📚 A-Level Chemistry: Reaction Mechanisms from June 2018 Paper 3 | A-Level 化学：2018年6月试卷3反应机理
The June 2018 A-Level Chemistry Paper 3 placed a significant focus on reaction mechanisms. Students were required not only to recall reagents and conditions but also to illustrate stepwise electron movements using curly arrows, interpret energy profiles, and justify the regioselectivity observed in unfamiliar reactions. This article revisits the core mechanisms tested in that paper, providing a bilingual study guide for revision.
2018年6月A-Level化学试卷3重点考查了反应机理。考生不仅要回忆试剂和反应条件，还要用弯曲箭头表示分步的电子转移过程，解释能量曲线，并说明在陌生反应中观察到的区域选择性背后的原理。本文回顾了试卷中涉及的核心机理，提供一份中英双语备考指南。
1. Importance of Mechanisms in Paper 3 | 试卷3中机理的重要性
In Question 4 of June 2018 Paper 3, students were asked to propose a mechanism for the electrophilic addition of hydrogen bromide to propene, while Question 6 required the hydrolysis mechanism of a secondary halogenoalkane. These questions tested the ability to draw curly arrows and to predict major products using carbocation stability.
在2018年6月试卷3的第4题中，考生需要提出丙烯与溴化氢的亲电加成机理；第6题则考查了二级卤代烷的水解机理。这些题目不仅测试弯曲箭头的绘制能力，还要求运用碳正离子稳定性预测主要产物。
2. Electrophilic Addition: General Pattern | 亲电加成：一般模式
Electrophilic addition is the characteristic reaction of alkenes. The π bond acts as an electron-rich source that attacks an electron-deficient species – the electrophile. A carbocation intermediate is formed, which is then rapidly attacked by a nucleophile (often the conjugate base generated in the first step).
亲电加成是烯烃的特征反应。富电子的π键进攻缺电子物种（亲电试剂），生成碳正离子中间体，随后该中间体被亲核试剂（通常是第一步产生的共轭碱）迅速进攻。
Overall: CH₂=CH₂ + HBr → CH₃CH₂Br
总反应：CH₂=CH₂ + HBr → CH₃CH₂Br
3. Example from June 2018 Paper 3: Propene + HBr | 案例：丙烯与溴化氢
Step 1: The curly arrow (→) from the C=C bond attacks the δ⁺ hydrogen of H–Br, heterolytically breaking the H– drogen adds to the less substituted end, generating the more stable secondary carbocation.
第一步：C=C 键的弯曲箭头 (→) 进攻 H–Br 中带 δ⁺ 的氢原子，使 H–Br 键异裂，生成溴离子。氢加到取代较少的碳上，形成更稳定的二级碳正离子。
Step 2: A curly arrow from the lone pair on Br⁻ attacks the positively charged carbon, forming 2-bromopropane as the major product.
第二步：Br⁻ 上孤对电子的弯曲箭头进攻带正电荷的碳，生成主要产物 2-溴丙烷。
The regioselectivity follows Markownikov's rule: hydrogen adds to the carbon with more hydrogen atoms already attached, because the more substituted carbocation is more stable due to the electron-donating inductive effect of alkyl groups.
区域选择性遵循马氏规则：氢加在原来氢较多的碳上，因为烷基的给电子诱导效应使更高级的碳正离子更稳定。
4. Nucleophilic Substitution: SN1 vs SN2 | 亲核取代：SN1与SN2对比
Haloalkanes undergo nucleophilic substitution via two distinct mechanisms. SN2 is a one-step process where the nucleophile attacks from the back side, leading to inversion of configuration. SN1 proceeds via a planar carbocation intermediate, giving racemisation.
卤代烷的亲核取代有两种不同机理。SN2 是双分子一步过程，亲核试剂从背面进攻，导致构型翻转；SN1 则通过平面碳正离子中间体进行，产物外消旋化。
| Factor 因素 | SN2 | SN1 |
|---|---|---|
| Substrate 底物 | Primary > secondary 一级优于二级 | Tertiary > secondary 三级优于二级 |
| Kinetics 动力学 | Rate = k[RX][Nu⁻] | Rate = k[RX] |
| Stereochemistry 立体化学 | Inversion 翻转 | Racemisation 外消旋 |
5. Mechanism of Alkaline Hydrolysis of Halogenoalkanes | 卤代烷的碱性水解机理
In Paper 3, students had to show the mechanism for the hydrolysis of 2-bromobutane with aqueous sodium hydroxide. The OH⁻ ion acts as a nucleophile: a curly arrow from its lone pair points to the electron-deficient α-carbon, while a second curly arrow shows the C–Br bond breaking to form Br⁻.
试卷3中，考生需要展示 2-溴丁烷与氢氧化钠水溶液的水解机理。OH⁻ 作为亲核试剂：孤对电子的弯曲箭头指向缺电子的 α-碳，同时另一个弯曲箭头表示 C–Br 键断裂生成 Br⁻。
The product is butan-2-ol. Because the substrate is secondary, both SN2 and SN1 can operate under appropriate conditions, but with a strong nucleophile like OH⁻, the reaction favours an SN2 pathway producing inversion of configuration.
产物是 2-丁醇。由于底物为二级卤代烷，SN2 和 SN1 在一定条件下均可进行，但在强亲核试剂 OH⁻ 存在下，反应倾向 SN2 途径，导致构型翻转。
6. Free Radical Substitution: Chlorination of Alkanes | 自由基取代：烷烃的氯化
Free radical substitution is the key mechanism for the chlorination of methane. It proceeds in three stages: initiation, propagation and termination. UV light is required to homolytically break the Cl–Cl bond, generating two chlorine radicals.
自由基取代是甲烷氯化的关键机理，包括链引发、链增长和链终止三个阶段。需要紫外光使 Cl–Cl 键均裂，产生两个氯自由基。
Propagation step 1: Cl• + CH₄ → •CH₃ + HCl
链增长第一步：Cl• + CH₄ → •CH₃ + HCl
Propagation step 2: •CH₃ + Cl₂ → CH₃Cl + Cl•
链增长第二步：•CH₃ + Cl₂ → CH₃Cl + Cl•
ctron (fishhook arrow). Termination occurs when any two radicals combine, for example 2 Cl• → Cl₂.
增长步骤中的每个弯曲箭头表示单电子转移（半箭头）。任何两个自由基结合即发生终止，例如 2 Cl• → Cl₂。
7. Elimination: Dehydration of Alcohols | 消除反应：醇的脱水
Alcohols can be dehydrated to alkenes using concentrated sulfuric acid or phosphoric acid. The mechanism is an E1 elimination: the hydroxy group is protonated, water leaves to form a carbocation, and a neighbouring C–H bond is broken to create the alkene π bond.
醇可用浓硫酸或磷酸脱水生成烯烃。该机理属于 E1 消除：羟基先被质子化，离去水生成碳正离子，邻近的 C–H 键断裂形成烯烃 π 键。
The final step requires a curly arrow from a C–H bond to the C–C region, forming the double bond, and another arrow taking the proton away. Zaitsev's rule predicts the most substituted alkene is the major product.
最后一步需要从 C–H 键指向 C–C 区域的弯曲箭头形成双键，同时另一个箭头带走质子。根据扎伊采夫规则，取代基最多的烯烃为主要产物。
8. Electrophilic Substitution in Benzene | 苯的亲电取代
Benzene undergoes electrophilic substitution rather than addition because the aromatic π system is stabilised by delocalisation. An electrophile (E⁺) must be generated first, for example the nitronium ion NO₂⁺ in nitration.
苯环发生亲电取代而非加成，因为离域大 π 键使芳环稳定。必须先产生亲电试剂（E⁺），如硝化反应中的硝鎓离子 NO₂⁺。
A curly arrow from the benzene ring π cloud attacks the electrophile, forming a positively charged intermediate called the Wheland intermediate (arenium ion). Loss of a proton restores aromaticity and generates the substituted product.
苯环 π 电子云的弯曲箭头进攻亲电试剂，形成带正电的中间体威兰德中间体（芳正离子）。失去一个质子恢复芳香性，得到取代产物。
9. Curly Arrows: Rules and Common Errors | 弯曲箭头：规则与常见错误
In June 2018 Paper 3, many students lost marks by drawing curly arrows incorrectly. Arrows must start from a bond or a lone pair, and point towards an atom or a bond being formed. They should never originate from a positive charge.
在2018年6月试卷3中，许多考生因弯曲箭头画错而失分。箭头必须从一个键或孤对电子出发，指向正形成的键或原子，绝不能从正电荷出发。
Common mistakes include: omitting the carbocation intermediate, showing a one-step addition for alkene + HBr without a carbocation, and forgetting to balance charges. Always label δ+ and δ– on polar bonds to justify the attack direction.
常见错误包括：漏掉碳正离子中间体，将烯烃 + HBr 加成画成一步而不显示碳正离子，以及忘记平衡电荷。务必在极性键上标出 δ+ 和 δ–，以解释进攻方向。
10. Applying Mechanisms to Multi‑Step Synthesis | 机理在多步合成中的应用
Understanding mechanisms allows chemists to design synthetic routes. For instance, converting propene to propan-2-ol requires electrophilic addition of water (hydration), which proceeds via a carbocation and follows Markovnikov's rule.
理解机理可以帮助设计合成路线。例如，将丙烯转化为 2-丙醇需要进行水的亲电加成（水合），该过程经过碳正离子并遵循马氏规则。
If a different alcohol is targeted, a halide substitution–hydrolysis sequence might be used: radical bromination followed by nucleophilic substitution. Knowing when SN2 predominates is critical for controlling stereochemistry.
若目标为另一种醇，可采用卤代–水解的序列：先进行自由基溴化，再亲核取代。明确何时 SN2 为主，对于控制立体化学至关重要。
11. Exam Tips for Drawing Mechanisms | 绘制机理的应试技巧
Always read the question carefully: note whether the reaction is in aqueous or alcoholic conditions, whether the reagent is an electrophile or a nucleophile, and whether the substrate is primary, secondary or tertiary. This determines the mechanistic pathway.
仔细审题：注意反应是在水溶液还是醇溶液中进行，试剂是亲电试剂还是亲核试剂，底物是一级、二级还是三级卤代烷，这些决定了机理路径。
Show all lone pairs and formal charges. Use balanced equations and include the word 'curly arrow' in your annotation. For elimination, remember to draw the proton removed by a base, often a water molecule or HSO₄⁻.
画出所有孤对电子和形式电荷。使用配平方程式，在说明中标注 "弯曲箭头"。对于消除反应，别忘了画出被碱（通常是水分子或 HSO₄⁻）夺走的质子。
12. Summary Table of Key Mechanisms | 关键机理总结表
| Reaction 反应 | Reagents/Conditions 试剂/条件 | Mechanism Type 机理类型 | Key Feature 关键特征 |
|---|---|---|---|
| Alkene + HX | HX, room temp 室温 | Electrophilic Addition 亲电加成 | Carbocation intermediate, Markownikov 碳正离子，马氏规则 |
| Haloalkane + NaOH(aq) | NaOH, water, heat 水加热 | Nucleophilic Substitution 亲核取代 | SN2 (primary) or SN1 (tertiary) 一级SN2，三级SN1 |
| Alcohol → Alkene | Conc. H₂SO₄, 170°C | Elimination (E1) 消除 | Carbocation, Zaitsev product 碳正离子，扎氏产物 |
| Benzene + HNO₃ | Conc. H₂SO₄/HNO₃, 50°C | Electrophilic Substitution 亲电取代 | NO₂⁺ electrophile, Wheland intermediate NO₂⁺亲电体，威兰德中间体 |
| Alkane + Cl₂ | UV light 紫外光 | Free Radical Substitution 自由基取代 | Chain reaction, homolytic fission 链反应，均裂 |
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