定义Definition
亲电芳香取代是亲电试剂进攻芳环 π 电子，取代环上的一个氢原子（并保持芳香性）的反应，包括卤代、硝化、磺化、傅–克烷基化和酰基化等。
Electrophilic aromatic substitution replaces a ring hydrogen of an arene by an electrophile while preserving aromaticity; it includes halogenation, nitration, sulfonation and Friedel–Crafts alkylation and acylation.
机理Mechanism
两步：①亲电试剂与 π 体系形成 σ 络合物（惠兰德中间体，芳香性暂时丧失，决速步）；②碱夺取 sp³ 碳上的质子，恢复芳香性。所以芳环倾向取代而不是像烯烃那样加成。
Two steps: (1) the electrophile forms a σ-complex (Wheland intermediate) with temporary loss of aromaticity — rate-determining; (2) a base removes the proton from the sp³ carbon, restoring aromaticity. Arenes therefore substitute rather than add.
定位效应Directing effects
活化的邻、对位定位基：–NH₂、–OH、–OR、–R、–NHCOR；钝化的邻、对位定位基：卤素（诱导吸电子、共振给电子）；钝化的间位定位基：–NO₂、–CN、–COR、–COOH、–SO₃H、–NR₃⁺。可由 σ 络合物共振式的稳定性解释：给电子基在邻、对位进攻时能直接稳定正电荷。
Activating ortho/para directors: –NH₂, –OH, –OR, alkyl, –NHCOR; deactivating ortho/para: halogens (inductively withdrawing, resonance donating); deactivating meta directors: –NO₂, –CN, –COR, –COOH, –SO₃H, –NR₃⁺. σ-Complex resonance explains them: donors stabilise the cation directly only for ortho/para attack.
定量Quantification
哈米特 σ⁺ 常数、分速率因子（如甲苯硝化 f_o ≈ 40、f_p ≈ 60、f_m ≈ 2.5）描述取代基效应；杂环中吡咯、呋喃比苯活泼，吡啶比苯钝化得多（3 位取代）。
Hammett σ⁺ and partial rate factors (toluene nitration: f_o ≈ 40, f_p ≈ 60, f_m ≈ 2.5) quantify effects; pyrrole and furan are more reactive than benzene, pyridine far less (substituting at C-3).
反应Reactions
需生成强亲电试剂（路易斯酸、混酸等）
A strong electrophile must be generated (Lewis acids, mixed acid, etc.)
生成亲电试剂 E⁺。
Generate E⁺.
芳环进攻 E⁺ 生成 σ 络合物（决速步）。
The ring attacks E⁺ to form the σ-complex (rate-determining).
失去质子恢复芳香性。
Loss of H⁺ restores aromaticity.
甲氧基是邻、对位定位基。
Methoxy is an ortho/para director.
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.
苯环 π 电子进攻硝酰阳离子 NO₂⁺
The ring π electrons attack the nitronium ion NO₂⁺
σ 络合物（芳基正离子）失去质子，恢复芳香性
The σ-complex (arenium ion) loses H⁺ and regains aromaticity
硝基苯
Nitrobenzene
年表Timeline
- 1834
米切利希硝化、磺化苯
Mitscherlich nitrates and sulfonates benzene
- 1877
傅–克反应
Friedel–Crafts reactions
- 1942
惠兰德提出 σ 络合物中间体
Wheland proposes the σ-complex intermediate