![]() This is the only orientation supporting the resonance stabilization of the negative charge in the transition state. One requirement for these reactions is the ortho or para position of the electron-withdrawing group to the leaving group. Notice that the nitro group is typically used as an electron-withdrawing group even though other groups such as carbonyls can also activate the ring toward a nucleophilic attack.Īs for the nucleophile, a variety of charged and neutral strong nucleophiles such as –OH, –OR, –SR, NH 3, and other amines can be used. Nucleophilic Aromatic Substitution – The Addition-Elimination Mechanism Let’s put some common examples of these mechanisms and look into the details of the main patterns. The benzyne mechanism works mainly with a very strong base and nucleophile, NH 2 – and does not require an EWG group, although its presence is not a hurdle. There are two main mechanisms: addition-elimination or elimination-addition, also known as the benzyne mechanism which is sometimes considered as a separate reaction. X here is the leaving group and the EWG stands for an electron-withdrawing group (most often nitro) which is there to activate the ring by making it electron-deficient. ![]() However, some aryl halides with a strong electron-withdrawing substituent(s) on the ring can undergo nucleophilic substitution ( S N Ar) instead of electrophilic substitution: We have seen that most reactions of aromatic compounds involve electrophilic substitutions because the π electrons make the aromatic ring electron-rich and therefore, nucleophilic.
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