Electrophilic Addition: Key Points for CCEA A-Level Chemistry | 亲电加成 考点精讲

📚 Electrophilic Addition: Key Points for CCEA A-Level Chemistry | 亲电加成 考点精讲

Electrophilic addition is the characteristic reaction of alkenes and alkynes, involving the attack of an electron-deficient species on a π bond. For CCEA A-Level Chemistry, you must master the stepwise mechanisms, regioselectivity described by Markovnikov’s rule, carbocation stability, and the stereochemical outcomes of these additions. This article unpacks every key point to ensure you are fully prepared for the exam.

亲电加成是烯烃和炔烃的特征反应，涉及缺电子物种对π键的进攻。对于CCEA A-Level化学，你必须掌握分步机理、马氏规则描述的区域选择性、碳正离子稳定性以及这些加成反应的立体化学结果。本文拆解了每一个关键点，确保你为考试做好充分准备。

1. What is Electrophilic Addition? | 什么是亲电加成？

Electrophilic addition is a reaction in which an electrophile accepts an electron pair from the π bond of an unsaturated compound, typically an alkene, converting the π bond into two new σ bonds and producing a saturated product.

亲电加成是一种反应，亲电试剂接受不饱和化合物（通常是烯烃）π键的一对电子，将π键转化成两个新的σ键，生成饱和产物。

An electrophile is any electron-deficient species, such as a proton (H⁺), a carbocation, or the partially positive end of a polarised molecule. The π electrons of the double bond are exposed and electron‑rich, making alkenes attractive nucleophiles.

亲电试剂是任何缺电子物种，例如质子 (H⁺)、碳正离子，或者极性分子中带部分正电荷的一端。双键的π电子暴露出来且富含电子，这使得烯烃成为有吸引力的亲核试剂。

In CCEA exams, the most important electrophiles are hydrogen halides (HX), halogens (X₂), and the acidic species in hydration and sulfuric acid addition. The common feature is that the π bond donates a pair of electrons to form a covalent bond with the electrophile.

在CCEA考试中，最重要的亲电试剂是卤化氢 (HX)、卤素 (X₂) 以及水合和硫酸加成中的酸性物种。共同特征是π键提供一对电子与亲电试剂形成共价键。

2. General Mechanism of Electrophilic Addition | 亲电加成的一般机理

The mechanism of electrophilic addition to alkenes can be divided into two fundamental stages: (i) formation of a reactive intermediate by electrophilic attack, and (ii) nucleophilic capture of that intermediate to complete the addition.

烯烃亲电加成的机理可以分为两个基本阶段：(i) 亲电进攻形成活性中间体；(ii) 亲核试剂捕获该中间体完成加成。

When a symmetrical alkene reacts with HX, the electrophile H⁺ adds to one carbon of the double bond, generating a carbocation. The halide ion then acts as a nucleophile, rapidly bonding to the carbocation.

当对称烯烃与HX反应时，亲电试剂H⁺加到双键的一个碳上，生成碳正离子。然后卤离子作为亲核试剂迅速与碳正离子成键。

CH₂=CH₂ + H⁺ → CH₃CH₂⁺ (slow) ; CH₃CH₂⁺ + Br⁻ → CH₃CH₂Br (fast)

CH₂=CH₂ + H⁺ → CH₃CH₂⁺ (慢) ; CH₃CH₂⁺ + Br⁻ → CH₃CH₂Br (快)

For halogens such as Br₂, the electrophilic attack generates a cyclic bromonium ion, a three-membered ring with a positive charge on bromine. Subsequent attack by the bromide ion opens the ring to give the vicinal dibromide.

对于卤素如Br₂，亲电进攻生成环状溴鎓离子，即一个带正电荷的三元环。随后溴离子进攻开环，得到邻二溴代物。

Curly arrows must show the flow of electron pairs: from the π bond to the electrophile, and from the nucleophile to the electron-deficient centre. CCEA requires accurate mechanistic drawing, so always include all lone pairs and intermediate structures.

弯曲箭头必须标明电子对的流动：从π键流向亲电试剂，从亲核试剂流向缺电子中心。CCEA要求准确的机理图示，因此一定要画出所有孤对电子和中间体结构。

3. Addition of Hydrogen Halides (HX) | 卤化氢的加成

Hydrogen halides (HCl, HBr, HI) add across the double bond of alkenes at room temperature to form haloalkanes. The reaction is rapid and proceeds via a carbocation intermediate.

卤化氢 (HCl, HBr, HI) 在室温下与烯烃双键加成，形成卤代烷。反应快速进行，中间体为碳正离子。

The electrophile is the proton generated from the polar H–X bond. The regiochemistry of addition to unsymmetrical alkenes is governed by Markovnikov’s rule, leading to the more stable carbocation and thus the more substituted haloalkane as the major product.

亲电试剂是由极性H–X键产生的质子。不对称烯烃加成的区域化学由马氏规则控制，生成更稳定的碳正离子，因此主要

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