A-Level Chemistry: Reaction Mechanisms (Jun 18 Markscheme 2) | A-Level 化学：反应机理（2018年6月评分方案卷二）

📚 A-Level Chemistry: Reaction Mechanisms (Jun 18 Markscheme 2) | A-Level 化学：反应机理（2018年6月评分方案卷二）

Reaction mechanisms are the heart of organic chemistry at A-Level. In the June 2018 Paper 2, candidates were tasked with drawing mechanisms for several key reactions, from electrophilic addition to nucleophilic substitution. This article revisits those mechanisms, unpacks the mark scheme requirements, and clarifies common pitfalls so you can master mechanism questions.

反应机理是A-Level有机化学的核心。在2018年6月卷二考试中，考生需要为几类关键反应绘制机理，包括亲电加成和亲核取代。本文重温这些机理，解析评分方案的要求，并澄清常见错误，助你征服机理题。

1. What is a Reaction Mechanism? | 什么是反应机理？

A reaction mechanism breaks down a chemical transformation into individual bond-breaking and bond-forming steps, showing the movement of electrons using curly arrows. It reveals the pathway from reactants to products through intermediates and transition states.

反应机理将化学变化分解为单独的键断裂和键形成步骤，使用弯曲箭头表示电子移动。它通过中间体和过渡态揭示了从反应物到产物的路径。

In the Jun 18 markscheme, credit was given for correctly drawn curly arrows, identification of species like electrophiles and nucleophiles, and proper depiction of charges on intermediates.

在六月18的评分方案中，正确画出弯曲箭头、识别亲电试剂和亲核试剂等物种，以及正确标明中间体上的电荷均可得分。

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

Curly arrows show the movement of an electron pair. The arrow starts from a lone pair or a bond and points towards an electron-deficient atom. Full-headed arrows represent two-electron movements, while half-headed (fish-hook) arrows indicate the movement of a single electron in radical reactions.

弯曲箭头表示电子对的移动。箭头起始于孤对电子或一根键，指向缺电子的原子。实心箭头代表双电子移动，而半箭头（鱼钩箭头）代表自由基反应中单个电子的移动。

Common mistake: drawing the arrow from a positive charge or forgetting to show the tail on the correct electron source. The Jun 18 paper penalised arrows starting from atoms rather than bonds.

常见错误：从正电荷出发画箭头，或忘记把箭尾画在正确的电子源上。六月18的试卷中，箭头起始于原子而非键会被扣分。

3. Electrophilic Addition to Alkenes | 烯烃的亲电加成

Alkenes react with electrophiles via an electrophilic addition mechanism. The π bond acts as a nucleophile, attracting the electrophile. The typical example tested in Jun 18 was the reaction of ethene with bromine.

烯烃通过亲电加成机理与亲电试剂反应。π键充当亲核体，吸引亲电试剂。六月18试卷中测试的典型例子是乙烯与溴的反应。

Step 1: The electrophile Br₂ is polarised by the alkene’s electron density. A curly arrow from the double bond attacks the partially positive bromine, breaking the Br–Br bond heterolytically.

第一步：亲电试剂Br₂被烯烃的电子云极化。来自双键的弯曲箭头进攻部分正电的溴原子，导致Br–Br键异裂。

Step 2: The resulting carbocation intermediate reacts with the bromide ion. A curly arrow from the bromide lone pair to the carbocation forms the C–Br bond.

第二步：生成的碳正离子中间体与溴离子反应。来自溴离子孤对电子的弯曲箭头指向碳正离子，形成C–Br键。

4. Example: Bromination of Ethene | 实例：乙烯的溴化

The overall equation:

总方程式：

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

Mechanism details: The addition is anti, meaning the two bromine atoms add to opposite faces of the alkene. However, in simple mechanism questions at A-Level, stereochemistry is not always required unless specified.

机理细节：该加成是反式的，即两个溴原子加在烯烃的两侧。不过在A-Level的简单机理题中，除非特别要求，通常不涉及立体化学。

Mark Scheme Point: In Jun 18, marks were awarded for showing the curly arrows, the intermediate carbocation, and correct charges (Br⁻).

评分要点：六月18中，画出弯曲箭头、中间体碳正离子和正确电荷（Br⁻）即可得分。

5. Nucleophilic Substitution: SN1 vs SN2 | 亲核取代：SN1与SN2

Nucleophilic substitution occurs when a nucleophile replaces a leaving group on a carbon atom. The two limiting mechanisms are SN1 and SN2, both featured in the Jun 18 markscheme.

当亲核试剂取代碳原子上的离去基团时，发生亲核取代。两种极限机理为SN1和SN2，六月18评分方案中均有涉及。

The table below summarises the crucial differences that A-Level candidates must be able to recall and apply when analysing rate data or selecting a mechanism.

下表总结了关键区别，A-Level考生在分析速率数据或选择机理时必须能够回忆并应用。