📚 Electrophilic Substitution on Benzene | 苯的亲电取代反应
Benzene (C₆H₆) is a planar, cyclic molecule with a delocalised π-electron system, conferring exceptional stability known as aromaticity. Rather than undergoing addition reactions that would disrupt this aromatic stabilisation, benzene typically reacts via electrophilic substitution, where an electrophile replaces a hydrogen atom on the ring. This mechanism is a cornerstone of IB Chemistry HL, linking structure, bonding, and reactivity.
苯 (C₆H₆) 是一种平面的环状分子，具有离域的 π 电子体系，因此具有特殊的稳定性，即芳香性。与破坏芳香稳定的加成反应不同，苯通常发生亲电取代反应，即亲电试剂取代环上的一个氢原子。该机理是 IB 化学高水平课程的核心内容，将结构、键合和反应性紧密联系起来。
1. Introduction to Benzene and Aromaticity | 苯和芳香性简介
lar formula C₆H₆ and a hexagonal ring structure with alternating double bonds in the Kekulé representation. However, all carbon–carbon bonds are equivalent, with a bond length of 0.139 nm, intermediate between C– rons are delocalised in a ring above and below the plane of the carbon atoms, satisfying Hückel' bilisation energy is about 150 kJ mol⁻¹, making addition reactions energetically unfavourable.
苯的分子式为 C₆H₆，凯库勒式中呈现交替单双键的六元环结构。但所有碳碳键是等价的，键长为 0.139 nm，介于碳碳单键和双键之间。6 个 π 电子在碳原子平面上下形成离域环，符合休克尔规则 (4n+2，n=1)。芳香稳定化能约为 150 kJ mol⁻¹，使得加成反应在能量上不利。
2. Why Electrophilic Substitution? | 为何发生亲电取代？
ing a carbocation intermediate (the σ-complex or Wheland intermediate), followed by loss of a proton to restore aromaticity. The overall reaction is E⁺ + C₆H₆ → C₆H₅E + H⁺.
在苯的双键上发生加成反应会破坏连续的离域 π 电子环，使体系能量升高。而亲电取代保留了芳香六偶体。典型的机理是亲电试剂 (E⁺) 进攻 π 体系，生成碳正离子中间体（σ 络合物或 Wheland 中间体），随后失去一个质子恢复芳香性。总反应为 E⁺ + C₆H₆ → C₆H₅E + H⁺。
3. General Mechanism of Electrophilic Aromatic Substitution | 亲电芳香取代通用机理
Step 1: Generation of the electrophile. A catalyst often converts a neutral reagent into a stronger electrophile (e.g., NO₂⁺, Br⁺, R⁺). Step 2: The electrophile accepts an electron pair from the π system, forming a σ-complex in which the ring carbon carrying E⁺ becomes sp³-hybridised. This intermediate is stabilised by resonance (three canonical forms) but is not aromatic. Step 3: A base (usually the conjugate base of the catalyst) removes the proton from the sp³ carbon, re-establishing the delocalised π system and releasing the catalyst.
第 1 步：产生亲电试剂。催化剂常将中性试剂转化为更强的亲电试剂（如 NO₂⁺、Br⁺、R⁺）。第 2 步：亲电试剂接受 π 体系的电子对，形成 σ 络合物，其中带有 E⁺ 的环碳原子变为 sp³ 杂化。该中间体通过共振（三种经典结构）稳定，但不具有芳香性。第 3 步：碱（通常是催化剂的共轭碱）从 sp³ 碳上脱去质子，重新建立离域 π 体系并释放催化剂。
C₆H₆ + E⁺ ⇌ [C₆H₆E]⁺ → C₆H₅E + H⁺
4. Nitration of Benzene | 苯的硝化反应
Nitration uses a mixture of concentrated nitric acid and concentrated sulfuric acid. The electrophile is the nitronium ion, NO₂⁺, generated by the following equilibrium: HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + 2HSO₄⁻ + H₃O⁺. Benzene reacts with NO₂⁺ at 50–55 °C to form nitrobenzene (C₆H₅NO₂). The reaction must be carefully temperature-controlled to avoid multiple nitration.
硝化反应使用浓硝酸和浓硫酸的混酸。亲电试剂为硝酰正离子 NO₂⁺，通过下列平衡产生：HNO₃ + 2H₂SO₄ ⇌ NO₂⁺ + 2HSO₄⁻ + H₃O⁺。苯在 50–55 ℃ 下与 NO₂⁺ 反应生成硝基苯 (C₆H₅NO₂)。需小心控制温度以避免多硝基化。
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (浓 H₂SO₄ 催化)
5. Halogenation of Benzene | 苯的卤化反应
trophile attacks the ring, giving bromobenzene and HBr. Chlorination follows a similar pathway with AlCl₃ or FeCl₃.
苯不与溴水反应，但在 FeBr₃ 或 AlBr₃ 等路易斯酸催化剂存在下可发生溴代。催化剂极化 Br₂ 产生亲电性更强的溴正离子 (有效 Br⁺) 和四溴合铁(Ⅲ)酸根离子：Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻。亲电试剂进攻苯环，得到溴苯和 HBr。氯化反应以类似方式进行，使用 AlCl₃ 或 FeCl₃。
C₆H₆ + Br₂ → C₆H₅Br + HBr (FeBr₃ 催化)
6. Friedel-Crafts Alkylation | 傅-克烷基化反应
Alkylation introduces an alkyl group onto the benzene ring using a haloalkane and a Lewis acid catalyst (commonly AlCl₃). The electrophile is
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