Electron Pushing for Nucleophilic Aromatic Substitution

The kinetics of nucleophilic aromatic substitution is almost always second order—first order in nucleophile and first-order in the aromatic electrophile. The intermediate structure is called the Meisenheimer complex (or Jackson-Meisenheimer complex). The Meisenheimer complex can sometimes be directly observed at low temperatures. In aprotic sol- 

On rare occasions, the reaction is first order in the aromatic structure and zero order in the nucleophile. This is reminiscent of an SnI reaction. This mechanism occurs with diazo-nium salts, where the leaving group is so good (N2) that it can depart without assistance, leaving behind an aryl cation that is trapped by a nucleophile (Eq. 10.115). The nucleophile can be water to make a phenol, or CuX salts that place the X group on the ring (the Sand-meyer reaction). The Sandmeyer reaction actually involves electron transfer, as we describe in a Connections highlight in Section 12.2.3. 

Another substitution reaction that occurs on aromatic rings in the presence of nucleophiles involves the intermediacy of benzyne. Benzyne is benzene minus two adjacent hydrogens, producing a formal triple bond (C6H4). The structure of benzyne has been examined both experimentally and theoretically, and the alkyne representation is most widely accepted, although the cummulene and biradical structures are significant resonance contributors. 

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