AS Chemistry Unit 2 Insert Jan19: Reaction Mechanisms | AS化学单元2 插入页(2019年1月)：反应机理

📚 AS Chemistry Unit 2 Insert Jan19: Reaction Mechanisms | AS化学单元2 插入页(2019年1月)：反应机理

The January 2019 insert for AS Chemistry Unit 2 provides a concise summary of the key organic reaction mechanisms that every candidate must master. This article breaks down those mechanisms step by step, using curly arrow notation and clear explanations to ensure you can confidently apply them to any exam question. We will cover free radical substitution, electrophilic addition, and nucleophilic substitution, highlighting the essential reagents, conditions, and electron movements that define each pathway.

2019年1月AS化学单元2的插入页精要地总结了每位考生必须掌握的有机反应机理。本文逐步拆解这些机理，使用弯箭头符号和清晰的解释，确保你能自信地将它们应用于任何考题。我们将涵盖自由基取代、亲电加成和亲核取代，重点突出定义每条反应途径的关键试剂、条件以及电子移动。

1. Understanding Reaction Mechanisms | 理解反应机理

A reaction mechanism describes the step-by-step sequence of bond breaking and bond making that converts reactants into products. In AS Chemistry, we use curly arrows to show the movement of electron pairs. A full curly arrow represents the movement of two electrons, while a half-headed arrow (often depicted as a fish-hook) shows the movement of a single electron in radical processes.

反应机理描述了将反应物转化为产物的键断裂和键形成的逐步过程。在AS化学中，我们用弯箭头表示电子对的移动。一个完整的弯箭头代表两个电子的移动，而半箭头（通常画作鱼钩状）则表示自由基过程中单个电子的移动。

Homolytic fission breaks a covalent bond so that each atom retains one electron, forming two radicals. Heterolytic fission breaks a bond so that both electrons go to one atom, generating a cation and an anion. Identifying which type of fission occurs is the first step in classifying a mechanism.

均裂使共价键断裂，每个原子各保留一个电子，形成两个自由基。异裂则使两个电子都归到其中一个原子上，产生一个阳离子和一个阴离子。识别发生的裂解类型是划分机理类别的第一步。

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

Curly arrows must start at a source of electrons, such as a lone pair, a bond pair, or a negative charge, and end at an electron-deficient centre, such as a carbocation or a partially positive atom. Never draw a curly arrow starting from a positive charge. The arrow head indicates where the electrons end up, forming a new bond or becoming a lone pair.

弯箭头必须始于电子源，例如孤对电子、键对电子或负电荷，止于缺电子中心，比如碳正离子或带部分正电荷的原子。千万不要从正电荷开始画弯箭头。箭头指示电子最终到达的位置，形成新键或变为孤对电子。

In electrophilic addition, the first arrow starts from the π bond of the alkene towards the electrophile. In nucleophilic substitution, the arrow starts from the nucleophile’s lone pair and attacks the partially positive carbon attached to the halogen. Practising accurate curly arrow placement is essential for full marks in mechanism questions.

在亲电加成中，第一个箭头从烯烃的π键指向亲电试剂。在亲核取代中，箭头从亲核试剂的孤对电子出发，进攻连接卤素的部分正电荷碳。准确放置弯箭头的练习对于在机理题中取得满分至关重要。

3. Free Radical Substitution of Alkanes | 烷烃的自由基取代

The free radical substitution mechanism is how alkanes react with halogens in the presence of ultraviolet (UV) light. A typical example is the chlorination of methane: CH₄ + Cl₂ → CH₃Cl + HCl. This reaction proceeds through three stages: initiation, propagation, and termination.

自由基取代机理是烷烃在紫外光存在下与卤素反应的途径。典型例子是甲烷的氯化：CH₄ + Cl₂ → CH₃Cl + HCl。该反应经历三个阶段：引发、增长和终止。

This mechanism is unique because it involves homolytic bond fission and neutral radical intermediates. The overall equation is a substitution reaction, but the detailed steps show how chlorine atoms and methyl radicals are generated and consumed in a chain reaction.

此机理独特之处在于其涉及均裂和中性自由基中间体。总方程式是一个取代反应，但详细的步骤展示了氯原子和甲基自由基如何在链式反应中产生和被消耗。

4. Mechanism Steps: Initiation, Propagation, Termination | 机理步骤：引发、增长、终止

Initiation: The Cl–Cl bond undergoes homolytic fission under UV light to form two highly reactive chlorine radicals. Cl₂ → 2 Cl•. No free radicals are present before this step.

引发：在紫外光照射下，Cl–Cl键发生均裂，生成两个高活性的氯自由基。Cl₂ → 2 Cl•。在此步骤之前没有自由基存在。

Propagation (step 1): A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical. Cl• + CH₄ → HCl + •CH₃.

增长（第1步）：一个氯自由基从甲烷中夺取一个氢原子，生成HCl和一个甲基自由基。Cl• + CH₄ → HCl + •CH₃。

Propagation (step 2): The methyl radical reacts with a chlorine molecule, producing chloromethane and regenerating a chlorine radical. •CH₃ + Cl₂ → CH₃Cl + Cl•. This chlorine radical can then continue the chain reaction by repeating step 1.

增长（第2步）：甲基自由基与氯分子反应，生成氯甲烷并再生一个氯自由基。•CH₃ + Cl₂ → CH₃Cl + Cl•。这个氯自由基随后可以重复第1步，使链式反应继续。

Termination: Any two radicals combine to form a stable molecule, stopping the chain. Possible termination steps include Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, and •CH₃ + •CH₃ → C₂H₆.

终止：任意两个自由基结合形成稳定分子，使链终止。可能的终止步骤包括 Cl• + Cl• → Cl₂，•CH₃ + Cl• → CH₃Cl，以及 •CH₃ + •CH₃ → C₂H₆。

5. Electrophilic Addition of Alkenes | 烯烃的亲电加成

Alkenes contain a carbon–carbon double bond (C=C) that is electron-rich due to the π bond. Electrophiles, which are species that accept an electron pair, are attracted to this region of high electron density. The electrophilic addition mechanism turns the alkene into a saturated compound by adding two atoms or groups across the double bond.

烯烃含有碳碳双键（C=C），由于π键的存在使其电子密度较高。亲电试剂是接受电子对的物种，它们被这个高电子密度区域所吸引。亲电加成机理通过向双键两端加上两个原子或基团，将烯烃转化为饱和化合物。

The general mechanism involves two steps: attack by the electrophile to form a carbocation intermediate, followed by rapid combination with a nucleophile. Understanding the stability of carbocations (tertiary > secondary > primary) helps predict major products when unsymmetrical alkenes are used.

该通用机理包含两步：亲电试剂的进攻形成碳正离子中间体，随后快速与亲核试剂结合。理解碳正离子的稳定性（三级 > 二级 > 一级）有助于在不对称烯烃反应中预测主要产物。

6. Addition of Bromine (Br₂) | 溴的加成反应

When ethene reacts with bromine (dissolved in an inert organic solvent like tetrachloromethane), the Br–Br molecule acts as the electrophile. As the bromine molecule approaches the double bond, the π electrons induce a dipole in Br₂, making one bromine atom partially positive and the other partially negative.

当乙烯与溴（溶于惰性有机溶剂如四氯化碳中）反应时，Br–Br分子充当亲电试剂。当溴分子接近双键时，π电子在Br₂中诱导出偶极，使一个溴原子显部分正电荷，另一个溴原子显部分负电荷。

The electron-rich double bond attacks the partially positive bromine, breaking the Br–Br bond heterolytically and forming a cyclic bromonium ion (or a carbocation if not cyclic) and a bromide ion. In the second step, the bromide ion attacks the bromonium ion from the opposite side, giving overall anti addition and forming 1,2-dibromoethane.

富电子的双键进攻部分正电荷的溴，使Br–Br键发生异裂，生成一个环状溴鎓离子（若非环状则是碳正离子）和一个溴离子。第二步，溴离子从反面进攻溴鎓离子，整体为反式加成，生成1,2-二溴乙烷。

Overall: C₂H₄ + Br₂ → C₂H₄Br₂

总反应：C₂H₄ + Br₂ → C₂H₄Br₂

7. Addition of Hydrogen Halides (HX) | 卤化氢的加成

Hydrogen halides, such as HBr, add to alkenes to form halogenoalkanes. The H–X bond is already polarised, with the hydrogen carrying a partial positive charge. The π electrons of the double bond form a bond to H⁺, releasing a halide ion and creating a carbocation. The halide ion then quickly combines with the carbocation.

卤化氢（例如HBr）与烯烃加成生成卤代烷。H–X键本身已经极化，氢原子带部分正电荷。双键的π电子与H⁺形成键，释放出卤离子并产生碳正离子。卤离子随后迅速与碳正离子结合。

For unsymmetrical alkenes like propene, the major product follows Markovnikov’s rule: the hydrogen attaches to the carbon with the greater number of hydrogen atoms already attached, because this pathway forms the more stable carbocation intermediate. You must be able to draw the mechanism for both the major and minor products.

对于如丙烯这样的不对称烯烃，主要产物遵循马尔科夫尼科夫规则：氢加在已经连有较多氢原子的碳上，因为这一路径生成更稳定的碳正离子中间体。你必须能够同时画出主要产物和次要产物的机理。

8. Nucleophilic Substitution of Haloalkanes | 卤代烷的亲核取代

Haloalkanes contain a polar carbon–halogen bond, making the carbon atom electron-deficient and susceptible to attack by nucleophiles. A nucleophile is a species that donates an electron pair to form a new covalent bond. Common nucleophiles in AS Chemistry include hydroxide ions (OH⁻), cyanide ions (CN⁻), and ammonia (NH₃).

卤代烷含有极性的碳-卤键，使得碳原子缺电子，易受亲核试剂进攻。亲核试剂是提供电子对以形成新共价键的物种。AS化学中常见的亲核试剂包括氢氧根离子(OH⁻)、氰根离子(CN⁻)和氨(NH₃)。

The reaction involves the nucleophile attacking the partially positive carbon from the opposite side of the halogen, as the carbon–halogen bond breaks heterolytically. The mechanism is called bimolecular nucleophilic substitution (S_N2) when a primary or secondary haloalkane is used, and it proceeds in a single concerted step with no intermediate.

该反应涉及亲核试剂从卤素原子的反方向进攻部分正电荷碳，同时碳-卤键异裂。当使用一级或二级卤代烷时，该机理称为双分子亲核取代（S_N2），它在一个协同步骤中完成，没有中间体。

9. Hydrolysis with Hydroxide Ions | 与氢氧根离子的水解

Haloalkanes can be converted into alcohols by heating with aqueous sodium hydroxide or potassium hydroxide under reflux. The hydroxide ion acts as the nucleophile. For example, the reaction of bromoethane with NaOH(aq):

卤代烷可以与氢氧化钠或氢氧化钾水溶液在回流加热下反应转化为醇。氢氧根离子充当亲核试剂。例如，溴乙烷与NaOH水溶液的反应：

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

The lone pair on the OH⁻ ion attacks the δ+ carbon, pushing the C–Br bond electrons onto the bromine atom. The transition state involves a partially bonded carbon to both oxygen and bromine, and finally ethanol is produced and bromide ions are released. This reaction is slower with chloroalkanes due to the stronger C–Cl bond.

OH⁻离子上的孤对电子进攻δ+碳，将C–Br键的电子推向溴原子。过渡态中碳同时与氧和溴部分成键，最终生成乙醇并释放出溴离子。由于C–Cl键更强，氯代烷发生此反应的速度更慢。

10. Reaction with Ammonia | 与氨的反应

When a haloalkane is heated with excess ethanolic ammonia in a sealed tube, an amine is formed. Ammonia acts as a nucleophile via its lone pair. The initial step produces a primary amine (RNH₂). However, because the amine produced is itself nucleophilic, further substitution can occur to give secondary and tertiary amines unless ammonia is used in large excess.

当卤代烷与过量乙醇氨在密封管中加热时，会生成胺。氨通过其孤对电子充当亲核试剂。起始步骤生成一级胺(RNH₂)。但是，由于生成的胺本身也具有亲核性，除非氨大量过量，否则会发生进一步的取代反应，生成二级和三级胺。

Mechanism for the first substitution: The lone pair on NH₃ attacks the partially positive carbon of the haloalkane, displacing the halide ion and forming an alkylammonium ion. A second ammonia molecule then removes a proton to yield the neutral primary amine. In the exam, you are typically required to show the formation of the primary amine only.

第一次取代的机理：NH₃的孤对电子进攻卤代烷中部分正电荷碳，取代卤离子，形成烷基铵离子。随后另一个氨分子脱去一个质子，生成中性一级胺。在考试中，通常只需画出生成一级胺的过程。

11. Summary of Conditions and Reagents | 条件与试剂总结

Success in mechanism questions often depends on recalling the correct reagents and conditions:

在机理题中取得成功常常取决于能否准确回忆试剂和条件：

Free radical substitution: Alkane + halogen, UV light, room temperature. No ionic intermediates.
自由基取代：烷烃 + 卤素，紫外光，室温。没有离子中间体。
Electrophilic addition with Br₂: Alkene + bromine dissolved in an inert organic solvent, room temperature, anti addition.
溴的亲电加成：烯烃 + 溶于惰性有机溶剂的溴，室温，反式加成。
Electrophilic addition with HX: Alkene + hydrogen halide, room temperature, follows Markovnikov’s rule for unsymmetrical alkenes.
HX的亲电加成：烯烃 + 卤化氢，室温，不对称烯烃遵循马尔科夫尼科夫规则。
Nucleophilic substitution with OH⁻: Haloalkane + aqueous NaOH or KOH, heat under reflux. Produces alcohol.
与OH⁻的亲核取代：卤代烷 + NaOH或KOH水溶液，回流加热。生成醇。
Nucleophilic substitution with NH₃: Haloalkane + excess ethanolic ammonia, heat in sealed tube. Produces primary amine.
与NH₃的亲核取代：卤代烷 + 过量乙醇氨，密封管中加热。生成一级胺。

12. Practice Pitfalls and Exam Tips | 常见错误与考试技巧

Drawing a curly arrow starting from a positive charge is a common mistake — always begin from a lone pair, a bond pair, or a negative charge. Also, ensure the arrow head points exactly to the atom that will accept the electrons. Avoid drawing extra bonds or forgetting to show charges on intermediates.

从正电荷开始画弯箭头是一个常见错误——务必始于孤对电子、键对电子或负电荷。此外，确保箭头准确指向将要接受电子的原子。避免画出额外的键或忘记标出中间体上的电荷。

When illustrating the free radical substitution propagation steps, always show the regeneration of the halogen radical. For electrophilic addition, show the formation of the carbocation and the attack by the nucleophile clearly. For unsymmetrical alkenes with HBr, the major product requires justification based on carbocation stability.

在描绘自由基取代的增长步骤时，务必展示卤素自由基的再生。对于亲电加成，清晰显示碳正离子的形成和亲核试剂的进攻。对于与HBr反应的不对称烯烃，主要产物需基于碳正离子稳定性进行论证。

Finally, check your curly arrows carefully. Each arrow represents the donation of two electrons (or one for a half-arrow). If the number of electrons around an atom exceeds the octet rule during the movement, your mechanism is incorrect. Practise past paper insert examples repeatedly until the electron flow becomes second nature.

最后，仔细检查你的弯箭头。每个箭头代表两个电子的给予（或半个箭头代表一个电子）。如果在移动过程中某原子周围的电子数超出八隅体规则，你的机理就是错误的。反复练习历年真题中的插入页示例，直到电子流动变得像呼吸一样自然。

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AS Chemistry Unit 2 Insert Jan19: Reaction Mechanisms | AS化学 单元2 插入页(2019年1月)：反应机理