📚 Reaction Mechanisms in A-Level WJEC Chemistry: Key Exam Points | A-Level WJEC 化学：反应机理 考点精讲
In A-Level WJEC Chemistry, reaction mechanisms are the backbone of organic chemistry. They show step‑by‑step how bonds break and form, where electrons move, and why certain products dominate. This article covers the essential exam points, from bond fission and curly arrows to free‑radical substitution, electrophilic addition, nucleophilic substitution, and elimination. You will learn how to draw mechanisms accurately, interpret kinetic evidence, and avoid common pitfalls.
在 A-Level WJEC 化学中，反应机理是有机化学的主干。它们逐步展示化学键如何断裂与形成、电子如何移动、以及为何某些产物占主导。本文涵盖关键考点，从键的断裂和弯箭头到自由基取代、亲电加成、亲核取代和消除反应。你将学会如何准确绘制机理、解释动力学证据并避开常见错误。
1. Types of Bond Fission | 键断裂的类型
Covalent bonds can break in two fundamentally different ways during a reaction. Homolytic fission occurs when a bond breaks symmetrically, each atom taking one electron from the shared pair. This generates two neutral free radicals, each with an unpaired electron (e.g. Cl–Cl → 2Cl•). Heterolytic fission involves an unsymmetrical break where both electrons go to the more electronegative atom, producing a cation and an anion (e.g. H–Br → H⁺ + :Br⁻). Recognizing the type of fission is the first step in predicting the reaction pathway.
共价键在反应中可以两种根本不同的方式断裂。均裂是指键对称地断裂，每个原子从共享电子对中各取一个电子，生成两个各带一个未成对电子的中性自由基（例如 Cl–Cl → 2Cl•）。异裂涉及不对称断裂，两个电子都归电负性更强的原子，产生一个阳离子和一个阴离子（例如 H–Br → H⁺ + :Br⁻）。识别断裂类型是预测反应路径的第一步。
2. Overview of Reaction Mechanisms | 反应机理概述
A reaction mechanism is a detailed sequence of elementary steps that describes which bonds are broken and formed, and in what order. In WJEC exams you must use curly arrows to show the movement of electron pairs. A full‑headed curly arrow (→) starts from a lone pair or a bond and points towards an atom or a new bond being formed. A half‑headed arrow (fish‑hook) shows the movement of a single electron in radical reactions. The reactants, intermediates, transition states and products together map the energetic pathway of the reaction.
反应机理是描述哪些化学键断裂和形成、以及按什么顺序发生的基元步骤的详细序列。在 WJEC 考试中，你必须使用弯箭头表示电子对的移动。双头弯箭头（→）从孤对电子或一个键出发，指向原子或正在形成的键。在自由基反应中半箭头（鱼钩箭头）表示单个电子的移动。反应物、中间体、过渡态和产物共同描绘出反应的能量路径。
3. Free‑Radical Substitution | 自由基取代反应
Alkanes react with halogens in the presence of UV light via a free‑radical chain mechanism. The classic example is the chlorination of methane: CH₄ + Cl₂ → CH₃Cl + HCl. The mechanism has three stages. Initiation: UV light breaks the Cl–Cl bond homolytically to produce two chlorine radicals (Cl₂ → 2Cl•). Propagation: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical (Cl• + CH₄ → HCl + •CH₃). The methyl radical then reacts with a chlorine molecule to give chloromethane and regenerate a chlorine radical (•CH₃ + Cl₂ → CH₃Cl + Cl•). Termination: Two radicals combine to form a stable molecule, e.g. Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, •CH₃ + •CH₃ → C₂H₆. The exam will ask you to draw these steps using the correct curly arrows for single‑electron movement.
烷烃在紫外光下与卤素通过自由基链式机理发生反应。经典例子是甲烷的氯化：CH₄ + Cl₂ → CH₃Cl + HCl。该机理包含三个阶段。引发：紫外光使 Cl–Cl 键均裂，产生两个氯自由基（Cl₂ → 2Cl•）。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（Cl• + CH₄ → HCl + •CH₃）。甲基自由基随后与氯分子反应，生成氯甲烷并再生氯自由基（•CH₃ + Cl₂ → CH₃Cl + Cl•）。终止：两个自由基结合成稳定分子，如 Cl• + Cl• → Cl₂、•CH₃ + Cl• → CH₃Cl、•CH₃ + •CH₃ → C₂H₆。考试会要求你使用正确的单电子移动弯箭头画出这些步骤。
4. Electrophilic Addition to Alkenes | 烯烃的亲电加成
Alkenes are nucleophilic because of the high electron density in the π bond. They undergo electrophilic addition with reagents such as HBr, Br₂ and concentrated H₂SO₄. In the first step, the π electrons attack the electrophile (e.g. Hᵟ⁺ from HBr), forming a carbocation intermediate and the halide ion. In the second step, the nucleophile (Br⁻) rapidly attacks the carbocation. For unsymmetrical alkenes, Markovnikov’s rule applies: the hydrogen adds to the carbon with the greater number of hydrogen atoms already attached, because the more stable (more substituted) carbocation is formed preferentially. When bromine adds to ethene, the intermediate is a cyclic bromonium ion, and the bromide ion attacks from the opposite face to give anti addition. You must be able to draw the bromonium ion and show the stereochemical consequence clearly.
烯烃因 π 键的高电子密度而具有亲核性。它们与 HBr、Br₂ 和浓 H₂SO₄ 等试剂发生亲电加成。第一步中，π 电子进攻亲电试剂（如 HBr 中的 Hᵟ⁺），形成碳正离子中间体和卤离子。第二步，亲核试剂（Br⁻）迅速进攻碳正离子。对于不对称烯烃，适用马氏规则：氢加到已经带有较多氢原子的碳上，因为优先生成更稳定（取代更多）的碳正离子。当溴与乙烯加成时，中间体为环状溴鎓离子，溴离子从背面进攻，得到反式加成产物。你必须能够画出溴鎓离子并清楚地展示立体化学结果。
5. Nucleophilic Substitution: SN1 and SN2 | 亲核取代：SN1 与 SN2
Halogenoalkanes undergo nucleophilic substitution with reagents such as aqueous hydroxide ions, cyanide ions and ammonia. Two distinct mechanisms operate: SN2 (bimolecular nucleophilic substitution) is a one‑step process where the nucleophile attacks the carbon bearing the leaving group from the back, simultaneously displacing the halogen. The rate = k[RX][Nu⁻], and the reaction proceeds with inversion of configuration (Walden inversion). SN2 is favoured for primary halogenoalkanes where steric hindrance is minimal. SN1 (unimolecular nucleophilic substitution) proceeds in two steps: slow ionisation of the C–X bond to give a planar carbocation, followed by fast attack of the nucleophile from either side. Rate = k[RX] only. SN1 is favoured for tertiary halogenoalkanes because the tertiary carbocation is relatively stable. The stereochemical outcome is racemisation if the carbon is chiral.
卤代烷与 OH⁻(aq)、CN⁻ 和 NH₃ 等试剂发生亲核取代。存在两种不同的机理：SN2（双分子亲核取代）为一步过程，亲核试剂从背面进攻连有离去基团的碳，同时取代卤素。速率 = k[RX][Nu⁻]，反应伴随构型翻转（瓦尔登翻转）。SN2 对空间位阻较小的伯卤代烷有利。SN1（单分子亲核取代）分两步进行：C–X 键缓慢电离生成平面型碳正离子，随后亲核试剂可从平面两侧快速进攻。速率 = k[RX]。SN1 对叔卤代烷有利，因为叔碳正离子相对稳定。若碳为手性中心，立体化学结果为外消旋化。
WJEC often asks you to compare SN1/SN2: provide kinetic orders, explain relative rates for 1°, 2°, 3° substrates, and draw the energy profiles. In SN2, the transition state is pentacoordinate with the nucleophile and leaving group partially bonded; in SN1, the energy profile shows two humps with a carbocation intermediate sitting in a shallow well.
WJEC 常要求比较 SN1 与 SN2：给出反应级数，解释伯、仲、叔底物的相对速率，并画出能量曲线。在 SN2 中，过渡态为五配位，亲核试剂与离去基团部分成键；在 SN1 中，能量曲线呈现两个峰，中间以浅谷中的碳正离子中间体相连。
6. Elimination Reactions | 消除反应
When a halogenoalkane is heated with a strong base in ethanol, elimination competes with substitution. The base removes a proton from a β‑carbon, forcing out the halide ion and creating a C=C double bond. The E2 mechanism is concerted: the base abstracts the β‑hydrogen as the leaving group departs, with the transition state requiring the H–C and C–X bonds to be anti‑periplanar. Rate = k[RX][base]. E1 is stepwise, with initial carbocation formation followed by deprotonation. Zaitsev’s rule states that the major alkene product is the more substituted one. For example, 2‑bromobutane yields but‑2‑ene as the major product, not but‑1‑ene. You should be able to rationalise this by the hyperconjugative stability of the more substituted alkene. Always consider the reaction conditions: a bulky base and ethanolic solvent favour elimination; aqueous conditions and a good nucleophile favour substitution.
当卤代烷与强碱在乙醇中加热时，消除反应与取代反应竞争。碱从 β‑碳上夺取一个质子，同时卤离子离去，生成 C=C 双键。E2 机理为协同过程：碱夺取 β‑氢的同时离去基团离去，过渡态要求 H–C 和 C–X 键处于反式共平面。速率 = k[RX][碱]。E1 为分步进行，首先生成碳正离子，然后脱质子。扎伊采夫规则指出，主产物是取代更多的烯烃。例如，2‑溴丁烷的主要消除产物是丁-2-烯，而非丁-1-烯。你应能通过取代更多烯烃的超共轭稳定性来解释。务必注意反应条件：大位阻碱和乙醇溶剂有利于消除；水溶液条件和优良亲核试剂有利于取代。
7. Drawing Curly Arrows Correctly | 正确绘制弯箭头
Marks in WJEC mechanism questions depend heavily on precise curly‑arrow drawing. A full arrow must start from a lone pair or the centre of a covalent bond, never from an atom. It must end at an atom that accepts the electron pair, or between two atoms to indicate a new bond. Arrows for proton transfer should start at the base’s lone pair and go to the hydrogen, with a second arrow from the H–X bond to the electronegative atom. In radical mechanisms, use a half‑arrow to show single‑electron shift. Always show all relevant lone pairs and formal charges on reacting species and intermediates. A common error is to draw a reversible arrow (⇌) where the mechanism step is actually irreversible under the given conditions.
在 WJEC 机理题中，分数的取得极度依赖准确的弯箭头画法。双头箭头必须始于孤对电子或共价键的中心，绝不能从原子出发。终点必须指向接受电子对的原子，或两原子之间以表示新键。质子转移的箭头应从碱的孤对电子出发指向氢，同时第二支箭头从 H–X 键指向电负性原子。自由基机理中使用半箭头表示单电子移动。始终标出反应物种和中间体上所有相关的孤对电子与形式电荷。常见错误是在给定条件下机理步骤实际不可逆时画可逆箭头（⇌）。
8. Reaction Profiles and the Rate‑Determining Step | 反应能量曲线与决速步
A reaction energy diagram plots potential energy against the reaction coordinate. Multistep mechanisms contain several energy maxima (transition states) and minima (intermediates). The rate‑determining step is the elementary step with the highest activation energy. For an SN1 reaction, this is the formation of the carbocation; for SN2, the single step is itself the rate‑determining step. Understanding the shape of the energy profile helps link the mechanism to the observed rate law: the number of molecules involved before and at the rate‑determining transition state determines the molecularity and kinetic order. WJEC may ask you to sketch and label these profiles, showing the effect of a catalyst or the difference between an endothermic and an exothermic overall reaction.
反应能量图以势能对反应坐标作图。多步机理包含若干个能量极大值（过渡态）和极小值（中间体）。决速步骤是具有最高活化能的基元步骤。在 SN1 反应中，决速步是碳正离子的生成；在 SN2 中，单一步骤本身就是决速步。理解能量曲线的形状有助于将机理与实测速率方程联系起来：在决速过渡态之前和之中参与反应的分子数决定了反应的分子数和动力学级数。WJEC 可能要求你画出并标注这些曲线，展示催化剂的影响或总反应吸热与放热的区别。
9. Evidence for Reaction Mechanisms | 反应机理的证据
Kinetic data provide the most direct evidence for distinguishing SN1 from SN2. An SN2 reaction shows second‑order kinetics (first order in both substrate and nucleophile), whereas SN1 shows first‑order kinetics dependent only on the substrate. Stereochemical observations are also crucial: complete inversion in SN2, and racemisation in SN1 for chiral substrates. Isotopic labelling, such as using ¹⁸O in water for ester hydrolysis, can confirm which bond is broken. In electrophilic addition, the anti‑addition observed with bromine supports the bromonium ion intermediate. For free‑radical substitution, the detection of trace amounts of ethane in methane chlorination corroborates the termination steps and the presence of methyl radicals.
动力学数据是区分 SN1 与 SN2 最直接的证据。SN2 反应呈现二级动力学（底物和亲核试剂均为一级），而 SN1 仅依赖底物浓度，为一级反应。立体化学观察同样关键：SN2 完全翻转，SN1 对手性底物则发生外消旋化。同位素标记，如酯水解中使用 H₂¹⁸O，可确认哪根键断裂。在亲电加成中，溴加成观测到的反式加成支持溴鎓离子中间体。对于自由基取代，甲烷氯化中微量乙烷的检出证实了终止步骤和甲基自由基的存在。
10. Common Misconceptions and Exam Technique | 常见误解和应考技巧
Many students lose marks by forgetting to include formal charges on intermediates (e.g. the carbocation has a positive charge) or by drawing the curly arrow starting from a positive charge rather than from the electron source. Always check that the number of electrons is conserved in each step and that the overall charge balances. Another misconception is that nucleophiles always attack the carbon bearing the leaving group; in elimination, the base attacks the β‑hydrogen. In WJEC exams, you must also be explicit about the type of bond fission when writing a mechanism — state “heterolytic” or “homolytic” in your answer where required. Practice drawing the mechanisms from memory several times, paying special attention to the geometry of transition states and the stereochemistry of products.
许多学生因忘记在中间体上标注形式电荷（如碳正离子带正电）或将弯箭头起点画在正电荷上而不是电子源上而失分。务必检查每步的电子数守恒，且总电荷平衡。另一个误解是亲核试剂总是进攻连有离去基团的碳；在消除反应中，碱进攻的是 β‑氢。在 WJEC 考试中，书写机理时你还需明确指出键断裂的类型——在需要的地方写上“异裂”或“均裂”。反复默画机理数次，特别留意过渡态的几何构型和产物的立体化学。
11. Key Mechanisms Checklist for WJEC | WJEC 重点机理自检清单
- Free‑radical substitution (CH₄ + Cl₂) — initiation, propagation, termination; single‑electron arrows
- 自由基取代（CH₄ + Cl₂）——引发、增长、终止；单电子箭头
- Electrophilic addition of HBr to propene — carbocation stability, Markovnikov orientation, two‑step arrows
- HBr 与丙烯的亲电加成——碳正离子稳定性、马氏取向、两步箭头
- Electrophilic addition of Br₂ to ethene — bromonium ion, anti‑addition, stereochemistry
- Br₂ 与乙烯的亲电加成——溴鎓离子、反式加成、立体化学
- SN1 hydrolysis of (CH₃)₃CBr — carbocation intermediate, racemisation, first‑order kinetics
- (CH₃)₃CBr 的 SN1 水解——碳正离子中间体、外消旋化、一级动力学
- SN2 reaction of CH₃CH₂Br with OH⁻ — inversion, second‑order kinetics, transition state drawing
- CH₃CH₂Br 与 OH⁻ 的 SN2 反应——翻转、二级动力学、过渡态画法
- E2 elimination of 2‑bromobutane with ethanolic KOH — Zaitsev product, anti‑periplanar requirement
- 2‑溴丁烷与 KOH 乙醇溶液的 E2 消除——扎伊采夫产物、反式共平面要求
- Energy profiles for SN1 vs SN2 — labelling intermediates, transition states and activation energies
- SN1 与 SN2 的能量曲线——标注中间体、过渡态和活化能
Going through this checklist and being able to reproduce each mechanism with correct curly arrows, charges and stereochemistry will put you in an excellent position for the WJEC A‑Level Chemistry exam.
完成这份清单，并能用正确的弯箭头、电荷与立体化学复现每个机理，将使你在 WJEC A‑Level 化学考试中处于极佳位置。
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