Reaction Mechanisms: CH05 Unit 5 | 反应机理：CH05 第五单元

📚 Reaction Mechanisms: CH05 Unit 5 | 反应机理：CH05 第五单元

Reaction mechanisms lie at the heart of A-Level organic chemistry, and Unit 5 of the International Chemistry specification (CH05) demands deep understanding of how electrons flow during chemical transformations. Mastering mechanisms is not about memorising every step, but about recognising patterns and applying logical curly-arrow conventions. This article unpacks the key mechanisms tested in the CH05 question paper, from electrophilic addition to free-radical substitution, and shows you how to write flawless mechanisms that gain full marks.

反应机理是 A-Level 有机化学的核心，国际化学大纲第五单元（CH05）要求深入理解化学反应中电子的流动方式。掌握机理并非死记硬背每一步，而是要识别规律并运用弯箭头作图逻辑。本文详细解读 CH05 试卷中常考的反应机理类型——从亲电加成到自由基取代——并教你如何写出完美无缺的机理步骤，获取满分。

1. What Are Reaction Mechanisms? | 什么是反应机理？

A reaction mechanism breaks down an overall chemical reaction into a series of elementary steps, each showing electron movement with curly arrows. Every curly arrow starts from a source of electrons (a lone pair or a bond) and points to an electron-deficient site. In CH05, you must be able to draw mechanisms for addition, substitution, and elimination reactions, correctly identifying nucleophiles, electrophiles, and free radicals.

反应机理将总化学反应拆解成一系列基元步骤，每一步都用弯箭头标示电子移动。每一个弯箭头都从电子源（孤对电子或化学键）出发，指向缺电子位点。在 CH05 考试中，你必须能够绘制加成、取代和消除反应的机理，并准确识别亲核试剂、亲电试剂和自由基。

2. Electrophilic Addition: Alkenes | 亲电加成：烯烃

Alkenes undergo electrophilic addition because the π‑bond is an electron‑rich region that attracts electrophiles. The general two‑step mechanism for addition of HX (e.g. HBr) begins with the π‑electrons attacking the partially positive hydrogen, forming a carbocation intermediate. In the second step, the bromide ion attacks the carbocation to give the saturated product. Remember that unsymmetrical alkenes form the more stable carbocation, following Markovnikov’s rule.

烯烃发生亲电加成是因为 π 键是富电子区域，吸引亲电试剂。加成 HX（如 HBr）的一般两步机理以 π 电子进攻带部分正电的氢开始，形成碳正离子中间体。第二步溴离子进攻碳正离子，得到饱和产物。记住，不对称烯烃会生成更稳定的碳正离子，遵循马氏规则。

CH₃–CH=CH₂ + H–Br → CH₃CHBrCH₃ (major) via CH₃–C⁺H–CH₃

3. Electrophilic Addition: Curly Arrows and Carbocation Stability | 亲电加成：弯箭头与碳正离子稳定性

The first curly arrow always starts from the middle of the double bond and goes to the electrophilic atom, while the second arrow shows the H–X bond breaking heterolytically, with the electron pair moving to X. When drawing the second step, the curly arrow originates from the lone pair on the bromide ion and ends on the positive carbon. The stability of carbocations follows the order tertiary > secondary > primary > methyl, due to inductive and hyperconjugation effects.

第一个弯箭头总是从双键中央出发，指向亲电原子；第二个箭头表示 H–X 键异裂，电子对移向 X。绘制第二步时，弯箭头从溴离子的孤对电子出发，指向带正电的碳。碳正离子的稳定性顺序为叔碳 > 仲碳 > 伯碳 > 甲基，这是诱导效应和超共轭效应的结果。

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

Nucleophilic substitution is a cornerstone of Unit 5. SN2 is a concerted, one‑step mechanism where the nucleophile attacks the carbon from the opposite side of the leaving group, inverting the stereochemistry. The rate depends on both substrate and nucleophile concentrations. SN1, on the other hand, proceeds via a planar carbocation intermediate, leading to racemisation. The rate‑determining step is the departure of the leaving group, so the rate depends only on the substrate.

亲核取代是第五单元的基石。SN2 是协同一步机理，亲核试剂从离去基团背面进攻碳原子，导致立体化学反转。速率取决于底物和亲核试剂的浓度。而 SN1 则经由平面碳正离子中间体进行，引起外消旋化。决速步骤是离去基团离去，因而速率只取决于底物浓度。

5. Factors Influencing SN1 and SN2 Pathways | 影响 SN1 与 SN2 途径的因素

Substrate structure is the dominant factor: tertiary haloalkanes favour SN1 because the bulky alkyl groups hinder backside attack, while primary substrates favour SN2. The nature of the nucleophile matters more for SN2 – strong, small nucleophiles like OH⁻ and CN⁻ accelerate SN2. Solvent polarity also plays a role; polar protic solvents stabilise the carbocation and leaving group in SN1, whereas polar aprotic solvents enhance SN2 by leaving the nucleophile unsolvated.

底物结构是主导因素：叔卤代烷倾向于 SN1，因为庞大的烷基阻碍背面进攻，而伯卤代烷倾向于 SN2。亲核试剂的性质对 SN2 影响更大——如 OH⁻ 和 CN⁻ 等强亲核、体积小的试剂加速 SN2。溶剂极性也起作用：极性质子溶剂稳定 SN1 的碳正离子和离去基团，而极性非质子溶剂使亲核试剂不被溶剂化，从而促进 SN2。

6. Free Radical Substitution: Alkanes | 自由基取代：烷烃

Alkanes react with halogens under UV light via a free radical chain mechanism. The three stages are initiation (homolytic cleavage of Cl₂ to form chlorine radicals), propagation (a chlorine radical abstracts a hydrogen to form HCl and an alkyl radical, which then reacts with a Cl₂ molecule to regenerate a chlorine radical), and termination (two radicals combine). This mechanism explains why a mixture of products is often obtained.

烷烃在紫外光下与卤素通过自由基链式机理反应。三个阶段分别为：引发（Cl₂ 均裂生成氯自由基）、增长（氯自由基夺取一个氢原子，生成 HCl 和一个烷基自由基，后者再与 Cl₂ 分子反应，重新生成氯自由基）、终止（两个自由基结合）。该机理解释了为何常得到混合产物。

Cl–Cl → 2 Cl• (initiation)
CH₄ + Cl• → •CH₃ + HCl
•CH₃ + Cl–Cl → CH₃Cl + Cl•

7. Electrophilic Aromatic Substitution: Benzene | 亲电芳香取代：苯

Benzene resists addition and instead undergoes electrophilic substitution to preserve its aromatic stability. In nitration, the electrophile NO₂⁺ is generated from HNO₃ and H₂SO₄. The π‑electrons of the benzene ring attack the electrophile, forming a positively charged intermediate (Wheland intermediate), which then loses H⁺ to restore aromaticity. This two‑step mechanism is also used for Friedel–Crafts alkylation and acylation.

苯不易发生加成反应，而是发生亲电取代以保持其芳香稳定性。在硝化反应中，亲电试剂 NO₂⁺ 由 HNO₃ 和 H₂SO₄ 产生。苯环的 π 电子进攻亲电试剂，形成带正电荷的中间体（惠兰中间体），随后脱去 H⁺ 恢复芳香性。这一两步机理同样适用于傅‑克烷基化和酰基化反应。

8. Drawing Mechanisms with Curly Arrows: Exam Tips | 用弯箭头绘制机理：应试技巧

Always start the curly arrow at the electron‑rich site – a lone pair, a π‑bond, or a negative charge – and point the arrowhead directly at the electron‑deficient atom. Never draw arrows heading into empty space. Show all relevant charges on intermediates (e.g. carbocations), and clearly indicate bond‑breaking with a second arrow flowing to the leaving group. In CH05 mark schemes, missing a lone pair or an arrow often costs marks.

始终从富电子位置（孤对电子、π 键或负电荷）出发绘制弯箭头，箭头直指缺电子原子。切勿将箭头画向空白处。中间体上要标出所有相关电荷（如碳正离子），并用第二个箭头清晰表示离去基团的离去过程。在 CH05 评分标准中，漏画孤对电子或箭头通常会扣分。

9. Common Pitfalls in Reaction Mechanism Questions | 反应机理题中的常见错误

Students frequently mistakenly use a nucleophile as an electrophile or vice versa. Another error is drawing the arrow from the electrophile to the nucleophile, which shows the wrong direction of electron flow. For SN2, failing to show the inversion of configuration (wedge‑dash notation) can lose marks. In free radical substitution, forgetting to put the half‑arrow (fish‑hook) for single‑electron movements is a classic mistake.

学生常误把亲核试剂当成亲电试剂使用，或反之。另一个错误是箭头画反——从亲电试剂指向亲核试剂，弄错了电子流动方向。对于 SN2，如果未用楔形式表示构型反转，也可能丢分。自由基取代中，忘记用半箭头（鱼钩箭头）表示单电子移动也是一个典型错误。

10. Applying Mechanisms in Unfamiliar Contexts | 在陌生情境中应用机理

CH05 often presents reactions with unseen reagents or complex molecules. The key is to identify the functional group transformation and map it onto a familiar mechanism. Ask yourself: is this an addition, substitution, or elimination? Which site is electron‑rich? Where is the electrophile? Once you recognise the mechanistic pattern, drawing the arrows becomes systematic. Regular practice with past‑paper mechanisms builds this intuition.

CH05 试卷常出现不熟悉的试剂或复杂分子的反应。关键是识别官能团转化，并将其对应到熟悉的机理。问自己：这是加成、取代还是消除？哪个位点富电子？亲电试剂在哪儿？一旦识别出机理模式，绘制箭头就变得系统化。通过历年真题反复训练机理作图，可以培养这种直觉。

Published by TutorHao | Chemistry Revision Series | aleveler.com

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