双语化学辞典Chem Lexicon
机理 · 有机化学 · 反应机理 Organic chemistry · Reaction mechanisms

SNAr反应Nucleophilic aromatic substitution

拼音Pinyin SNAr fǎn yìng　别名Also 亲核芳香取代； SNAr reaction

定义Definition

SNAr 反应是亲核试剂取代芳环上离去基团（通常为卤素）的反应。典型机理为加成–消除，需要离去基团邻、对位有强吸电子基（–NO₂、–CN 等）活化。

Nucleophilic aromatic substitution replaces a leaving group (usually a halide) on an aromatic ring with a nucleophile. The usual addition–elimination mechanism needs strong electron-withdrawing groups (–NO₂, –CN) ortho or para to the leaving group.

加成–消除机理Addition–elimination

亲核试剂加成到连有离去基团的碳上，生成负电荷离域的迈森海默络合物（决速步）；离去基团离开恢复芳香性。由于决速步是加成，C–F 最快（F 电负性最强使碳最缺电子）：F ≫ Cl ≈ Br > I，与 SN2 相反。

The nucleophile adds to the carbon bearing the leaving group, forming a delocalised Meisenheimer complex (rate-determining), which then expels the leaving group. Because addition limits the rate, fluoride reacts fastest (F ≫ Cl ≈ Br > I) — the opposite of SN2.

苯炔机理Benzyne mechanism

未活化的卤苯与极强碱（NaNH₂/液氨）反应时，经消除–加成生成苯炔中间体，亲核试剂可加到原位或邻位（罗伯茨 1953 年用 ¹⁴C 标记证明）。

Unactivated aryl halides with very strong bases (NaNH₂/NH₃) react by elimination–addition via benzyne, the nucleophile entering at the original or adjacent carbon (Roberts’s ¹⁴C labelling, 1953).

应用Applications

桑格试剂 2,4-二硝基氟苯标记多肽 N 端（桑格 1958 年诺贝尔奖）；工业上合成二苯醚、苯胺衍生物；药物合成中含氮杂芳环（吡啶、嘧啶）的氯常被胺取代。氯苯与 NaOH 高温高压水解（道氏法）曾用于制苯酚。

Sanger’s reagent (2,4-dinitrofluorobenzene) labels peptide N-termini (Sanger, Nobel 1958); industrial diaryl ethers and aniline derivatives; amination of chloropyridines and chloropyrimidines in drug synthesis. The Dow process once hydrolysed chlorobenzene with NaOH at high temperature and pressure to make phenol.

反应Reactions

Ar(EWG)–X + Nu− ⟶ [迈森海默络合物]− ⟶ Ar(EWG)–Nu + X−Ar(EWG)–X + Nu− ⟶ [Meisenheimer complex]− ⟶ Ar(EWG)–Nu + X−

邻/对位吸电子基活化，极性非质子溶剂（DMSO、DMF），加热

ortho/para electron-withdrawing activation, polar aprotic solvents (DMSO, DMF), heating

  1. 亲核加成生成迈森海默络合物。

    Nucleophilic addition gives the Meisenheimer complex.

  2. 负电荷离域到吸电子基。

    Negative charge delocalises onto the acceptor.

  3. 离去基团离去，恢复芳香性。

    The leaving group departs, restoring aromaticity.

1-氯-2,4-二硝基苯 + NaOH ⟶ 2,4-二硝基苯酚1--2,4-Nitrobenzene + NaOH ⟶ 2,4-Nitrobenzene

活化芳环的水解。

Hydrolysis of an activated aryl chloride.

反应机理图Mechanism

红色弯箭头表示一对电子的移动方向：从电子来源（孤对电子或化学键）指向电子去向。Red curly arrows show where an electron pair moves: from its source (lone pair or bond) to its destination.

1

CH₃O⁻ 加到连 F 的碳上，负电荷推到硝基所连的碳

CH₃O⁻ adds to the C–F carbon; the negative charge moves to the carbon bearing NO₂

慢slow
2

迈森海默络合物：负电荷由硝基稳定；F⁻ 离去恢复芳香性

Meisenheimer complex, stabilised by NO₂; loss of F⁻ restores aromaticity

快fast
3

对硝基苯甲醚；F 虽是差的离去基团，却因加成是决速步而最快

4-Nitroanisole; F reacts fastest because addition, not loss of F⁻, is rate-determining

年表Timeline

  • 1902

    迈森海默表征 σ 络合物

    Meisenheimer characterises the σ-complex

  • 1945

    桑格引入 DNFB 标记

    Sanger introduces DNFB labelling

  • 1953

    罗伯茨证明苯炔中间体

    Roberts proves the benzyne intermediate