📚 Electrophilic Addition | 亲电加成
Electrophilic addition is the characteristic reaction of alkenes, in which the π bond is broken and two new σ bonds are formed. An electrophile — an electron‑deficient species — attacks the electron‑rich double bond. This mechanism is fundamental to AQA A‑level Chemistry and explains the reactivity of unsaturated hydrocarbons with halogens, hydrogen halides, and sulfuric acid.
亲电加成是烯烃的特征反应，π 键断裂并形成两个新的 σ 键。亲电试剂（缺电子物种）进攻富电子的双键。这一机理是 AQA A‑level 化学的基础，解释了不饱和烃与卤素、卤化氢和硫酸的反应。
1. The Electron‑Rich Double Bond | 富电子的双键
The C=C double bond consists of one σ bond and one π bond. The π electrons lie above and below the plane of the molecule and are more exposed, creating a region of high electron density. This makes the double bond attractive to electrophiles (electron‑pair acceptors).
C=C 双键由一个 σ 键和一个 π 键组成。π 电子分布在分子平面的上下方，更为暴露，形成高电子密度区域。这使得双键对亲电试剂（电子对受体）具有吸引力。
Common electrophiles include H⁺ (from H–X), Br⁺ (from Br–Br with polarisation), and SO₃ (in the sulfonation of benzene, though here we focus on alkenes). The polarisability of the π cloud facilitates the initial attack.
常见亲电试剂包括 H⁺（来自 H–X）、Br⁺（来自极化的 Br–Br）和 SO₃（用于苯的磺化，但这里我们聚焦于烯烃）。π 电子云的可极化性促进了最初的进攻。
2. General Mechanism of Electrophilic Addition | 亲电加成的一般机理
The mechanism proceeds in two steps. Step 1: The electrophile accepts a pair of electrons from the double bond, forming a bond to one carbon and leaving the other carbon with a positive charge — this is the carbocation intermediate. Step 2: A nucleophile (often the leftover anion) quickly donates a pair of electrons to the carbocation, forming the second bond.
机理分两步进行。第一步：亲电试剂从双键接受一对电子，与一个碳成键，使另一个碳带正电荷——形成碳正离子中间体。第二步：亲核试剂（通常是残留的阴离子）迅速向碳正离子提供一对电子，形成第二个键。
The overall process converts an unsaturated alkene into a saturated product. For example, ethene + HBr → bromoethane.
总过程将不饱和烯烃转化为饱和产物。例如，乙烯 + HBr → 溴乙烷。
3. Addition of Hydrogen Halides (HX) | 卤化氢 (HX) 的加成
When gaseous HCl, HBr, or HI is bubbled through an alkene, the H–X bond polarises. The hydrogen, carrying a δ+ charge, acts as the electrophile. The first step produces the carbocation, and the halide ion (X⁻) acts as the nucleophile in the second step.
当气态 HCl、HBr 或 HI 通入烯烃时，H–X 键发生极化。带 δ+ 电荷的氢作为亲电试剂。第一步生成碳正离子，卤离子 (X⁻) 在第二步中充当亲核试剂。
For symmetrical alkenes (ethene, but‑2‑ene) only one product is possible. For unsymmetrical alkenes, the orientation of addition follows Markovnikov’s rule.
对于对称烯烃（乙烯、2‑丁烯）仅有一种产物。对于不对称烯烃，加成取向遵循马氏规则。
4. Markovnikov’s Rule and Carbocation Stability | 马氏规则与碳正离子稳定性
Markovnikov’s rule states: The hydrogen atom (or electrophile) attaches to the carbon that already has the greater number of hydrogen atoms. In mechanistic terms, the more stable carbocation is formed in the rate‑determining step.
马氏规则指出：氢原子（或亲电试剂）加到本身含氢较多的碳原子上。从机理上讲，在决速步骤中生成更稳定的碳正离子。
Carbocation stability increases with alkyl substitution: methyl < 1° < 2° < 3°. Alkyl groups donate electron density through positive inductive effect (+I), dispersing the positive charge.
碳正离子稳定性随烷基取代增加：甲基 < 1° < 2° < 3°。烷基通过正诱导效应 (+I) 提供电子密度，分散正电荷。
| Carbocation Type | Example | Relative Stability |
|---|---|---|
| Methyl (CH₃⁺) | No alkyl groups | Least stable |
| Primary (1°) | CH₃CH₂⁺ | Low |
| Secondary (2°) | (CH₃)₂CH⁺ | Moderate |
| Tertiary (3°) | (CH₃)₃C⁺ | Most stable |
When propene reacts with HBr, the secondary carbocation (CH₃–C⁺H–CH₃) forms preferentially over the primary (CH₃–CH₂–CH₂⁺), leading to 2‑bromopropane as the major product.
当丙烯与 HBr 反应时，倾向于生成二级碳正离子 (CH₃–C⁺H–CH₃) 而非一级 (CH₃–CH₂–CH₂⁺)，因此 2‑溴丙烷为主要产物。
5. Addition of Halogens (Br₂ and Cl₂) | 卤素 (Br₂ 和 Cl₂) 的加成
Halogens add to alkenes at room temperature. As the Br₂ molecule approaches the double bond, the π electrons induce a dipole, making the nearer Br atom δ+ and the farther δ−. This polarised Br–Br acts as the electrophile.
卤素在室温下与烯烃加成。当 Br₂ 分子接近双键时，π 电子诱导出偶极，使得较近的 Br 原子呈 δ+，较远的呈 δ−。这种极化的 Br–Br 作为亲电试剂。
The first step forms a cyclic bromonium ion (or chloronium ion, though bromonium is the classic example). The three‑membered ring contains a positively charged bromine. In the second step, the Br⁻ nucleophile attacks from the opposite side, giving anti stereochemistry.
第一步生成环状溴鎓离子（或氯鎓离子，但溴鎓是典型例子）。三元环含有一个带正电荷的溴。第二步中，Br⁻ 亲核试剂从反面进攻，得到反式立体化学。
This anti addition can be observed when cyclopentene reacts with Br₂ to give trans‑1,2‑dibromocyclopentane exclusively. In aqueous solution, competing nucleophiles like water can open the bromonium ion, leading to bromohydrin formation.
当环戊烯与 Br₂ 反应时，反式加成只生成反‑1,2‑二溴环戊烷。在水溶液中，水等竞争性亲核试剂可打开溴鎓离子，生成溴代醇。
6. Bromine Water Test for Unsaturation | 不饱和烃的溴水测试
The addition of bromine to an alkene is accompanied by a rapid colour change from orange‑brown to colourless. This qualitative test distinguishes alkenes (and other unsaturated compounds) from alkanes. Alkanes require UV light for free‑radical substitution and do not decolourise bromine water in the dark.
溴与烯烃加成伴随着从橙棕色到无色的快速颜色变化。这一定性测试可区别烯烃（及其他不饱和化合物）与烷烃。烷烃需要紫外光进行自由基取代，在黑暗中不会使溴水褪色。
The decolourisation is a result of the π bond breaking and the formation of the colourless dibromoalkane. The test is highly specific for carbon‑carbon multiple bonds under normal conditions.
褪色是由于 π 键断裂并生成无色的二溴代烷。该测试在正常条件下对碳‑碳重键具有高度专一性。
7. Addition of Sulfuric Acid | 硫酸的加成
Cold concentrated sulfuric acid adds to alkenes in an electrophilic addition. The electrophile is H⁺ (from H₂SO₄), and the nucleophile is the hydrogen sulfate ion HSO₄⁻.
冷的浓硫酸通过亲电加成与烯烃反应。亲电试剂是 H⁺（来自 H₂SO₄），亲核试剂是硫酸氢根离子 HSO₄⁻。
- Step 1: Alkene + H⁺ → carbocation (Markovnikov orientation).
- Step 2: Carbocation + HSO₄⁻ → alkyl hydrogen sulfate.
第一步：烯烃 + H⁺ → 碳正离子（马氏取向）。第二步：碳正离子 + HSO₄⁻ → 硫酸氢烷酯。
The product is an alkyl hydrogen sulfate. This is soluble in sulfuric acid, allowing alkenes to be absorbed from gaseous mixtures. Subsequent hydrolysis with water produces an alcohol — this is the basis of the industrial hydration of alkenes to make alcohols.
产物是硫酸氢烷酯，可溶于硫酸，从而使烯烃能从气体混合物中被吸收。随后加水水解生成醇——这是烯烃工业化水合制醇的基础。
8. Regioselectivity and Major vs Minor Products | 区域选择性与主、次产物
In unsymmetrical alkenes, the carbocation intermediate can be formed at two different carbons. The pathway that proceeds via the more stable carbocation has a lower activation energy and is favoured. This leads to regioselectivity — the preference for one constitutional isomer over another.
在不对称烯烃中，碳正离子中间体可在两个不同的碳上形成。经由更稳定碳正离子的路径具有较低的活化能，因此更有利。这导致了区域选择性——优先形成一种构造异构体而非另一种。
For example, addition of HCl to 2‑methylpropene gives predominantly 2‑chloro‑2‑methylpropane (via a 3° carbocation) rather than 1‑chloro‑2‑methylpropane (via a 1° carbocation).
例如，HCl 与 2‑甲基丙烯加成主要得到 2‑氯‑2‑甲基丙烷（经由 3° 碳正离子），而非 1‑氯‑2‑甲基丙烷（经由 1° 碳正离子）。
9. Carbocation Rearrangements | 碳正离子重排
Sometimes the initially formed carbocation can rearrange to a more stable one through a hydride (H⁻) or alkyl shift. This occurs when an adjacent carbon bears a hydrogen or alkyl group that can migrate with its bonding pair of electrons.
有时最初生成的碳正离子可通过氢负离子 (H⁻) 或烷基迁移重排为更稳定的碳正离子。当相邻碳上带有可连同其键合电子对一起迁移的氢或烷基时，就会发生这种情况。
A classic example is the addition of HCl to 3‑methyl‑1‑butene. The secondary carbocation initially produced can undergo a 1,2‑shift to give a tertiary carbocation, leading to unexpected products like 2‑chloro‑2‑methylbutane.
一个经典例子是 HCl 与 3‑甲基‑1‑丁烯的加成。最初生成的二级碳正离子可发生 1,2‑迁移生成三级碳正离子，从而产生如 2‑氯‑2‑甲基丁烷等意料之外的产物。
Recognising potential rearrangements is important when predicting products in exam scenarios.
在考试中预测产物时，识别潜在的重排很重要。
10. Summary of Key Electrophilic Addition Reactions | 关键亲电加成反应总结
| Reagent | Electrophile | Product Type | Regiochemistry |
|---|---|---|---|
| HX (X=Cl, Br, I) | H⁺ | Haloalkane | Markovnikov |
| Br₂ / Cl₂ | Br⁺ (via Br–Br dipole) | Vicinal dihalide (anti addition) | Not applicable for symmetrical alkenes |
| H₂SO₄ (cold, conc.) | H⁺ | Alkyl hydrogen sulfate | Markovnikov |
| H₂O (steam, H₃PO₄ cat.) | H⁺ | Alcohol | Markovnikov |
In each case the first step is attack by the electrophile, generating the most stable carbocation or a cyclic ion intermediate. The second step is rapid nucleophilic capture.
在每种情况下，第一步是亲电试剂的进攻，生成最稳定的碳正离子或环状离子中间体。第二步是快速的亲核捕获。
11. Common Exam Pitfalls | 常见考试易错点
- Curly arrows: Always show movement of an electron pair from the double bond to the electrophile, and from the nucleophile to the carbocation. Arrows must start from the bond or lone pair.
弯箭头：始终显示电子对从双键向亲电试剂的移动，以及从亲核试剂向碳正离子的移动。箭头必须始于键或孤对电子。 - Carbocation stability: Avoid drawing a primary carbocation when a more stable secondary or tertiary option exists.
碳正离子稳定性：存在更稳定的二级或三级选项时，避免画出一级碳正离子。 - Bromination mechanism: Use the bromonium ion (not a planar carbocation) for addition of Br₂ to alkenes. Mention anti addition.
溴化机理：对于 Br₂ 与烯烃的加成，使用溴鎓离子（而非平面碳正离子），并提及反式加成。 - Markovnikov application: Only for unsymmetrical alkenes; symmetrical alkenes give a single product.
马氏规则的应用：仅适用于不对称烯烃；对称烯烃只得到一种产物。
12. Practice and Revision Tips | 练习与复习建议
Draw out the full mechanism for at least three different unsymmetrical alkenes reacting with HBr, including the cation intermediate and curly arrows. Then practise the bromine water mechanism with cyclopentene or cylohexene to master the bromonium ion structure.
至少画出三种不同的不对称烯烃与 HBr 反应的完整机理，包括阳离子中间体和弯箭头。然后练习环戊烯或环己烯的溴水机理，掌握溴鎓离子的结构。
Use molecular models or online simulations to visualise the planar carbocation and the three‑membered bromonium ring. This helps in understanding why anti addition occurs with halogens.
使用分子模型或在线模拟来可视化平面碳正离子和三元溴鎓环。这有助于理解为什么卤素会进行反式加成。
Always link the regiochemical outcome to the relative stability of carbocations, and use the phrase “lower activation energy via more stable intermediate” in explanations.
始终将区域化学结果与碳正离子的相对稳定性联系起来，并在解释中使用“通过更稳定的中间体，活化能更低”这一表述。
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