📚 AS Chemistry Insert 2 Jan 2022: Reaction Mechanisms | AS化学 2022年1月插入材料2：反应机理
The AQA AS Chemistry Paper 2 (7404/2) January 2022 insert provided essential reference material on organic reaction mechanisms. This article breaks down the core mechanisms that AS students must master, linking directly to the style of guidance found in the insert. Understanding the movement of electrons using curly arrows is central to predicting reaction outcomes and explaining reaction pathways.
AQA AS 化学试卷2（7404/2）2022年1月插入材料提供了有机反应机理的重要参考资料。本文详细解析AS学生必须掌握的核心机理，并直接呼应插入材料中的指导方式。理解用弯箭头表示的电子移动是预测反应结果和解释反应路径的关键。
1. Understanding Reaction Mechanisms | 理解反应机理
A reaction mechanism shows the step-by-step sequence of bond breaking and bond making. Curly arrows represent the movement of electron pairs, never atoms. At AS level, you need to be able to interpret given mechanisms and draw simple ones for common reactions, using the correct arrow notation.
反应机理展示了化学键断裂与生成的逐步过程。弯箭头代表电子对的移动，而不是原子的移动。在AS阶段，你需要能够解释所给出的机理，并用正确的箭头标注方式为常见反应画出简单的机理。
There are two fundamental types of bond fission: homolytic fission, where each atom takes one electron from the bond, producing radicals; and heterolytic fission, where both electrons go to one atom, forming a cation and an anion. Homolytic fission uses half-headed curly arrows (often drawn as a single barb), while heterolytic fission uses full-headed curly arrows.
化学键断裂有两种基本类型：均裂，每个原子从共价键中拿走一个电子，产生自由基；异裂，两个电子都归一方原子，形成阳离子和阴离子。均裂使用半弯箭头（常画作单钩箭头），异裂则使用完整的弯箭头。
In the insert, you will see curly arrow diagrams with clear labelling of lone pairs, partial charges, and intermediates. Always follow the electron flow from an electron-rich site (nucleophile or double bond) to an electron-poor site (electrophile or positive carbon).
在插入材料中，你会看到弯箭头示意图，清晰标注孤对电子、部分电荷和中间体。始终遵循电子流从富电子位点（亲核试剂或双键）流向缺电子位点（亲电试剂或带正电的碳）。
2. Free-Radical Substitution: Chlorination of Methane | 自由基取代：甲烷的氯化
Alkanes react with halogens in the presence of UV light through a free-radical substitution mechanism. The overall equation for chlorination of methane is CH₄ + Cl₂ → CH₃Cl + HCl. The mechanism involves three stages: initiation, propagation, and termination.
烷烃在紫外光存在下与卤素发生自由基取代反应。甲烷氯化的总方程式为 CH₄ + Cl₂ → CH₃Cl + HCl。该机理包括三个阶段：链引发、链传递和链终止。
Initiation: Ultraviolet light provides energy to break the Cl–Cl bond homolytically. Two chlorine radicals are formed: Cl–Cl ⤻(UV)⤻ 2 Cl•. A half-headed curly arrow originates from the bond and points to each chlorine atom, showing one electron going to each.
链引发： 紫外光提供能量使 Cl–Cl 键发生均裂，形成两个氯自由基：Cl–Cl ⤻(紫外)⤻ 2 Cl•。一个半弯箭头从键出发，指向每个氯原子，表示每个原子带走一个电子。
Propagation step 1: A chlorine radical abstracts a hydrogen atom from methane, forming hydrogen chloride and a methyl radical: Cl• + H–CH₃ → H–Cl + •CH₃. The half-arrow shows one electron from the C–H bond moving onto the Cl• to form the H–Cl bond, while the other electron remains on carbon, producing the radical •CH₃.
链传递步骤1： 一个氯自由基从甲烷中夺取氢原子，生成氯化氢和甲基自由基：Cl• + H–CH₃ → H–Cl + •CH₃。半箭头表示 C–H 键的一个电子移到 Cl• 上形成 H–Cl 键，另一个电子留在碳上，产生 •CH₃ 自由基。
Propagation step 2: The methyl radical reacts with a chlorine molecule to form chloromethane and a new chlorine radical: •CH₃ + Cl–Cl → CH₃Cl + Cl•. This regenerates the radical species, continuing the chain reaction.
链传递步骤2： 甲基自由基与氯分子反应，生成氯甲烷和一个新的氯自由基：•CH₃ + Cl–Cl → CH₃Cl + Cl•。这重新生成了自由基物种，使链反应继续进行。
Termination: Any two radicals can combine to form a stable molecule, removing radicals from the system. Examples: Cl• + Cl• → Cl₂; •CH₃ + Cl• → CH₃Cl; •CH₃ + •CH₃ → CH₃CH₃.
链终止： 任意两个自由基可以结合形成稳定分子，将自由基从体系中移除。例子：Cl• + Cl• → Cl₂；•CH₃ + Cl• → CH₃Cl；•CH₃ + •CH₃ → CH₃CH₃。
3. Electrophilic Addition in Alkenes | 烯烃的亲电加成
Alkenes undergo electrophilic addition because the π‑bond is an area of high electron density. This attracts electrophiles — species that can accept a pair of electrons. Typical reagents include HBr, H₂SO₄, and Br₂. The C=C double bond breaks and two σ‑bonds are formed to the adjacent carbons.
烯烃发生亲电加成反应的原因是 π 键区域电子密度高，能吸引亲电试剂——即能接受一对电子的物种。常见的试剂包括 HBr、H₂SO₄ 和 Br₂。C=C 双键断裂，相邻的两个碳各自形成新的 σ 键。
The mechanism proceeds via a carbocation intermediate when the electrophile is unsymmetrical. The electrophilic attack generates a positively charged carbon atom, which is then rapidly attacked by a nucleophile (often the anion released in the first step).
当亲电试剂不对称时，该机理经由碳正离子中间体进行。亲电进攻产生一个带正电荷的碳原子，紧接着被亲核试剂（通常是第一步释放的阴离子）迅速进攻。
4. Mechanism of Electrophilic Addition: Ethene and HBr | 亲电加成机理：乙烯与溴化氢
The reaction of ethene with hydrogen bromide is a classic example. Step 1: The π‑electrons of the ethene C=C bond attack the hydrogen of H–Br. A curly arrow ⟶ starts from the middle of the double bond and points to the hydrogen, while another curly arrow from the H–Br bond goes to the bromine, generating Br⁻ and the ethyl carbocation CH₃–CH₂⁺.
乙烯与溴化氢的反应是一个经典范例。第一步：乙烯 C=C 键的 π 电子进攻 H–Br 中的氢。一根弯箭头 ⟶ 从双键中间出发指向氢，同时另一根弯箭头从 H–Br 键指向溴，生成 Br⁻ 和乙基碳正离子 CH₃–CH₂⁺。
Step 2: The bromide ion, acting as a nucleophile, donates a lone pair to the positively charged carbon. A curly arrow starts from the lone pair on Br⁻ and points to the carbocation, forming the C–Br bond and giving the product bromoethane, CH₃CH₂Br.
第二步：溴离子作为亲核试剂，将一对孤对电子给予带正电的碳。弯箭头从 Br⁻ 上的孤对电子出发指向碳正离子，形成 C–Br 键，得到产物溴乙烷 CH₃CH₂Br。
Note that the curly arrow for the nucleophilic attack must start at the lone pair or the negative charge symbol, not at the atom symbol itself. The carbocation is an intermediate that exists fleetingly but is not isolated.
请注意，亲核进攻的弯箭头必须从孤对电子或负电荷符号出发，而不是从原子符号本身出发。碳正离子是一种短暂存在的中间体，无法分离。
5. Markovnikov’s Rule and Carbocation Stability | 马氏规则与碳正离子稳定性
When an unsymmetrical alkene like propene reacts with HBr, two carbocation intermediates are possible: a primary carbocation and a secondary carbocation. Markovnikov’s rule states that the hydrogen atom attaches to the carbon that already has more hydrogen atoms, yielding the more stable carbocation intermediate.
当不对称烯烃（如丙烯）与 HBr 反应时，可能生成两种碳正离子中间体：伯碳正离子和仲碳正离子。马氏规则指出，氢原子加到含氢较多的碳原子上，从而生成更稳定的碳正离子中间体。
The stability order of carbocations is tertiary > secondary > primary > methyl. Tertiary carbocations are stabilised by the inductive effect of three alkyl groups pushing electron density towards the positive centre. Hyperconjugation also contributes to stability. Thus, the major product from propene + HBr is 2‑bromopropane, not 1‑bromopropane.
碳正离子的稳定性顺序为：叔碳正离子 > 仲碳正离子 > 伯碳正离子 > 甲基碳正离子。叔碳正离子因三个烷基的诱导效应将电子密度推向正电荷中心而得到稳定。超共轭效应也有贡献。因此，丙烯与 HBr 反应的主产物是 2‑溴丙烷，而不是 1‑溴丙烷。
In the insert, you may be provided with energy profile diagrams or product distribution data. Always relate regioselectivity to the relative stability of the possible carbocations.
在插入材料中，你可能会看到能量曲线图或产物分布数据。始终将区域选择性与可能碳正离子的相对稳定性联系起来。
6. Nucleophilic Substitution: Hydrolysis of Haloalkanes | 亲核取代：卤代烷的水解
Haloalkanes can be converted to alcohols by heating with aqueous sodium hydroxide. The hydroxide ion, OH⁻, acts as a nucleophile and attacks the electron‑deficient carbon atom bonded to the halogen. The overall reaction for bromoethane is: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻.
卤代烷与氢氧化钠水溶液共热可转化为醇。氢氧根离子 OH⁻ 作为亲核试剂，进攻与卤素相连的缺电子碳原子。溴乙烷的总反应为：CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻。
The mechanism: a full curly arrow ⟶ originates from a lone pair on the oxygen of OH⁻ and points to the carbon attached to bromine. Simultaneously, the C–Br bond breaks heterolytically, with the electron pair moving onto the bromine atom, represented by another curly arrow from the C–Br bond to Br. This is an example of an Sₙ2 process for primary haloalkanes.
机理：一根完整的弯箭头 ⟶ 从 OH⁻ 中氧的孤对电子出发，指向与溴相连的碳。与此同时，C–Br 键发生异裂，电子对移向溴原子，用另一根弯箭头从 C–Br 键指向 Br。这是伯卤代烷发生 Sₙ2 过程的一个实例。
For tertiary haloalkanes, an Sₙ1 mechanism may compete, involving rate‑determining formation of a carbocation before nucleophilic attack. However, at AS the focus is typically on the Sₙ2 mechanism, which occurs in one concerted step with inversion of configuration at a chiral centre.
对于叔卤代烷，可能会竞争 Sₙ1 机理，该机理涉及速率决定步骤的碳正离子形成，随后发生亲核进攻。但在 AS 阶段，重点通常是 Sₙ2 机理，它通过一个协同步骤发生，手性中心的构型发生翻转。
7. SN1 vs SN2 Mechanisms (AS Context) | SN1与SN2机理 (AS背景)
Although detailed kinetic studies appear more at A2, AS students should recognise the key differences:
| Feature | SN1 | SN2 | 特征 |
|---|---|---|---|
| Kinetics | Rate = k[haloalkane]; unimolecular | Rate = k[haloalkane][Nu]; bimolecular | 动力学 |
| Intermediate | Carbocation formed | Transition state only | 中间体 |
| Stereochemistry | Racemisation possible | Inversion of configuration | 立体化学 |
| Preferred substrate | Tertiary haloalkane | Primary haloalkane | 优选底物 |
The insert may include model mechanisms for haloalkane substitution. Identify the type by considering the structure of the haloalkane and the strength of the nucleophile. A strong nucleophile and an unhindered carbon favour Sₙ2, whereas a stable carbocation favours Sₙ1.
插入材料可能包含卤代烷取代反应的模型机理。通过考虑卤代烷的结构和亲核试剂的强度来识别反应类型。强亲核试剂和空间位阻小的碳有利于 Sₙ2，而稳定的碳正离子则有利于 Sₙ1。
8. Elimination Reactions: Dehydrohalogenation | 消除反应：脱卤化氢
When a haloalkane is heated with potassium hydroxide dissolved in ethanol, an elimination reaction occurs, producing an alkene. For example, bromoethane yields ethene: CH₃CH₂Br + OH⁻ → CH₂=CH₂ + H₂O + Br⁻. The hydroxide ion acts as a base rather than a nucleophile.
当卤代烷与溶在乙醇中的氢氧化钾共热时，发生消除反应生成烯烃。例如，溴乙烷生成乙烯：CH₃CH₂Br + OH⁻ → CH₂=CH₂ + H₂O + Br⁻。此时氢氧根离子作为碱而非亲核试剂。
Mechanistically, a curly arrow starts from the lone pair on OH⁻ and points to a β‑hydrogen atom. Simultaneously, the electron pair from the C–H bond moves to form the C=C π‑bond, while the C–Br bond breaks heterolytically, with its electron pair moving onto the bromine. The concerted removal of HBr generates the alkene.
机理上，一根弯箭头从 OH⁻ 的孤对电子出发，指向 β‑氢原子。同时，C–H 键的电子对移动形成 C=C π 键，而 C–Br 键异裂，其电子对移向溴。这一定向的 HBr 消除步骤生成了烯烃。
Elimination and substitution often compete. Using a strong, sterically hindered base (like KOH/ethanol) and heating favours elimination, whereas aqueous conditions and primary haloalkanes favour substitution. The insert may indicate the conditions, so you must deduce the dominant pathway.
消除反应与取代反应常常相互竞争。使用强有位阻的碱（如 KOH/乙醇）并加热有利于消除，而水溶液条件和伯卤代烷有利于取代。插入材料可能标出条件，你必须由此推断主导路径。
9. Curly Arrows and Electron Movement | 弯箭头与电子移动
Correct curly arrow drawing is essential for marks. Rules: the arrow starts at an electron‑rich site – a lone pair, a negative charge, or the centre of a π‑bond. It ends at an electron‑poor site – an atom, a bond, or a positive charge. Never draw an arrow starting from a positive charge.
正确地绘制弯箭头对于得分至关重要。规则如下：箭头从富电子位点出发——孤对电子、负电荷或 π 键的中心。箭头指向缺电子位点——某个原子、化学键或正电荷。切勿从正电荷出发画箭头。
Full‑headed arrows ( ⟶ ) depict the movement of an electron pair, used for heterolytic processes. Half‑headed arrows ( ⤻ ) depict the movement of a single electron, used for homolytic processes. In the insert, you will see both types used appropriately.
完整的弯箭头（ ⟶ ）描绘一对电子的移动，用于异裂过程。半钩箭头（ ⤻ ）描述单个电子的移动，用于均裂过程。在插入材料中，你会看到两种箭头都被恰当地使用。
When drawing mechanisms, balance charges at each step. The total charge on the left must equal the total charge on the right. Use brackets to show transition states if required, but intermediates are drawn as distinct species. Practice copying and completing partial mechanisms from the insert.
绘制机理时，每一步都要平衡电荷。左侧的总电荷必须等于右侧的总电荷。如有需要，用括号表示过渡态，但中间体应作为独立的物种画出。通过模仿插入材料中的部分机理并完成它们来加强练习。
10. Inter
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