📚 Electrophilic Addition | 亲电加成
Electrophilic addition is the characteristic reaction of alkenes, where the carbon–carbon double bond acts as a region of high electron density and attracts electrophiles. This mechanism explains how unsaturated molecules become saturated and is a core part of GCSE Edexcel Chemistry.
亲电加成是烯烃的特征反应，碳碳双键作为高电子密度区域吸引亲电试剂。该机理解释了不饱和分子如何变为饱和分子，是 GCSE Edexcel 化学的核心内容。
1. What is Electrophilic Addition? | 什么是亲电加成？
Electrophilic addition is a reaction in which an electrophile attacks an electron-rich alkene, breaking the π bond and forming two new σ bonds. The product is a saturated compound with no double bond.
亲电加成是一个亲电试剂进攻富电子的烯烃、断裂π键并形成两个新σ键的反应。产物是没有双键的饱和化合物。
An electrophile is a species that is electron-deficient and can accept a pair of electrons. Common electrophiles include H⁺, Br₂ (polarised), and HBr.
亲电试剂是缺电子并能接受一对电子的物种。常见的亲电试剂有 H⁺、极化的 Br₂ 和 HBr。
2. Electrophiles and the Pi Bond | 亲电试剂与π键
d and a π bond. The π bond lies above and below the plane of the atoms and is more exposed, making it an electron-rich region that electrophiles find attractive.
C=C 双键由一个σ键和一个π键组成。π键位于原子平面的上下方，暴露在外，形成了一个富电子区域，吸引亲电试剂。
When an electrophile approaches, the π electrons are donated to form a new bond with the electrophile. This leaves the other carbon electron-deficient, generating a carbocation intermediate.
当亲电试剂靠近时，π电子被给出与亲电试剂形成新键。另一个碳原子因此缺电子，形成碳正离子中间体。
3. Basic Mechanism: A Two-Step Process | 基本机理：两步过程
Step 1: The electrophile accepts the π electrons, creating a carbocation and a negatively charged species. Step 2: The negatively charged species (or a nucleophile) attacks the carbocation, forming the final addition product.
第一步：亲电试剂接受π电子，生成碳正离子和一个带负电荷的物种。第二步：带负电荷的物种（或亲核试剂）进攻碳正离子，形成最终加成产物。
omide: CH₂=CH₂ + HBr → CH₃CH₂Br. The H⁺ first adds to one carbon, giving a carbocation CH₃–C⁺H₂, then Br⁻ attacks to give bromoethane.
例如，乙烯与溴化氢反应：CH₂=CH₂ + HBr → CH₃CH₂Br。H⁺ 先加到一个碳上，得到碳正离子 CH₃–C⁺H₂，然后 Br⁻ 进攻得到溴乙烷。
4. Reaction with Halogens: Bromination | 烯烃与卤素反应：溴化反应
Alkenes react with bromine (Br₂) at room temperature. The non-polar Br₂ molecule becomes polarised as it approaches the electron-rich double bond, making one end Brᵟ⁺ and the other Brᵟ⁻.
烯烃在室温下与溴 (Br₂) 反应。非极性的 Br₂ 分子靠近富电子双键时发生极化，一端变为 Brᵟ⁺，另一端变为 Brᵟ⁻。
The Brᵟ⁺ acts as an electrophile and adds to the alkene, forming a cyclic bromonium ion intermediate (or a carbocation, depending on conditions), followed by attack of Br⁻ to yield a dibromoalkane.
Brᵟ⁺ 作为亲电试剂加成到烯烃上，形成环状溴鎓离子中间体（或碳正离子，视条件而定），随后 Br⁻ 进攻生成二溴代烷。
CH₂BrCH₂Br. The orange-brown colour of bromine water disappears, a test for unsaturation.
典型方程：CH₂=CH₂ + Br₂ → CH₂BrCH₂Br。溴水的橙棕色褪去，这是检验不饱和性的方法。
5. Reaction with Hydrogen Halides | 烯烃与卤化氢反应
Hydrogen halides (HCl, HBr, HI) add across the double bond. The H–X bond is already polarised: Hᵟ⁺ and Xᵟ⁻. The H⁺ (or Hᵟ⁺) adds first as the electrophile.
卤化氢（HCl, HBr, HI）加成到双键上。H–X 键本身已极化：Hᵟ⁺ 和 Xᵟ⁻。H⁺（或 Hᵟ⁺）首先作为亲电试剂加成。
With ethene and HBr, the product is bromoethane. However, with unsymmetrical alkenes, two possible products can form, and the major product is determined by carbocation stability (Markovnikov's rule).
乙烯与 HBr 反应得到溴乙烷。对于不对称烯烃，可能生成两种产物，主要产物由碳正离子稳定性决定（马氏规则）。
6. Markovnikov's Rule | 马氏规则
Markovnikov's rule states that when a hydrogen halide adds to an unsymmetrical alkene, the hydrogen atom attaches to the carbon that already has more hydrogen atoms, while the halogen attaches to the more substituted carbon.
马氏规则指出，当卤化氢与不对称烯烃加成时，氢原子加到含氢较多的碳原子上，卤素加到取代较多的碳原子上。
us HBr gives mainly 2-bromopropane (CH₃CHBrCH₃) rather than 1-bromopropane. This is because the secondary carbocation formed is more stable than the primary one.
例如，丙烯 (CH₃CH=CH₂) 与 HBr 反应主要得到 2-溴丙烷 (CH₃CHBrCH₃) 而非 1-溴丙烷。因为生成的二级碳正离子比一级碳正离子更稳定。
7. Carbocation Stability | 碳正离子稳定性
Carbocations are positively charged carbon intermediates. Their stability increases with the number of alkyl groups attached: tertiary (3°) > secondary (2°) > primary (1°) > methyl.
碳正离子是带正电荷的碳中间体。稳定性随烷基数目增加而升高：三级碳正离子 (3°) > 二级 (2°) > 一级 (1°) > 甲基。
Alkyl groups push electron density towards the positive carbon, dispersing the charge (positive inductive effect). A more stable carbocation is formed faster, leading to the major product.
烷基将电子密度推向正电碳，分散电荷（正诱导效应）。更稳定的碳正离子生成更快，导致主要产物的形成。
8. Reaction with Water: Acid-Catalysed Hydration | 烯烃与水反应：酸催化水合
Alkenes can add water (H₂O) in the presence of an acid catalyst, typically concentrated sulfuric acid or phosphoric acid. The reaction follows an electrophilic addition mechanism with H⁺ as the electrophile.
烯烃可在酸催化剂（通常为浓硫酸或磷酸）存在下与水加成。反应遵循亲电加成机理，H⁺ 作为亲电试剂。
₅OH. In the mechanism, a carbocation forms and then water acts as a nucleophile, followed by loss of H⁺ to form the alcohol.
总方程式：C₂H₄ + H₂O → C₂H₅OH。机理中先形成碳正离子，水作为亲核试剂进攻，随后失 H⁺ 生成醇。
With unsymmetrical alkenes, Markovnikov's rule applies: the OH group attaches to the more substituted carbon so that the more stable carbocation is involved.
对于不对称烯烃，适用马氏规则：OH 基团连接到取代较多的碳上，涉及更稳定的碳正离子。
9. Summary and Comparison | 总结与对比
The key feature of electrophilic addition is the breaking of a π bond in an alkene by an electrophile, followed by capture of a nucleophile. Common reagents: halogens, hydrogen halides, and water/acid.
亲电加成的关键特征是烯烃的 π 键被亲电试剂断裂，随后捕获亲核试剂。常见试剂：卤素、卤化氢和水/酸。
| Reagent | Product | Regioselectivity |
|---|---|---|
| X₂ (e.g., Br₂) | Dihaloalkane | Not applicable (symmetrical product) |
| HX (e.g., HBr) | Haloalkane | Markovnikov (X to more substituted C) |
| H₂O/H⁺ | Alcohol | Markovnikov (OH to more substituted C) |
Understanding the mechanism allows you to predict products, explain the colour change with bromine water, and appreciate the role of carbocation stability.
理解机理有助于预测产物、解释溴水颜色变化以及认识碳正离子稳定性的作用。
10. Exam Tips for GCSE Edexcel | 考试提分要点
Always show the curly arrow from the double bond to the electrophile. Use clear structural formulas and label the partial charges on polarised molecules like HBr and Br₂.
务必画出从双键指向亲电试剂的弯箭头。使用清晰的结构式，并标出极性分子如 HBr 和 Br₂ 的部分电荷。
When explaining Markovnikov's rule, link your answer to carbocation stability – compare primary, secondary and tertiary carbocations with the number of alkyl groups.
解释马氏规则时，将答案与碳正离子稳定性联系起来——用烷基数目比较一级、二级、三级碳正离子。
For the test of unsaturation, state that the orange-brown bromine water turns colourless when shaken with an alkene. This is an addition reaction, not a substitution.
关于不饱和性检验，指出橙棕色溴水与烯烃振摇后变为无色。这是加成反应，不是取代反应。
Be ready to draw the mechanism for ethene + Br₂ or propene + HBr, showing the intermediate and the movement of electron pairs.
准备绘制乙烯与 Br₂ 或丙烯与 HBr 的机理，展示中间体及电子对移动。
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