OxfordAQA CH04 Chemistry: Reaction Mechanisms | OxfordAQA CH04 化学：反应机理

📚 OxfordAQA CH04 Chemistry: Reaction Mechanisms | OxfordAQA CH04 化学：反应机理

Welcome to this comprehensive revision guide on reaction mechanisms for the OxfordAQA International A-level Chemistry Unit 4 (CH04). Mastering electron-pushing arrows and understanding the step-by-step processes of organic reactions is essential for success in your written exam. This guide covers key mechanistic types including electrophilic addition, electrophilic substitution, nucleophilic addition, nucleophilic substitution, elimination, and free radical substitution. We also connect mechanisms to rate equations and the concept of the rate-determining step. Let’s dive deep into the curly arrow world.

欢迎阅读这份针对 OxfordAQA 国际 A-level 化学第四单元 (CH04) 的反应机理综合复习指南。掌握“电子推动”箭头并理解有机反应的分步过程对您的笔试成功至关重要。本指南涵盖关键的机理类型，包括亲电加成、亲电取代、亲核加成、亲核取代、消除反应和自由基取代。我们还将机理与速率方程及决速步的概念联系起来。让我们深入探究弯曲箭头的世界。

1. What Are Reaction Mechanisms? | 什么是反应机理？

A reaction mechanism describes the step-by-step sequence of elementary steps by which an overall chemical change occurs. Each step involves bond breaking and bond making, and the movement of electrons is shown using curly arrows. Understanding mechanisms allows chemists to predict products, design synthesis routes, and control reaction conditions. For example, the conversion of a halogenoalkane into an alcohol may look simple, but the mechanism reveals exactly how the hydroxide ion displaces the halide ion.

反应机理描述了整个化学变化发生的基本步骤的逐步顺序。每一步都涉及键的断裂和形成，电子移动用弯曲箭头表示。理解机理可以让化学家预测产物、设计合成路线并控制反应条件。例如，卤代烃转化为醇看似简单，但机理精确揭示了氢氧根离子如何取代卤离子。

2. Bond Fission: Homolytic vs Heterolytic | 键断裂：均裂与异裂

Bond breaking can be homolytic or heterolytic. Homolytic fission gives two radicals, each with one electron from the bond. A common example is the cleavage of a chlorine molecule under UV light: Cl₂ → 2Cl•. Heterolytic fission gives a cation and an anion; both electrons go to one atom. For instance, HBr can ionise as HBr → H⁺ + Br⁻. In mechanisms, curly arrows show electron pair movement in heterolytic processes, while fishhook arrows (single-barbed) are used for radical reactions.

键断裂可以是均裂或异裂。均裂产生两个自由基，每个各带一个来自化学键的电子。常见例子是氯分子在紫外光下裂解：Cl₂ → 2Cl•。异裂产生一个阳离子和一个阴离子；两个电子都流向一个原子。例如，HBr 可电离为 HBr → H⁺ + Br⁻。在机理中，弯曲箭头表示异裂过程中的电子对移动，而单钩箭头用于自由基反应。

3. Drawing Curly Arrows Correctly | 正确绘制弯曲箭头

Curly arrows must start from a source of electrons: a lone pair, a negative charge, or a bond (σ or π bond). They point towards an electron-deficient atom or a region of low electron density. Always draw the arrow from the electron pair to the atom, not the other way around. Double-headed arrows represent movement of an electron pair; single-headed arrows show movement of one electron in radical mechanisms. For example, in the nucleophilic attack of CN⁻ on propanone, the arrow starts at the lone pair on cyanide and ends at the carbonyl carbon.

弯曲箭头必须起始于电子来源：孤对电子、负电荷或化学键（σ键或π键）。箭头指向缺电子的原子或电子密度较低的区域。务必从电子对指向原子，而不是相反。双头箭头表示电子对的移动；单头箭头表示自由基机理中单个电子的移动。例如，在 CN⁻ 对丙酮的亲核进攻中，箭头起始于氰根离子上的孤对电子，指向羰基碳。

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

Alkenes react with electrophiles such as HBr, Br₂, or H₂SO

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