📚 Reaction Mechanisms in AS Chemistry | AS化学中的反应机理
The January 2023 International AS Chemistry Unit 1 question paper (CH01-QP) challenged students to demonstrate a clear understanding of organic reaction mechanisms. Free-radical substitution in alkanes and electrophilic addition in alkenes were core topics, requiring detailed recall of bond breaking, curly arrow conventions, and intermediate stabilities. This article unpacks the key mechanistic concepts tested, providing a bilingual revision guide suitable for any AS-level Chemistry student aiming to score high marks on mechanism questions.
2023年1月的国际AS化学单元一试卷(CH01-QP)要求学生清晰展示有机反应机理的理解。烷烃的自由基取代和烯烃的亲电加成是核心考点，需要准确回忆化学键断裂、弯箭头表示法和中间体稳定性等细节。本文剖析了试卷中考察的关键机理概念，为中英双语学习者提供一份适合争取高分的AS化学复习指南。
1. What Is a Reaction Mechanism? | 什么是反应机理？
A reaction mechanism is a step-by-step description of how bonds break and form during a chemical reaction. It shows the movement of electron pairs or single electrons using curly arrows, and identifies reactive intermediates such as free radicals or carbocations.
反应机理是对化学反应过程中化学键如何断裂和形成的逐步描述。它使用弯箭头表示电子对或单电子的运动，并识别出活泼中间体，如自由基或碳正离子。
In the AS Unit 1 exam, mechanisms are often worth several marks. Students must draw curly arrows precisely and show all relevant species, including partial charges where required.
在AS单元一考试中，机理题通常占好几分。学生必须准确画出弯箭头，并标出所有相关物种，必要时包括部分电荷。
The CH01 paper specifically required mechanisms for the chlorination of methane and the addition of HBr to propene. Both exemplify fundamental organic processes that must be memorised and understood.
CH01试卷明确要求写出甲烷氯化和丙烯与HBr加成的机理。这两个反应展示了必须熟记并理解的基础有机过程。
2. Bond Breaking: Homolysis and Heterolysis | 化学键断裂：均裂与异裂
Covalent bonds can break in two distinct ways. Homolytic fission occurs when a bond breaks symmetrically, each atom taking one electron from the shared pair, producing two free radicals. This is typical under UV light or high temperature.
共价键有两种不同的断裂方式。均裂发生在化学键对称地断裂时，每个原子从共享电子对中各取走一个电子，生成两个自由基。这通常在紫外线或高温条件下发生。
In contrast, heterolytic fission is unsymmetrical: one atom takes both electrons, forming a negative ion, while the other becomes a positive ion. This occurs in polar bonds and leads to ionic intermediates.
与此相反，异裂是不对称的：一个原子带走两个电子形成负离子，另一个原子成为正离子。这发生在极性键中，并生成离子型中间体。
| Homolytic Fission | Heterolytic Fission |
|---|---|
| Equal electron distribution; forms two radicals. | Unequal electron distribution; forms cation and anion. |
| 均裂：电子平均分配，生成两个自由基。 | 异裂：电子不均分配，生成阳离子和阴离子。 |
| Example: Cl-Cl → 2 Cl• under UV light | Example: HCl → H⁺ + Cl⁻ in polar solvent |
Knowing which fission operates in a given mechanism is the first step to correctly drawing the curly arrows. Free-radical reactions depend on homolysis, while electrophilic addition starts with heterolysis of the electrophile.
弄清楚给定机理中发生的是哪种断裂，是正确画出弯箭头的第一步。自由基反应依赖于均裂，而亲电加成起始于亲电试剂的异裂。
3. Free Radical Substitution: An Overview | 自由基取代反应概述
Free radical substitution is the characteristic reaction of alkanes with halogens under UV light. The overall reaction replaces a hydrogen atom with a halogen atom, producing a haloalkane and a hydrogen halide.
自由基取代是烷烃在紫外光下与卤素发生的特征反应。总反应是用卤素原子取代一个氢原子，生成卤代烷和卤化氢。
The January 2023 paper featured the chlorination of methane. The equation is:
2023年1月的试卷考查了甲烷的氯化反应。反应方程式为：
CH₄ + Cl₂ → CH₃Cl + HCl
This seemingly simple transformation proceeds via a chain reaction involving radical intermediates. Understanding the three distinct stages — initiation, propagation, and termination — is essential for full marks.
这个看似简单的转化其实是通过涉及自由基中间体的链反应进行的。理解三个不同阶段——链引发、链增长和链终止——是拿到满分的关键。
4. Mechanism of Methane Chlorination | 甲烷氯化反应机理
Initiation: The chlorine molecule undergoes homolytic fission under UV light to generate two chlorine free radicals. This step requires energy and is represented with a curly half-arrow (fish-hook arrow) moving from the bond to each chlorine atom.
链引发： 氯分子在紫外光下发生均裂，生成两个氯自由基。这一步需要能量，用半箭头（鱼钩箭头）表示从化学键指向每个氯原子的单电子移动。
Cl₂ + UV light → 2 Cl•
Propagation: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical •CH₃. The methyl radical then reacts with a fresh Cl₂ molecule, producing chloromethane and regenerating a Cl• radical, which continues the chain.
链增长： 一个氯自由基从甲烷中夺取一个氢原子，生成 HCl 和甲基自由基 •CH₃。甲基自由基随后与另一个 Cl₂ 分子反应，生成氯甲烷并再生一个 Cl• 自由基，使链反应继续。
Cl• + CH₄ → HCl + •CH₃
•CH₃ + Cl₂ → CH₃Cl + Cl•
Termination: Two radicals combine to form a stable molecule, removing radicals from the system. Possible termination steps include coupling of two chlorine radicals, two methyl radicals, or a chlorine and a methyl radical.
链终止： 两个自由基结合形成一个稳定分子，从体系中移除自由基。可能的终止步骤包括两个氯自由基结合、两个甲基自由基结合，或一个氯自由基与一个甲基自由基结合。
Cl• + Cl• → Cl₂
•CH₃ + •CH₃ → C₂H₆
Cl• + •CH₃ → CH₃Cl
5. Initiation, Propagation, Termination Steps in Detail | 链引发、链增长、链终止步骤详解
Each stage must be written carefully in an exam. Marks are allocated for correct use of fish-hook arrows (single electron movement) in initiation and propagation, and for showing the correct intermediates. Never use full curly arrows in radical mechanisms unless it is heterolysis in a non-radical context.
每个阶段在考试中都必须详细书写。分数分配给在引发和增长步骤中正确使用鱼钩箭头（单电子移动），以及显示正确的中间体。在自由基机理中切勿使用全箭头，除非是在非自由基背景下异裂。
In the CH01 paper, students were expected to show at least two propagation steps and two plausible termination steps. Misplacing the arrow from the Cl• to the hydrogen atom instead of to the electron being transferred was a common error.
在CH01试卷中，要求学生至少写出两个增长步骤和两个合理的终止步骤。常见的错误是把箭头从Cl•指向氢原子，而不是指向正在转移的那个电子。
A strong exam tip is to always draw the radical with the unpaired electron clearly shown as a dot on the atom, for example, •CH₃, not CH₃• unless the radical is on carbon. The dot should be placed on the carbon atom to indicate the radical site.
一个重要的考试技巧是始终清晰地画出自由基的未成对电子，例如 •CH₃ 而不是 CH₃•，除非自由基在碳原子上。点要画在碳原子上以表明自由基位置。
6. Electrophilic Addition: Mechanism of Bromination of Ethene | 亲电加成：乙烯溴化机理
Alkenes undergo electrophilic addition because the electron-rich π-bond attacks an electron-deficient species (electrophile). With bromine, the non-polar Br-Br bond becomes polarised as it approaches the double bond, leading to heterolytic fission and formation of a cyclic bromonium ion and a bromide ion.
烯烃发生亲电加成是由于其富电子的π键进攻缺电子物种（亲电试剂）。在溴的存在下，非极性的 Br-Br 键在靠近双键时被极化，导致异裂，形成一个环状溴鎓离子和一个溴离子。
The mechanism is drawn using curly arrows: one arrow from the π-bond to one bromine atom, and a second arrow from the Br-Br bond to the other bromine atom to show departure of Br⁻. This concerted step forms the bromonium intermediate.
该机理用弯箭头表示：一个箭头从π键指向一个溴原子，另一个箭头从Br-Br键指向另一个溴原子以表示Br⁻的离去。这个协同步骤形成了溴鎓中间体。
H₂C=CH₂ + Br₂ → [cyclic C₂H₄Br]⁺ + Br⁻
In the second step, the bromide ion attacks the bromonium ring from the opposite side (backside attack), opening the ring and giving the anti-addition product, 1,2-dibromoethane.
在第二步中，溴离子从溴鎓环的反面进攻（背面进攻），打开环并给出反式加成产物 1,2-二溴乙烷。
[cyclic C₂H₄Br]⁺ + Br⁻ → BrCH₂CH₂Br
7. Electrophilic Addition of Hydrogen Halides | 卤化氢的亲电加成
When an alkene reacts with a hydrogen halide like HBr, the electrophile is the hydrogen ion (H⁺) generated by heterolysis of the H-X bond. The mechanism proceeds via a carbocation intermediate, not a halonium ion.
当烯烃与卤化氢（如 HBr）反应时，亲电试剂是由 H-X 键异裂生成的氢离子（H⁺）。该机理通过碳正离子中间体进行，而非卤鎓离子。
In the first step, the π-electrons attack H⁺, forming a carbocation and releasing X⁻. The more stable carbocation is formed according to Markovnikov’s rule, where the hydrogen adds to the carbon with more hydrogens initially.
在第一步中，π电子进攻 H⁺，形成碳正离子并释放出 X⁻。根据马氏规则，氢原子加到初始含氢较多的碳原子上，由此生成更稳定的碳正离子。
For example, propene with HBr yields mainly 2-bromopropane through the more stable secondary carbocation intermediate.
例如，丙烯与 HBr 反应主要生成 2-溴丙烷，因为经过更稳定的二级碳正离子中间体。
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major)
8. Carbocation Stability and Markovnikov’s Rule | 碳正离子稳定性与马氏规则
Carbocation stability follows the order: tertiary (3°) > secondary (2°) > primary (1°) > methyl. Alkyl groups donate electron density through the inductive effect and hyperconjugation, stabilising the positive charge.
碳正离子稳定性顺序为：叔碳正离子 (3°) > 仲碳正离子 (2°) > 伯碳正离子 (1°) > 甲基碳正离子。烷基通过诱导效应和超共轭提供电子密度，稳定正电荷。
Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atom bonds to the carbon that already holds more hydrogen atoms. This leads to the most stable carbocation intermediate and hence the major product.
马氏规则指出，在 HX 与不对称烯烃的加成中，氢原子连接到已经拥有更多氢原子的碳上。这会导致最稳定的碳正离子中间体，从而生成主要产物。
In the CH01 paper, students had to explain why 2-bromopropane was the major product from propene and HBr, referencing the stability of the secondary carbocation over the primary alternative.
在CH01试卷中，学生需要解释为什么丙烯与 HBr 反应的主要产物是 2-溴丙烷，要提及二级碳正离子比可能生成的一级碳正离子更稳定。
9. Curly Arrows in Mechanisms | 反应机理中的弯箭头
Curly arrows are the universal language of mechanisms. They always show movement of an electron pair, starting from a lone pair or a bond and pointing towards the atom or bond where the electrons are going. In radical mechanisms, half-headed (fish-hook) arrows show single-electron movement.
弯箭头是机理的通用语言。它们始终表示电子对的运动，从孤对电子或化学键出发，指向电子到达的原子或键。在自由基机理中，半箭头（鱼钩箭头）表示单电子的运动。
Common mistakes include: arrows that start from an atom rather than a bond or lone pair, arrows that go to the wrong atom, and forgetting to show the formation of ions. Always draw the arrow from the electron source to the electron sink.
常见错误包括：箭头从原子而非化学键或孤对电子出发，箭头指向错误的原子，以及忘记表示离子的形成。始终要从电子源画向电子汇。
Examiners reward precise curly-arrow diagrams. In the CH01 mark scheme, marks were explicitly given for correct arrow drawing even if the product was slightly wrong, so accuracy here cannot be overstated.
阅卷人青睐精确的弯箭头图示。在CH01评分方案中，即使产物稍微有误，箭头画对也能明确得分，因此箭头准确性再怎么强调也不为过。
10. Exam Tips and Common Pitfalls | 考试技巧与常见误区
• Always identify the type of bond fission (homolytic or heterolytic) before starting the mechanism.
• 在开始画机理之前，始终要先判断键断裂的类型（均裂还是异裂）。
• Show partial charges (δ⁺/δ⁻) on the electrophile where useful, especially in bromination of alkenes.
• 在适当的地方标出亲电试剂上的部分电荷（δ⁺/δ⁻），特别是在烯烃溴化反应中。
• For radical substitution, include the UV light label over the initiation arrow, and never write “chain carrier” without correctly identifying the radical.
• 对于自由基取代，在引发步骤的箭头上标注紫外光，且在没有正确识别自由基的情况下不要写“链载体”。
• In electrophilic addition to unsymmetrical alkenes, draw both possible carbocations and indicate which is more stable, crossing out the minor pathway if required.
• 在不对称烯烃的亲电加成中，画出两种可能的碳正离子并指出哪个更稳定，需要时可划掉次要路径。
• Avoid mixing full and half arrows within the same mechanistic sequence unless justified by the step.
• 避免在同一个机理序列中混用全箭头和半箭头，除非步骤本身提供了理由。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导