CHAPTER TWENTY-THREE Aryl Halides Overall reaction NaOCH3 p-Fluoronitrobenzene Sodium methoxide Sodium fluoride Step 1: Addition stage. The nucleophile, in this case methoxide ion, adds to the carbon atom that bears the leaving group to give a cyclohexadienyl anion intermediate OCH3 H O2 O, p-Fluoronitrobenzene Methoxide ion Cyclohexadienyl Step 2: Elimination stage. Loss of halide from the cyclohexadienyl intermediate restores the aromaticity of the ring and gives the product of nucleophilic aromatic substitution OCI H H O N p-Nitroanisole FIGURE 23. 3 The addition-elimination mechanism of nucleophilic aromatic substitution FIGURE 23. 4 Stru ture of the rate-dete ntermediate in the of 1-fluoro-4-nitrobenzene Back Forward Main MenuToc Study Guide ToC Student o MHHE Website924 CHAPTER TWENTY-THREE Aryl Halides FIGURE 23.4 Struc￾ture of the rate-determining intermediate in the reaction of 1-fluoro-4-nitrobenzene with methoxide ion. Overall reaction: Step 1: Addition stage. The nucleophile, in this case methoxide ion, adds to the carbon atom that bears the leaving group to give a cyclohexadienyl anion intermediate. NO2 NO2 NO2 NO2 F p-Fluoronitrobenzene NaOCH3 Sodium methoxide OCH3  OCH3 OCH3 OCH3 OCH3 p-Nitroanisole NaF Sodium fluoride H H H H F F p-Fluoronitrobenzene Methoxide ion slow H H H H  Step 2: Elimination stage. Loss of halide from the cyclohexadienyl intermediate restores the aromaticity of the ring and gives the product of nucleophilic aromatic substitution. fast H H H H NO2 F  H H H H NO2 p-Nitroanisole F  Fluoride ion Cyclohexadienyl anion intermediate Cyclohexadienyl anion intermediate FIGURE 23.3 The addition–elimination mechanism of nucleophilic aromatic substitution. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website