AS Chemistry: Reaction Mechanisms | AS 化学：反应机理考点精讲

📚 AS Chemistry: Reaction Mechanisms | AS 化学：反应机理考点精讲

Reaction mechanisms are the heart of organic chemistry at AS level. Understanding how bonds break and form, and how electron movement dictates the outcome of a reaction, is essential for predicting products and explaining chemical behaviour.

反应机理是 AS 阶段有机化学的核心。理解化学键的断裂与形成、电子如何移动并决定反应的结果，对于预测产物和解释化学行为至关重要。

1. What Is a Reaction Mechanism? | 什么是反应机理？

A reaction mechanism describes the step-by-step sequence of elementary reactions by which an overall chemical change occurs. Each step involves bond breaking or bond making, often with transient intermediates. At AS level, you will encounter mechanisms involving radicals, carbocations, or nucleophilic attack.

反应机理描述了整个化学变化所经历的一步一步的基元反应序列。每一步都涉及化学键的断裂或形成，并常伴随有不稳定的中间体。在 AS 阶段，你会接触到涉及自由基、碳正离子或亲核进攻的机理。

2. Bond Breaking: Homolytic vs Heterolytic Fission | 共价键断裂：均裂与异裂

When a covalent bond breaks, the two electrons in the bond can be distributed evenly or unevenly. Homolytic fission gives each atom one electron, producing two free radicals. Heterolytic fission gives both electrons to one atom, generating a cation and an anion.

共价键断裂时，键中的两个电子可以平均分配或不平均分配。均裂使每个原子各得到一个电子，产生两个自由基。异裂将两个电子都给了其中一个原子，生成一个阳离子和一个阴离子。

Homolytic fission: A−B → A· + B· (radicals). Typical in free radical substitution.
均裂： A−B → A· + B·（自由基）。常见于自由基取代反应。
Heterolytic fission: A−B → A⁺ + :B⁻ (or A:⁻ + B⁺). Typical in polar reactions like nucleophilic substitution.
异裂： A−B → A⁺ + :B⁻（或 A:⁻ + B⁺）。常见于极性反应如亲核取代。

3. Curly Arrows and Electron Movement | 弯箭头与电子移动

Curly arrows show the movement of an electron pair during bond breaking or bond making. The tail of the arrow starts at the electron source (lone pair or bond) and the head points to the electron-deficient atom. For radical processes, a half‑headed ‘fish‑hook’ arrow (⟿) shows the movement of a single electron.

弯箭头用于表示在化学键断裂或形成过程中电子对的移动。箭头尾端从电子源（孤电子对或化学键）出发，箭头指向缺电子原子。对于自由基过程，则使用半箭头“鱼钩箭头” (⟿) 表示单个电子的移动。

Example: In nucleophilic substitution, OH⁻ attacks the carbon atom. The arrow ⟶ starts at the lone pair on O and points to C, while a second arrow ⟶ shows the C−Br bond breaking and the electron pair moving onto Br.

例如：在亲核取代中，OH⁻ 进攻碳原子。弯箭头 ⟶ 从 O 上的孤对电子出发指向 C，同时另一个弯箭头 ⟶ 表示 C−Br 键断裂，电子对移向 Br。

4. Free Radical Substitution Mechanism | 自由基取代机理

The reaction between methane and chlorine in UV light proceeds via free radical substitution. The mechanism has three stages: initiation, propagation, and termination.

甲烷与氯气在紫外光下的反应通过自由基取代进行。该机理包含三个阶段：引发、增长和终止。

Initiation: Cl₂ ⟶ 2Cl· (homolytic fission by UV light).

引发： Cl₂ ⟶ 2Cl·（紫外光引发均裂）。

Propagation: Cl· + CH₄ → HCl + ·CH₃, then ·CH₃ + Cl₂ → CH₃Cl + Cl·. The Cl· is regenerated, keeping the chain going.

增长： Cl· + CH₄ → HCl + ·CH₃，然后 ·CH₃ + Cl₂ → CH₃Cl + Cl·。Cl· 再生，使链反应持续进行。

Termination: radicals combine: 2Cl· → Cl₂, 2·CH₃ → C₂H₆, or Cl· + ·CH₃ → CH₃Cl.

终止： 自由基结合：2Cl· → Cl₂，2·CH₃ → C₂H₆，或 Cl· + ·CH₃ → CH₃Cl。

Key exam point: The propagation steps must reproduce the radical used in the first step to sustain the chain reaction.

关键考点：增长步骤必须再生第一步消耗的自由基，以维持链反应。

5. Electrophilic Addition Mechanism | 亲电加成机理

Alkenes undergo electrophilic addition because the π‑bond is an electron‑rich region. A typical example is the addition of HBr to ethene. The mechanism involves a carbocation intermediate.

烯烃因 π 键电子云密度较高而发生亲电加成。典型例子是 HBr 与乙烯的加成。该机理涉及碳正离子中间体。

Step 1: The H−Br bond undergoes heterolytic fission. The electrophile H⁺ uses electrons from the π‑bond to form a C−H bond, generating a carbocation on the more substituted carbon and a Br⁻ ion.

第一步：H−Br 键发生异裂。亲电试剂 H⁺ 利用 π 键电子形成 C−H 键，在取代基较多的碳上生成碳正离子和 Br⁻ 离子。

Step 2: The Br⁻ ion acts as a nucleophile and attacks the carbocation, forming a C−Br bond.

第二步：Br⁻ 作为亲核试剂进攻碳正离子，形成 C−Br 键。

Overall: CH₂=CH₂ + HBr → CH₃CH₂Br (if unsymmetrical alkene, see Markovnikov’s rule).

总反应：CH₂=CH₂ + HBr → CH₃CH₂Br（若为不对称烯烃，需考虑马氏规则）。

6. Markovnikov’s Rule and Carbocation Stability | 马氏规则与碳正离子稳定性

When HX adds to an unsymmetrical alkene, the hydrogen attaches to the carbon with the greater number of hydrogen atoms already present, and the halogen attaches to the more substituted carbon. This is because the more substituted carbocation is more stable: tertiary > secondary > primary > methyl.

当 HX 与不对称烯烃加成时，氢原子加在原有氢原子较多的碳上，卤素加在取代基较多的碳上。这是因为取代基越多的碳正离子越稳定：叔碳正离子 > 仲碳正离子 > 伯碳正离子 > 甲基碳正离子。

Stability arises from the inductive effect and hyperconjugation of alkyl groups donating electron density to the positively charged carbon. In exam questions, you must draw the intermediate carbocation and justify the major product using stability arguments.

稳定性源于烷基的诱导效应和超共轭效应，为带正电荷的碳提供电子密度。在考题中，必须画出中间体碳正离子并用稳定性论证主要产物。

7. Nucleophilic Substitution: Basics | 亲核取代反应基础

Nucleophilic substitution involves a nucleophile replacing a leaving group on a saturated carbon. Typical nucleophiles include OH⁻, CN⁻, and NH₃. Common leaving groups are halide ions (e.g., Br⁻, I⁻). The carbon‑halogen bond is polar, making the carbon δ+ and susceptible to attack.

亲核取代是亲核试剂取代饱和碳原子上的离去基团。典型的亲核试剂有 OH⁻、CN⁻ 和 NH₃。常见的离去基团是卤离子（如 Br⁻、I⁻）。碳‑卤键极性使得碳带部分正电荷，易受进攻。

Example: Hydrolysis of bromoethane: CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr. The mechanism is drawn with a curly arrow from the OH⁻ lone pair to the carbon, and a second arrow showing the C−Br bond breaking.

例子：溴乙烷的水解：CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr。画机理时，弯箭头从 OH⁻ 的孤对电子指向碳，另一个箭头表示 C−Br 键断裂。

8. Comparing SN1 and SN2 | SN1 与 SN2 机理对比

At AS level, you may be asked to recognise the difference between SN1 and SN2 pathways depending on the structure of the halogenoalkane.

根据卤代烷结构的不同，AS 阶段可能要求你识别 SN1 和 SN2 路径的区别。

Feature	SN2	SN1
Steps	One (concerted)	Two (carbocation intermediate)
Rate equation	Rate = k[Nu][RX]	Rate = k[RX]
Prefers	Primary RX, strong nucleophile	Tertiary RX, weak nucleophile
Stereochemistry	Inversion	Racemisation

In SN2, the nucleophile attacks from the opposite side of the leaving group, leading to inversion of configuration. In SN1, the planar carbocation can be attacked from either side, so a mixture of retention and inversion products may result.

在 SN2 中，亲核试剂从离去基团背面进攻，导致构型翻转。在 SN1 中，平面型碳正离子可从任一侧被进攻，产物可能是保持和翻转构型的混合物。

9. Stability of Intermediates | 中间体的稳定性

Carbocation stability is key for Markovnikov addition and SN1. The more alkyl groups attached to the positively charged carbon, the more stable it is. Free radicals follow a similar trend: tertiary > secondary > primary > methyl, due to the electron‑donating inductive effect of alkyl groups.

碳正离子稳定性对马氏加成和 SN1 至关重要。连接在带正电荷碳上的烷基越多，越稳定。自由基也遵循类似规律：叔碳自由基 > 仲碳自由基 > 伯碳自由基 > 甲基自由基，因为烷基具有给电子诱导效应。

For example, (CH₃)₃C· is more stable than CH₃·. This stability influences the product distribution in free radical substitution of longer alkanes, where more substituted radicals are favoured.

例如，(CH₃)₃C· 比 CH₃· 更稳定。这种稳定性影响较长链烷烃自由基取代的产物分布，形成的取代基数越多越稳定，产物比例越高。

10. Common Mistakes in Mechanism Drawing | 机理画图常见错误

Many marks are lost in exams due to poor arrow drawing. Always ensure arrows start from a lone pair or a bond, never just from a negative charge symbol. Charges must be placed on the correct atoms at each step. For free radical steps, use fish‑hook arrows (⟿) explicitly.

考试中因箭头绘制不当而失分的情况很多。务必使箭头起始于孤对电子或化学键，绝不能仅从负电荷符号出发。电荷必须标注在每一步的正确原子上。对于自由基步骤，要明确使用鱼钩箭头 (⟿)。

Never draw arrows from a negative charge sign alone – start from an electron pair.
箭头不能从负电荷符号出发 – 必须从电子对出发。
Show all relevant lone pairs and formal charges on intermediates.
画出中间体上的所有相关孤对电子和形式电荷。
For electrophilic addition, ensure the carbocation is on the correct carbon (the one that gives the most stable intermediate).
对于亲电加成，确保碳正离子位于正确的碳上（形成最稳定中间体的那个）。
For SN2, draw the transition state with a dashed line C—Nu and C—X if required, but at AS a single step with arrow pushing is usually sufficient.
对于 SN2，如果需要可画出带虚线 C—Nu 和 C—X 的过渡态，但 AS 通常只需用箭头表示一步过程。

11. Summary Table of AS Mechanisms | AS 机理汇总表

Mechanism	Reactant Type	Key Feature	Intermediate
Free Radical Substitution	Alkane + Halogen (UV)	Chain reaction, radicals	Alkyl radical (R·)
Electrophilic Addition	Alkene + HX / X₂	Carbocation formation, Markovnikov	Carbocation (R⁺)
Nucleophilic Substitution (SN2)	Primary RX + Nu⁻	One step, inversion	Transition state only
Nucleophilic Substitution (SN1)	Tertiary RX + Nu⁻ (or weak Nu)	Two steps, carbocation	Carbocation (R⁺)

Refer to this table when deciding which mechanism to apply in a given question. The nature of the organic substrate, the reagent, and the conditions are your clues.

在决定某个题目用哪种机理时，可参考本表。有机底物的结构、试剂和反应条件就是你的线索。

12. Revision Checklist | 复习清单

Can I define homolytic and heterolytic fission and give examples of each?
我能定义均裂和异裂并举例说明吗？
Do I know how to draw full‑headed and half‑headed curly arrows correctly?
我会正确画出全箭头和半箭头的弯箭头吗？
Can I write the three stages of free radical substitution with chlorine and methane?
我能写出甲烷与氯气自由基取代的三个阶段吗？
Am I able to explain why Markovnikov’s rule works based on carbocation stability?
我能用碳正离子稳定性解释马氏规则的原理吗？
Can I draw the mechanism for electrophilic addition of HBr to propene, showing the intermediate?
我能画出 HBr 与丙烯亲电加成的机理并标出中间体吗？
Do I understand the difference between SN1 and SN2 and which halogenoalkanes favour each?
我理解 SN1 与 SN2 的区别及不同卤代烷倾向于哪种机理吗？
Have I practised drawing mechanisms with correct charges, lone pairs, and arrows?
我是否练习过画机理并正确标注电荷、孤对电子和箭头？

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

AS Chemistry: Reaction Mechanisms | AS 化学：反应机理 考点精讲