Reaction Mechanisms in Organic Chemistry | 有机化学反应机理

📚 Reaction Mechanisms in Organic Chemistry | 有机化学反应机理

Reaction mechanisms are the step-by-step sequences that describe how starting materials transform into products at the molecular level. They show the making and breaking of bonds, the movement of electrons, and the fleeting intermediates that determine the outcome of a chemical process. For A-level chemistry students, mastering mechanisms is the key to predicting reactivity, explaining selectivity, and designing synthetic routes.

反应机理是在分子水平上描述起始物如何转化为产物的逐步序列。它们展示了化学键的生成与断裂、电子的移动以及决定化学过程结果的短寿命中间体。对于 A 级化学学生而言，掌握机理是预测反应性、解释选择性和设计合成路线的关键。

1. Introduction to Reaction Mechanisms | 反应机理导论

A reaction mechanism comprises a series of elementary steps, each representing a single molecular event such as a bond fission or a collision that leads directly to products. The overall stoichiometric equation is the sum of these elementary steps. Curly arrows are the universal language for illustrating electron flow: a full arrow means two electrons move, while a half-arrow (fishhook) means one electron moves.

一个反应机理由一系列基元步骤组成，每一步都代表一个单一的分子事件，比如一次化学键断裂或一次直接生成产物的碰撞。总化学计量方程式是这些基元步骤的总和。弯箭头是描绘电子流动的通用语言：全箭头表示一对电子移动，而半箭头（鱼钩箭头）表示单个电子移动。

Intermediates such as carbocations, carbanions and free radicals appear inside the mechanism but never in the overall balanced equation. Transition states – high‑energy structures that exist only at the top of the energy barrier – are not isolated. The rate‑determining step is the slowest elementary step and controls the kinetic rate law.

碳正离子、碳负离子和自由基等中间体出现在机理内部，但绝不出现于总配平方程中。过渡态是仅存在于能垒顶部的极高能结构，无法被分离。决速步是最慢的基元步骤，控制着动力学速率方程。

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

Curly arrows always start from an electron‑rich source – a lone pair, a π bond or a negative charge – and point towards an electron‑deficient centre such as a positive charge, a δ⁺ atom or an empty orbital. The tail shows the origin of the electrons and the head shows their destination.

弯箭头始终起始于富电子源——孤对电子、π 键或负电荷——并指向缺电子中心，如正电荷、带 δ⁺ 的原子或空轨道。箭头尾部显示电子的来源，头部显示其去向。

In every mechanistic step, charge and mass must balance. After drawing an arrow, check that every atom has a complete octet (or the appropriate number of electrons for hydrogen and second‑row elements) and that any formal charges are correctly assigned. A common error is to move too many electrons, resulting in impossible pentavalent carbon intermediates.

在每个机理步骤中，电荷与质量必须守恒。画出箭头后，应检查每个原子是否具有完整的八隅体（或氢和第二周期元素应有的电子数），以及形式电荷是否正确标示。常见错误是移动了过多电子，产生不可能的五价碳中间体。

3. Homolytic and Heterolytic Fission | 均裂与异裂

Covalent bonds can break in two fundamentally different ways. Homolytic fission is symmetrical: the bond splits so that each atom retains one of the bonding electrons, producing two neutral free radicals. It is triggered by heat or ultraviolet light and is drawn with a single‑handed half‑arrow. For example, Cl₂ → 2 Cl•.

共价键可以以两种根本不同的方式断裂。均裂是对称的：化学键分裂，使每个原子各保留一个成键电子，生成两个中性自由基。它由热或紫外光引发，并用单钩半箭头表示。例如，Cl₂ → 2 Cl•。

Heterolytic fission is unsymmetrical: both electrons from the bond move to the more electronegative atom, creating a cation and an anion. This is illustrated with a full curly arrow. In a polar solvent, H–Br can heterolyse to H⁺ and Br⁻. Heterolytic processes dominate ionic mechanisms such as nucleophilic substitution and electrophilic addition.

异裂是不对称的：来自化学键的两个电子都移向电负性更强的原子，生成一个阳离子和一个阴离子。这用全弯箭头表示。在极性溶剂中，H–Br 可异裂为 H⁺ 和 Br⁻。异裂过程主导离子型机理，如亲核取代和亲电加成。

4. Free Radical Substitution | 自由基取代

Alkanes react with halogens in the presence of UV light via a radical chain mechanism. The initiation step uses homolytic fission of the halogen: Br₂ → 2 Br•. Propagation steps keep the chain going: a bromine radical abstracts a hydrogen from methane to give HBr and a methyl radical (•CH₃). The methyl radical then attacks a Br₂ molecule, forming CH₃Br and regenerating Br•.

烷烃在紫外光存在下与卤素通过自由基链机理反应。引发步骤利用卤素的均裂：Br₂ → 2 Br•。链增长步骤使链继续：溴自由基从甲烷夺取一个氢，生成 HBr 和甲基自由基 (•CH₃)。随后甲基自由基进攻 Br₂ 分子，形成 CH₃Br 并再生 Br•。

Termination removes radicals from the cycle when two radicals combine: Br• + Br• → Br₂, •CH₃ + Br• → CH₃Br, or •CH₃ + •CH₃ → C₂H₆. The reaction typically yields a mixture of mono‑, di‑ and tri‑substituted products because the radical can abstract hydrogen from unreacted alkane or from the newly formed haloalkane.

终止反应使自由基脱离循环：两个自由基结合，如 Br• + Br• → Br₂，•CH₃ + Br• → CH₃Br，或 •CH₃ + •CH₃ → C₂H₆。该反应通常得到一取代、二取代和三取代产物的混合物，因为自由基可从尚未反应的烷烃或新生成的卤代烷上夺取氢。

The overall equation for monobromination of methane is CH₄ + Br₂ → CH₃Br + HBr. Radical substitution is not suitable for synthesis when a single pure product is required because of poor selectivity.

甲烷一溴代的总方程式为 CH₄ + Br₂ → CH₃Br + HBr。当需要单一纯产物时，自由基取代因其低选择性而不适用于合成。

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

The π electrons of an alkene act as a nucleophile, making the double bond susceptible to attack by electrophiles. The general addition mechanism has two stages. First, the electrophile (e.g., H⁺ from HBr) attaches to one carbon of the double bond, breaking the π bond and leaving a carbocation on the other carbon.

烯烃的 π 电子充当亲核试剂，使双键易受亲电试剂进攻。一般的加成机理分为两个阶段。首先，亲电试剂（例如来自 HBr 的 H⁺）连接到双键的一个碳上，断裂 π 键并在另一个碳上留下碳正离子。

Second, the negatively charged or electron‑rich species (Br⁻) rapidly attacks the carbocation to complete the addition. With ethene, the product is bromoethane (CH₃CH₂Br). For unsymmetrical alkenes, Markovnikov’s rule applies: the hydrogen adds to the

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