NO2 Nitrobenzene (95%) H2O Water H2SO4 30–40°C 12.3 Nitration of Benzene 447 H H Benzene and nitronium ion slow O Step 1: Attack of nitronium cation on the π system of the aromatic ring Step 2: Loss of a proton from the cyclohexadienyl cation N O Cyclohexadienyl cation intermediate O H Cyclohexadienyl cation intermediate O N O H H O Water fast Nitrobenzene O H H H O Hydronium ion O N N O FIGURE 12.2 The me￾chanism of the nitration of benzene. An electrostatic po￾tential map of nitronium ion can be viewed on Learning By Modeling. The role of nitronium ion in the nitration of benzene was demonstrated by Sir Christo￾pher Ingold–the same person who suggested the SN1 and SN2 mechanisms of nucle￾ophilic substitution and who collaborated with Cahn and Prelog on the R and S nota￾tional system. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website