OxfordAQA CH04 Reaction Mechanisms | 牛津AQA CH04 反应机理

📚 OxfordAQA CH04 Reaction Mechanisms | 牛津AQA CH04 反应机理

In organic chemistry, a reaction mechanism describes the step-by-step sequence of bond-breaking and bond-making events that transform reactants into products. Understanding these pathways allows chemists to predict product outcomes, control reaction conditions, and explain why certain products form preferentially. For OxfordAQA CH04, you need to be able to draw mechanisms using curly arrows for heterolytic and homolytic processes, and recognise the roles of electrophiles, nucleophiles, and free radicals.

在有机化学中，反应机理描述了将反应物转化为产物的旧键断裂和新键形成的逐步过程。理解这些路径能让化学家预测产物结果、控制反应条件，并解释为何某种产物会优先生成。对于牛津AQA CH04单元，你需要能够使用弯曲箭头画出异裂和均裂过程的机理，并识别亲电试剂、亲核试剂和自由基的作用。

1. Curly Arrows and Electron Movement | 弯曲箭头与电子移动

A curly arrow is a universal tool for showing electron pair movement in mechanisms. A full-headed curly arrow starts from a lone pair or a bonding pair of electrons and points towards an electron-deficient atom or region. Half-headed (fishhook) arrows are used for the movement of a single electron in radical processes. In heterolytic bond breaking, a curly arrow starts from the bond and goes to the more electronegative atom, producing ions. In heterolytic bond making, the arrow starts from a nucleophile’s lone pair and goes to the electrophilic centre. Common mistakes include drawing arrows from a positive charge instead of a bond, or forgetting to show the formation of all products.

弯曲箭头是表示机理中电子对移动的通用工具。全箭头弯曲箭头从孤对电子或成键电子对出发，指向缺电子原子或区域。半箭头(鱼钩箭头)用于表示自由基过程中单个电子的移动。在异裂键断裂中，弯曲箭头从化学键出发，移向电负性更强的原子，产生离子。在异裂键形成中，箭头从亲核试剂的孤对电子指向亲电中心。常见错误包括从正电荷处画箭头而不是从化学键，或忘记显示所有产物的生成。

HO⁻ + CH₃Br → CH₃OH + Br⁻

The curly arrow originates from the lone pair on the oxygen of HO⁻ and points to the carbon of CH₃Br; simultaneously, a second arrow shows the electrons of the C–Br bond moving onto the bromine as it leaves.

弯曲箭头从HO⁻中氧的孤对电子出发，指向CH₃Br的碳；同时，第二个箭头表示C–Br键的电子移到离去的溴上。

2. Electrophilic Addition | 亲电加成反应

Alkenes react with electrophiles such as HBr, Br₂, or H₂SO₄ via electrophilic addition. The π bond is an electron-rich region that attacks the electrophile. Using the addition of HBr to ethene as an example: the π electrons attack the partially positive hydrogen (H with δ⁺) in HBr, forming a carbocation intermediate (CH₃CH₂⁺) and a bromide ion. The bromide ion then acts as a nucleophile, attacking the carbocation to give bromoethane. If the alkene is unsymmetrical, Markovnikov’s rule applies: the hydrogen attaches to the carbon with more hydrogen atoms already present, leading to the more stable carbocation.

烯烃与亲电试剂(如 HBr、Br₂ 或 H₂SO₄)发生亲电加成反应。π 键是富电子区域，会进攻亲电试剂。以 HBr 与乙烯的加成为例：π 电子进攻 HBr 中带部分正电荷的氢(d⁺)，形成碳正离子中间体(CH₃CH₂⁺)和溴离子。然后溴离子作为亲核试剂进攻碳正离子，生成溴乙烷。如果烯烃不对称，适用马氏规则：氢原子加到原先氢更多的碳上，从而生成更稳定的碳正离子。

CH₂=CH₂ + HBr → CH₃CH₂Br

In the addition of Br₂, a cyclic bromonium ion intermediate is formed, with the second bromide attacking from the opposite face, resulting in anti addition. This stereochemical outcome is often tested.

在 Br₂ 的加成中，生成环状溴鎓离子中间体，第二个溴离子从相反面进攻，得到反式加成产物。这个立体化学结果常常被考查。

3. Electrophilic Substitution in Benzene | 苯的亲电取代反应

Benzene undergoes electrophilic substitution rather than addition because the delocalised π system is stabilised by aromaticity. A strong electrophile (E⁺) must be generated in situ, such as NO₂⁺ for nitration or Br⁺ (carried by FeBr₃) for bromination. The mechanism proceeds via a Wheland intermediate (arenium ion), where the electrophile attaches to a ring carbon, breaking the aromaticity temporarily. Finally, loss of a proton restores the aromatic system. Drawing the mechanism requires showing the curly arrow from the benzene ring to the electrophile, the delocalised carbocation intermediate, and the removal of H⁺ by a base.

苯发生亲电取代而非加成反应，因为离域 π 体系因芳香性而稳定。必须在反应体系中现场生成强亲电试剂(E⁺)，例如用于硝化的 NO₂⁺，或用于溴化的 Br⁺(由 FeBr₃ 传递)。机理通过 Wheland 中间体(芳基正离子)进行，其中亲电试剂与环上的一个碳连接，暂时破坏芳香性。最后，失去一个质子恢复芳香体系。画出机理需要显示从苯环指向亲电试剂的弯曲箭头、离域碳正离子中间体，以及碱夺去 H⁺ 的过程。

C₆H₆ + NO₂⁺ → C₆H₅NO₂ + H⁺

Common electrophilic substitutions include halogenation (Cl₂/FeCl₃), nitration (HNO₃/H₂SO₄), and Friedel–Crafts alkylation/acylation. The same general pattern applies: generation of electrophile, attack on the ring, and loss of proton.

常见的亲电取代反应包括卤代(Cl₂/FeCl₃)、硝化(HNO₃/H₂SO₄)和傅列德尔-克拉夫茨烷基化/酰基化。遵循相同的模式：亲电试剂的生成、进攻芳环、脱去质子。

4. Nucleophilic Substitution: SN1 and SN2 | 亲核取代：SN1 与 SN2

Halogenoalkanes undergo nucleophilic substitution with reagents such as OH⁻, CN⁻, and NH₃. There are two limiting mechanisms: SN1 and SN2. In SN2, the nucleophile attacks the carbon bearing the halogen from the opposite side to the leaving group, leading to a single transition state with five groups around carbon. The reaction is concerted, with bond making and breaking happening simultaneously; the rate depends on both haloalkane and nucleophile concentrations (rate = k[RX][Nu⁻]). Inversion of configuration occurs if the carbon is chiral. In SN1, the leaving group departs first, forming a planar carbocation intermediate, which is then attacked by the nucleophile from either side. The rate depends only on the haloalkane concentration (rate = k[RX]), and racemisation is typical. Tertiary haloalkanes favour SN1 due to carbocation stability, while primary ones favour SN2.

卤代烷与 OH⁻、CN⁻、NH₃ 等试剂发生亲核取代反应。有两种极限机理：SN1 和 SN2。在 SN2 中，亲核试剂从离去基团的对侧进攻与卤素相连的碳，形成单一过渡态，碳周围有五个基团。反应是协同的，键的断裂和生成同时发生；速率取决于卤代烷和亲核试剂的浓度(速率 = k[RX][Nu⁻])。若

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