📚 Organic Reaction Types and Mechanisms | 有机反应类型与机理梳理
Organic chemistry can look like a long list of unrelated reactions. In fact, almost every transformation belongs to a small number of reaction types, and each type follows a recognisable mechanism. Mastering these patterns helps you predict products, choose the correct reagents, and explain exam mechanisms clearly.
有机化学表面上像是许多互不相关的反应，但几乎所有转化都属于有限的几种反应类型，而且每种类型都有清晰的机理规律。掌握这些规律，可以帮助你预测产物、选择合适的试剂，并在考试中清楚地解释反应机理。
1. Why Reaction Types and Mechanisms Matter | 为什么需要梳理反应类型与机理
A mechanism describes the step-by-step movement of electrons during a reaction. It tells you which bonds break, which bonds form, and which species donate or accept electron pairs.
机理描述反应中电子一步步移动的过程，它告诉你哪些键断裂、哪些键形成，以及哪种物种提供或接受电子对。
Curly arrows show electron movement. A full-headed arrow means a pair of electrons moves; a half-headed arrow means one electron moves in a radical step.
弯箭头表示电子移动：完整箭头表示一对电子移动，半箭头表示自由基步骤中单个电子移动。
Exam answers must include charges, lone pairs, and correct arrow positions. An arrow always starts from an electron-rich region and points to an electron-poor region.
考试答案必须包含电荷、孤对电子和正确的箭头位置。箭头总是从电子富集区域出发，指向电子缺乏区域。
If you can identify the reaction type, you can often predict the mechanism and therefore the product.
如果你能判断反应类型，通常就能推测出反应机理，从而预测产物。
2. Electrophiles, Nucleophiles and Radicals | 亲电试剂、亲核试剂与自由基
Organic mechanisms are easier to follow when you understand the main attacking species.
理解主要的进攻物种之后，有机机理会变得更容易掌握。
An electrophile is an electron-pair acceptor. It is often positively charged or polarised, for example H⁺, NO₂⁺, or the δ+ end of Br–Br.
亲电试剂是电子对的接受体，通常带正电或具有极性，例如 H⁺、NO₂⁺，或 Br–Br 中带 δ+ 的一端。
A nucleophile is an electron-pair donor. It carries a lone pair, for example OH⁻, CN⁻, NH₃, or H₂O.
亲核试剂是电子对的提供者，带有孤对电子，例如 OH⁻、CN⁻、NH₃ 或 H₂O。
A radical is a species with an unpaired electron, such as Cl· or ·CH₃. Radicals are usually formed by homolytic fission.
自由基是含有未成对电子的物种，例如 Cl· 或 ·CH₃。自由基通常由均裂产生。
In heterolytic fission, a covalent bond breaks unequally and forms ions. In homolytic fission, each atom keeps one electron and radicals form.
在异裂中，共价键不均匀断裂并生成离子；在均裂中，每个原子各保留一个电子，从而生成自由基。
3. Free Radical Substitution | 自由基取代反应
Alkanes are relatively unreactive, but they react with halogens in the presence of ultraviolet light by free radical substitution.
烷烃相对不活泼，但在紫外光作用下可与卤素发生自由基取代反应。
Example: CH₄ + Cl₂ → CH₃Cl + HCl under hν.
例如：CH₄ + Cl₂ → CH₃Cl + HCl，条件为光照 hν。
Initiation: Cl–Cl → 2Cl·. The chlorine–chlorine bond breaks homolytically.
链引发：Cl–Cl → 2Cl·。氯氯键发生均裂。
Propagation: Cl· + CH₄ → ·CH₃ + HCl, then ·CH₃ + Cl₂ → CH₃Cl + Cl·. These two steps repeat.
链增长：Cl· + CH₄ → ·CH₃ + HCl，然后 ·CH₃ + Cl₂ → CH₃Cl + Cl·。这两步不断重复。
Termination: any two radicals combine, for example ·CH₃ +
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