📚 Electrophilic Addition in Organic Chemistry | 有机化学中的亲电加成反应
Electrophilic addition is the characteristic reaction of alkenes, where the π bond acts as a nucleophile and attacks an electron‑deficient species — an electrophile. Mastering this mechanism is essential for IB and WJEC candidates, as it appears across multiple topics, including reaction mechanisms, isomerism, and synthetic routes. This article unpacks every crucial detail, from the general two‑step pathway to the nuances of Markovnikov orientation and carbocation rearrangements.
亲电加成是烯烃的特征反应，此时 π 键作为亲核试剂，进攻缺电子的物种——亲电试剂。掌握这一机理对 IB 和 WJEC 考生至关重要，因为它横跨反应机理、异构现象和合成路线等多个板块。本文将从通用的两步路径出发，逐一拆解马氏规则取向和碳正离子重排等所有关键细节。
1. Why Alkenes Undergo Electrophilic Addition | 烯烃为何能发生亲电加成
Alkenes contain a region of high electron density above and below the plane of the carbon‑carbon double bond. This π electron cloud is exposed and more loosely held than σ electrons, making it an attractive target for electrophiles — species that crave electrons, such as H⁺, Br₂, and H₃O⁺.
烯烃在碳碳双键平面的上下方拥有一个高电子密度区域。这个 π 电子云暴露在外，且比 σ 电子结合得更松散，使它成为亲电试剂（如 H⁺、Br₂ 和 H₃O⁺ 等渴求电子的物种）的绝佳目标。
The reaction is called “addition” because two reactants combine to form a single product, with the π bond breaking and two new σ bonds forming. The driving force is the conversion of a weaker π bond into two stronger σ bonds, releasing energy overall.
该反应被称为“加成”，是因为两个反应物结合生成一个产物，π 键断裂并形成两个新的 σ 键。其驱动力在于较弱的 π 键转化为两个更强的 σ 键，总体上释放能量。
2. The General Two‑Step Mechanism | 通用的两步机理
Electrophilic addition proceeds via two distinct steps. In the first, slow step, the electrophile is attacked by the π electrons, forming a carbocation intermediate and a negatively charged species. This step is rate‑determining because it requires the disruption of the π bond and the creation of an unstable carbocation.
亲电加成通过两个清晰的步骤进行。在第一步慢速步骤中，亲电试剂受到 π 电子的进攻，生成一个碳正离子中间体和一个带负电荷的物种。由于需要破坏 π 键并产生不稳定的碳正离子，该步骤是决速步。
In the second, fast step, the negatively charged species (often a nucleophile) donates a lone pair to the electron‑deficient carbocation, forming a new covalent bond. For example, with HBr, the Br⁻ ion attacks the carbocation to complete the addition.
在第二步快速步骤中，带负电荷的物种（通常为亲核试剂）将孤对电子提供给缺电子的碳正离子，形成新的共价键。例如，与 HBr 反应时，Br⁻ 离子进攻碳正离子，完成加成。
C₂H₄ + HBr → CH₃CH₂Br
C₂H₄ + HBr → CH₃CH₂Br
3. Electrophilic Attack: Heterolytic Bond Fission | 亲电进攻：异裂断键
When a symmetrical electrophile like Br₂ approaches the alkene, the electron‑rich double bond induces a dipole in the Br–Br molecule. The π electrons then form a bond with one bromine atom while the Br–Br bond breaks heterolytically — both electrons from the bond go to the leaving bromide ion. This results in a cyclic bromonium ion, not an open carbocation, as an intermediate.
当像 Br₂ 这样的对称亲电试剂靠近烯烃时，富电子的双键会在 Br–Br 分子中诱导出一个偶极。随后，π 电子与一个溴原子成键，同时 Br–Br 键异裂——键中的两个电子都转移到离去的溴负离子上。这会产生一个环状的溴鎓离子中间体，而非开链碳正离子。
With hydrogen halides, the H–X bond is already polarised, so the hydrogen atom acts as the electrophile directly. Heterolytic fission of H–X supplies the proton and leaves the halide ion ready for the second step.
对于氢卤酸，H–X 键已经极化，因此氢原子直接充当亲电试剂。H–X 的异裂提供质子，并留下卤负离子以待第二步反应。
4. Markovnikov's Rule and Regioselectivity | 马氏规则与区域选择性
When an unsymmetrical alkene reacts with an unsymmetrical electrophile, two regioisomers are theoretically possible. Markovnikov’s rule predicts that the hydrogen atom of the electrophile attaches to the carbon with the greater number of hydrogen atoms already attached, while the electrophilic fragment binds to the more substituted carbon.
当不对称烯烃与不对称亲电试剂反应时，理论上可能生成两种区域异构体。马氏规则预测，亲电试剂中的氢原子会连接到原先连有较多氢原子的碳上，而亲电碎片则连接到取代较多的碳上。
The modern justification rests on the stability of the carbocation intermediate. A more substituted carbocation (tertiary > secondary > primary) is stabilised by the +I (inductive) effect of surrounding alkyl groups, which donate electron density. Thus, the pathway proceeding through the more stable carbocation is favoured, directing the regiochemistry.
现代解释基于碳正离子中间体的稳定性。取代更多的碳正离子（三级 > 二级 > 一级）通过周围烷基的 +I（诱导）效应给出电子密度而得以稳定。因此，经过更稳定碳正离子的路径在能量上更有利，从而决定了区域化学取向。
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major) and not CH₃CH₂CH₂Br
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃（主要产物），而不是 CH₃CH₂CH₂Br
5. Carbocation Stability and Rearrangements | 碳正离子稳定性与重排
Carbocations are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of carbon atoms directly bonded to the positively charged carbon. The order of stability is 3° > 2° > 1° > methyl, driven by hyperconjugation and the inductive effect. This stability order is central to predicting major products.
碳正离子根据与带正电碳直接相连的碳原子数目，可分为一级（1°）、二级（2°）或三级（3°）。稳定性顺序为 3° > 2° > 1° > 甲基，这是由超共轭和诱导效应共同决定的。这一稳定性顺序对预测主产物至关重要。
Occasionally, a less stable carbocation formed initially can undergo a 1,2‑shift — a hydrogen atom or an alkyl group migrates with its bonding pair of electrons to the adjacent carbocation centre, creating a more stable carbocation. This leads to unexpected products, often tested in exam scenarios, for example with but‑1‑ene and HBr yielding some 2‑bromobutane via a hydride shift.
偶尔，最初形成的较不稳定的碳正离子可以发生 1,2‑迁移——一个氢原子或烷基带着其成键电子对迁移到相邻的碳正离子中心，生成一个更稳定的碳正离子。这会导致预料之外的产物，常常成为考试中的考点，例如丁‑1‑烯与 HBr 反应会经由氢负离子迁移生成一部分 2‑溴丁烷。
| Carbocation type / 碳正离子类型 | Example / 示例 | Relative stability / 相对稳定性 |
| Methyl / 甲基 | ⁺CH₃ | Least / 最低 |
| 1° primary / 一级 | CH₃CH₂⁺ | Low / 低 |
| 2° secondary / 二级 | (CH₃)₂CH⁺ | Moderate / 中等 |
| 3° tertiary / 三级 | (CH₃)₃C⁺ | Highest / 最高 |
6. Addition of Hydrogen Halides (HX) | 卤化氢 (HX) 的加成
Hydrogen halides (HCl, HBr, HI) add across alkenes in a straightforward electrophilic addition. The hydrogen halide bond is polar, with hydrogen being δ⁺. The alkene donates π electrons to the hydrogen, forming a carbocation on the more substituted carbon (following Markovnikov's rule), and the halide ion then quenches the cation.
卤化氢 (HCl、HBr、HI) 以直接的亲电加成方式加成到烯烃上。卤化氢键是极性的，氢带 δ⁺。烯烃将 π 电子提供给氢，在取代较多的碳上（遵循马氏规则）形成碳正离子，然后卤负离子猝灭该阳离子。
This reaction is often used to demonstrate regioselectivity. With propene and HBr, 2‑bromopropane dominates. Peroxide effect? Not here — only HBr under radical conditions gives anti‑Markovnikov products, which is a separate free‑radical addition mechanism, deliberately excluded from the electrophilic addition syllabus for typical exam boards, but worth knowing as a contrast.
该反应常被用来演示区域选择性。丙烯与 HBr 反应，2‑溴丙烷占主导。过氧化物效应？在此并不适用——只有 HBr 在自由基条件下才会给出反马氏产物，那属于独立的自由基加成机理，一般考纲明确排除在亲电加成之外，但作为对比仍然值得了解。
7. Addition of Halogens (Br₂ and Cl₂) | 卤素 (Br₂ 和 Cl₂) 的加成
When bromine or chlorine adds to an alkene, the mechanism proceeds through a cyclic halonium ion, not a discrete carbocation. This three‑membered ring intermediate ensures anti‑addition: the second halide ion must attack from the opposite face of the ring, giving a trans product.
溴或氯加成到烯烃上时，机理经过一个环状卤鎓离子，而不是离散的碳正离子。这个三元环中间体确保了反式加成：第二个卤负离子只能从环的背面进攻，生成反式产物。
The classic test for unsaturation involves the decolorisation of bromine water. Alkenes convert the orange‑brown bromine to a colourless dibromoalkane. This rapid colour change is a key qualitative test — but careful: phenols and enols also react, so context matters.
检验不饱和度的经典实验是溴水褪色。烯烃将橙棕色的溴转化为无色的二溴代烷。这一快速的颜色变化是关键的定性检验——但要注意：酚类和烯醇也会反应，因此情境很重要。
CH₂=CH₂ + Br₂ (in water) → CH₂BrCH₂Br
CH₂=CH₂ + Br₂（水中） → CH₂BrCH₂Br
8. Hydration of Alkenes: Acid‑Catalysed Addition of Water | 烯烃的水合反应：酸催化水加成
Alkenes can be hydrated to alcohols using steam and an acid catalyst, typically concentrated sulfuric or phosphoric acid. The mechanism mirrors that of HX addition: the acid provides a proton (electrophile), generating a carbocation, which is then attacked by a water molecule. Deprotonation yields the alcohol and regenerates the acid catalyst.
烯烃可通过水蒸气和酸催化剂（通常是浓硫酸或磷酸）水合生成醇。其机理与 HX 加成类似：酸提供一个质子（亲电试剂），生成碳正离子，随后水分子进攻该碳正离子。去质子化后得到醇并使酸催化剂再生。
Markovnikov's rule applies again: the major alcohol product is the one where the –OH group attaches to the more substituted carbon. For example, ethene gives ethanol, while propene gives propan‑2‑ol as the main product, not propan‑1‑ol.
马氏规则再次起作用：主要醇产物中，–OH 基团连在取代较多的碳上。例如，乙烯生成乙醇，而丙烯主要生成丙‑2‑醇，而不是丙‑1‑醇。
9. Stereochemistry of Electrophilic Addition | 亲电加成的立体化学
Stereochemical outcomes depend on the structure of the intermediate. With a planar carbocation, the nucleophile can attack from either face with equal probability, leading to a racemic mixture if a chiral centre is formed. This loss of stereochemical integrity is typical for HX additions where the carbocation is trigonal planar.
立体化学结果取决于中间体的结构。对于平面型碳正离子，亲核试剂可以从两个面以相等的概率进攻，因此若生成手性中心，就会得到外消旋混合物。这种立体化学完整性的丧失是 HX 加成（碳正离子为平面三角形）的典型特征。
In contrast, bromination via the bromonium ion gives exclusively anti‑addition. If the alkene is cyclic, the stereochemistry is fixed: a trans‑product results from a cycloalkene. This provides strong evidence for the cyclic bromonium ion mechanism and is a common exam question interpreting product stereochemistry.
相比之下，通过溴鎓离子进行的溴化反应专一地给出反式加成。如果烯烃为环状烯烃，立体化学就被锁定：环烯烃会生成反式产物。这为环状溴鎓离子机理提供了有力证据，也是常见的解释产物立体化学的考题。
10. Factors Affecting Reactivity of Alkenes | 影响烯烃反应活性的因素
The rate of electrophilic addition is directly influenced by the electron density of the double bond. Alkyl groups are electron‑donating via hyperconjugation and the inductive effect, so more substituted alkenes are more nucleophilic and react faster with electrophiles. A tetrasubstituted alkene adds an electrophile more rapidly than a monosubstituted one.
亲电加成的速率直接受双键电子密度的影响。烷基通过超共轭和诱导效应给出电子，因此取代更多的烯烃亲核性更强，与亲电试剂反应更快。四取代烯烃比单取代烯烃能更快地加成亲电试剂。
Conversely, electron‑withdrawing groups (e.g., –CF₃, –COOH) withdraw electron density from the double bond, making the alkene less reactive. Understanding these electronic effects helps predict yields and reaction conditions in multi‑step synthesis.
相反，吸电子基团（如 –CF₃、–COOH）从双键上撤走电子密度，使烯烃反应活性降低。理解这些电子效应有助于在多步合成中预测产率和反应条件。
11. Common Exam Pitfalls and How to Avoid Them | 常见考试陷阱及应对策略
Pitfall 1: Forgetting to draw the carbocation intermediate in the correct orientation. Always show the planar sp² hybridised carbocation with the empty p orbital clearly indicated, and draw the attacking nucleophile with a curved arrow showing lone‑pair donation.
陷阱 1：忘记以正确取向画出碳正离子中间体。务必画出平面 sp² 杂化的碳正离子，并清楚地标示空的 p 轨道，同时用弯箭头画出亲核试剂提供孤对电子的过程。
Pitfall 2: Mixing up Markovnikov and anti‑Markovnikov products. Unless free‑radical conditions are specified (e.g., peroxides), assume the reaction follows the normal electrophilic addition pathway, governed by carbocation stability.
陷阱 2：混淆马氏与反马氏产物。除非明确指定自由基条件（例如过氧化物），否则应假定反应遵循正常的亲电加成路径，受碳正离子稳定性控制。
Pitfall 3: Ignoring stereochemical details. When a bromonium ion is involved, you must indicate anti‑addition, and for open carbocations, acknowledge the formation of a mixture of stereoisomers where applicable.
陷阱 3：忽视立体化学细节。当涉及溴鎓离子时，必须标注反式加成；对于开链碳正离子，应注明在适当情况下会生成立体异构体的混合物。
12. Connecting Electrophilic Addition to Synthetic Pathways | 亲电加成与合成路线的关联
In IB and WJEC exams, electrophilic addition is rarely tested in isolation. It appears as a step in multi‑step synthesis schemes — converting alkenes into haloalkanes, alcohols, or further functionalised molecules. Being able to propose reagents and conditions, and predict intermediates, is a high‑level skill frequently assessed.
在 IB 和 WJEC 考试中，亲电加成很少孤立考查。它常作为多步合成路线中的一步出现——将烯烃转化为卤代烷、醇或进一步官能团化的分子。能够提出试剂和条件，并预测中间体，是经常被评估的高阶技能。
For example, converting ethene to ethanol requires H₂O/H⁺, while ethene to bromoethane goes via HBr, and ethene to 1,2‑dibromoethane uses Br₂. Drawing clear reaction maps linking starting materials to products with electrophilic addition as a key transformation is excellent revision practice.
例如，将乙烯转化为乙醇需要 H₂O/H⁺；乙烯至溴乙烷经由 HBr；乙烯至 1,2‑二溴乙烷使用 Br₂。画出清晰的反应路线图，以亲电加成为核心转化，将原料与产物连接起来，是绝佳的复习方法。
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