CHAPTER TWENTY-THREE Aryl Halides Before leaving this mechanistic discussion, we should mention that the addition- elimination mechanism for nucleophilic aromatic substitution illustrates a principle worth remembering. The words"activating"and"deactivating "as applied to substituent effects in organic chemistry are without meaning when they stand alone. When we say that a group is activating or deactivating, we need to specify the reaction type that is being considered. A nitro group is a strongly deactivating substituent in electrophilic aromatic substitution, where it markedly destabilizes the key cyclohexadienyl cation intermediate: N NO NO E Nitrobenzene and cation Product of electrophile intermediate; nitro group electrophilic is destabilizing A nitro group is a strongly activating substituent in nucleophilic aromatic substitution where it stabilizes the key cyclohexadienyl anion int O-Halonitrobenzene Cyclohexadienyl anion Product of (X= F, Cl. Br, or D) intermediate; nitro group and a nucleophile is stabilizing aromatic substitution A nitro group behaves the same way in both reactions: it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species(electrophilic aromatic substitution). Reaction is facilitated when electrons flow from the attacking pecies to the aromatic ring(nucleophilic aromatic substitution). By being aware of the connection between reactivity and substituent effects, you will sharpen your appreciation of how chemical reactions occur 23.7 RELATED NUCLEOPHILIC AROMATIC SUBSTITUTION REACTIONS The most common types of aryl halides in nucleophilic aromatic substitutions are those that bear o-or p-nitro substituents. Among other classes of reactive aryl halides, a few nerit special consideration. One class includes highly fluorinated aromatic compounds such as hexafluorobenzene, which undergoes substitution of one of its fluorines on reac- tion with nucleophiles such as sodium methoxide F F CH:OH, 65 OCH3 Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteBefore leaving this mechanistic discussion, we should mention that the addition– elimination mechanism for nucleophilic aromatic substitution illustrates a principle worth remembering. The words “activating” and “deactivating” as applied to substituent effects in organic chemistry are without meaning when they stand alone. When we say that a group is activating or deactivating, we need to specify the reaction type that is being considered. A nitro group is a strongly deactivating substituent in electrophilic aromatic substitution, where it markedly destabilizes the key cyclohexadienyl cation intermediate: A nitro group is a strongly activating substituent in nucleophilic aromatic substitution, where it stabilizes the key cyclohexadienyl anion intermediate: A nitro group behaves the same way in both reactions: it attracts electrons. Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution). Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution). By being aware of the connection between reactivity and substituent effects, you will sharpen your appreciation of how chemical reactions occur. 23.7 RELATED NUCLEOPHILIC AROMATIC SUBSTITUTION REACTIONS The most common types of aryl halides in nucleophilic aromatic substitutions are those that bear o- or p-nitro substituents. Among other classes of reactive aryl halides, a few merit special consideration. One class includes highly fluorinated aromatic compounds such as hexafluorobenzene, which undergoes substitution of one of its fluorines on reac￾tion with nucleophiles such as sodium methoxide. NaOCH3 CH3OH, 65°C F F F F F F Hexafluorobenzene F OCH3 F F F F 2,3,4,5,6-Pentafluoroanisole (72%) slow addition fast elimination NO2 X Y o-Halonitrobenzene (X  F, Cl, Br, or I) and a nucleophile NO2 X Y  Cyclohexadienyl anion intermediate; nitro group is stabilizing NO2 Y Product of nucleophilic aromatic substitution X very slow H fast NO2 H E Nitrobenzene and an electrophile NO2 H E Cyclohexadienyl cation intermediate; nitro group is destabilizing NO2 E Product of electrophilic aromatic substitution 926 CHAPTER TWENTY-THREE Aryl Halides Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website