A-Level Chemistry Unit 5 Reaction Mechanisms (June 2022 QP) | A-Level化学单元5反应机理（2022年6月试卷）

📚 A-Level Chemistry Unit 5 Reaction Mechanisms (June 2022 QP) | A-Level化学单元5反应机理（2022年6月试卷）

The June 2022 A-Level Chemistry Unit 5 paper presented a range of challenging reaction mechanism questions. Mastering mechanisms is essential for both understanding organic chemistry and securing high marks. This article revisits the core mechanisms examined, including electrophilic addition, nucleophilic substitution, elimination, electrophilic aromatic substitution, and addition-elimination, with detailed step-by-step breakdowns.

2022年6月的A-Level化学单元5试卷中，反应机理题目占据了重要地位。掌握机理对于理解有机化学以及获取高分至关重要。本文将回顾试卷中考查的核心机理，包括亲电加成、亲核取代、消去反应、亲电芳香取代和加成-消去，提供逐步的详细解析。

1. Introduction to Curly Arrow Mechanisms | 弯箭头机理简介

In A-Level mechanisms, a curly arrow (⟶) shows the movement of an electron pair. A full arrow indicates a pair of electrons moving, while a half-headed arrow (⇀) for radicals is less common at Unit 5. Always start the arrow from a lone pair, a negative charge, or a bond, and point towards an electron-deficient atom or to form a new bond. Heterolytic bond fission produces ions, whereas homolytic fission generates free radicals. Understanding these fundamentals is key to drawing valid mechanisms.

在A-Level机理中，弯箭头（⟶）表示电子对的移动。全箭头表示一对电子流动，而自由基机理中的半箭头（⇀）在单元5较少出现。弯箭头始终从孤对电子、负电荷或化学键出发，指向缺电子原子或形成新键。异裂产生离子，均裂则产生自由基。理解这些基础是画出正确机理的关键。

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

Electrophilic addition to alkenes, such as the reaction of ethene with HBr, proceeds via a two-step ionic mechanism. First, the π bond acts as a nucleophile, attacking the partially positive hydrogen (δ⁺) in HBr. The electrons move from the C=C bond to form a C–H bond, generating a carbocation and Br⁻. The curly arrow from the double bond to the H is shown, and another arrow from the H–Br bond to Br, breaking it heterolytically. In the second step, the bromide ion donates a lone pair to the carbocation to form the final haloalkane. The stability of the carbocation intermediate determines regioselectivity: tertiary > secondary > primary. Thus, with unsymmetrical alkenes, Markovnikov’s rule predicts the major product where the hydrogen attaches to the carbon with more hydrogens initially.

烯烃的亲电加成，例如乙烯与HBr反应，通过两步离子机理进行。首先，π键作为亲核体攻击HBr中部分正电荷的氢（δ⁺）。电子从C=C双键移动形成C–H键，同时生成碳正离子和Br⁻。弯箭头从双键指向H，另一个箭头从H–Br键指向Br，使其异裂。第二步，溴离子提供孤对电子给碳正离子，形成最终卤代烷。碳正离子中间体的稳定性决定区域选择性：三级 > 二级 > 一级。因此，对于不对称烯烃，马氏规则预测主要产物中氢加在原本氢较多的碳上。

3. Nucleophilic Substitution: SN1 and SN2 | 亲核取代：SN1与SN2

Nucleophilic substitution is central to Unit 5, with two distinct mechanisms: SN1 and SN2. SN2 is a concerted one-step mechanism where the nucleophile attacks the carbon from the opposite side of the leaving group, leading to inversion of configuration. The rate = k[haloalkane][nucleophile]. It requires a primary or methyl substrate and a good nucleophile. SN1 occurs in two steps: first, the leaving group departs, forming a planar carbocation; then the nucleophile attacks from either side, giving racemisation. Rate = k[haloalkane] only. It favours tertiary haloalkanes and polar protic solvents.

亲核取代在单元5中占据核心地位，有两种不同机理：SN1和SN2。SN2是协同一步机理，亲核体从离去基团背面进攻碳原子，引起构型翻转。速率方程：速率 = k[卤代烷][亲核体]。它需要一级或甲基底物和好的亲核体。SN1分两步：首先离去基团离去，形成平面碳正离子；然后亲核体从任一侧进攻，得到外消旋产物。速率 = k[卤代烷]。它倾向三级卤代烷和极性质子溶剂。

Exam questions often ask you to predict the mechanism based on the structure of the haloalkane and the nucleophile/conditions. Drawing the transition state or carbocation intermediate explicitly is essential.

考试常要求根据卤代烷结构和亲核体/条件预测机理。清晰地画出过渡态或碳正离子中间体至关重要。

4. Elimination Reactions | 消去反应

Elimination reactions often compete with nucleophilic substitution. In Unit 5, you encounter E2 elimination when a strong base (e.g., KOH in ethanol) reacts with a haloalkane. The base abstracts a β-proton while the leaving group departs, forming an alkene in a concerted process. E2 requires an anti-periplanar arrangement. For tertiary haloalkanes, E1 can also occur via carbocation intermediate, giving a mixture of alkenes following Zaitsev’s rule. Understanding the influence of base strength, steric hindrance, and temperature helps predict the major product.

消去反应常与

Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）