📚 Electrophilic Addition in Alkenes | 烯烃亲电加成考点精讲
Electrophilic addition is a fundamental reaction type in organic chemistry, especially for alkenes. In A-Level Edexcel Chemistry, you must master the mechanism, regioselectivity, and key reactions such as addition of hydrogen halides, halogens, and sulfuric acid. This guide covers all essential points tested in exams.
亲电加成是烯烃的典型反应，也是A-Level Edexcel化学的核心考点。你需要掌握反应机理、区域选择性以及卤化氢、卤素、硫酸等加成反应。本文将系统梳理所有必考知识点。
1. What is Electrophilic Addition? | 什么是亲电加成？
Alkenes contain a π bond which is an area of high electron density. This makes them attractive to electrophiles (electron-pair acceptors). Electrophilic addition involves the attack of an electrophile on the double bond, followed by addition of a nucleophile across the two carbon atoms.
烯烃的π键电子云密度高，容易受到亲电试剂（缺电子物种）的进攻。亲电加成反应中，亲电试剂首先进攻双键，然后亲核试剂加成到两个碳原子上，最终π键断裂形成两个新的σ键。
2. The Electrophile and the Double Bond | 亲电试剂与双键
The C=C double bond consists of a σ bond and a π bond. The π electrons are exposed and can be donated to an electrophile, which must have a positive charge or a partial positive charge. Common electrophiles include H⁺ (from HX), Brδ⁺ (from Br₂ after polarisation), and SO₃ (from H₂SO₄).
C=C双键由σ键和π键组成，π电子暴露在分子平面上下，容易给予缺电子的亲电试剂。常见的亲电试剂有H⁺（来自HX的极化）、Brδ⁺（Br₂极化后产生的部分正电荷）以及SO₃（来自浓硫酸）。
3. General Mechanism of Electrophilic Addition | 亲电加成的一般机理
The mechanism proceeds in two steps. Step 1 (slow): The electrophile accepts a pair of π electrons, forming a carbocation intermediate and breaking the π bond. Curly arrows must start from the middle of the C=C bond and point towards the electrophile. Step 2 (fast): A nucleophile (often the conjugate base of the electrophile) attacks the carbocation using a lone pair, forming a new σ bond. The curly arrow goes from the nucleophile’s lone pair to the positively charged carbon.
反应分两步进行：第一步（慢）亲电试剂接受π电子，形成碳正离子中间体，π键断裂。弯箭头必须从双键中间画向亲电试剂。第二步（快）亲核试剂利用孤对电子进攻碳正离子，形成新σ键。弯箭头从孤对电子画向带正电的碳。
All charges must be displayed on intermediates, and lone pairs should be shown on the nucleophile. Edexcel mark schemes are strict about curly arrow placement, so practice drawing them accurately.
所有中间体必须标出正电荷，亲核试剂上须显示孤对电子。Edexcel评分标准对弯箭头的位置要求严格，因此务必多加练习确保画法准确。
4. Carbocation Stability and Markovnikov’s Rule | 碳正离子稳定性与马氏规则
Carbocation stability follows the order: tertiary (3°) > secondary (2°) > primary (1°) > methyl. This is due to the positive inductive effect (+I effect) of alkyl groups, which donate electron density and help disperse the positive charge. The more alkyl groups attached to the charged carbon, the more stable the carbocation.
碳正离子稳定性顺序为：叔碳正离子 > 仲碳正离子 > 伯碳正离子 > 甲基正离子。这是因为烷基具有给电子诱导效应（+I效应），能分散正电荷。连接在带电碳上的烷基越多，碳正离子越稳定。
Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon that already carries more hydrogen atoms. This leads to the formation of the more stable carbocation intermediate and thus the major product. For example, with propene (CH₃CH=CH₂) and HBr, the secondary carbocation CH₃-C⁺H-CH₃ is favoured over the primary CH₃-CH₂-CH₂⁺, so 2-bromopropane is the major product.
马氏规则指出：不对称烯烃与HX加成时，氢原子加到原先含氢较多的碳上，这样经过的碳正离子中间体更稳定，从而决定主产物。例如丙烯（CH₃CH=CH₂）与HBr反应，仲碳正离子 CH₃-C⁺H-CH₃ 比伯碳正离子 CH₃-CH₂-CH₂⁺ 更稳定，因此2-溴丙烷为主产物。
5. Addition of Hydrogen Halides (HX) | 卤化氢的加成
Alkenes react with hydrogen halides (HCl, HBr, HI) at room temperature to form halogenoalkanes. The reactivity order is HI > HBr > HCl, correlating with the H–X bond strength (weaker bond reacts faster). The H–X bond is polarised, with hydrogen carrying a δ+ charge and acting as the electrophile. The overall equation for ethene with HBr is: CH₂=CH₂ + HBr → CH₃CH₂Br.
烯烃与卤化氢（HCl, HBr, HI）在室温下反应生成卤代烷。活性顺序为HI > HBr > HCl，这与H–X键的强度有关（键越弱反应越快）。H–X键极化后，氢带δ+电荷作为亲电试剂。乙烯与HBr的总反应式为：CH₂=CH₂ + HBr → CH₃CH₂Br。
A typical exam question might ask you to draw the mechanism for propene + HBr, including all charges and curly arrows, and to explain the regioselectivity using relative carbocation stability. Always show the correct dipole on HBr (Hδ⁺–Brδ⁻) before the first arrow.
常见考题要求画出丙烯与HBr的反应机理（含全部电荷和弯箭头），并用碳正离子稳定性解释区域选择性。务必在第一个弯箭头前标明HBr的偶极（Hδ⁺–Brδ⁻）。
6. Addition of Halogens (Br₂ and Cl₂) | 卤素的加成
Alkenes react with bromine or chlorine at room temperature in the dark. As the Br₂ molecule approaches the electron-rich double bond, the Br–Br bond becomes polarised, generating a Brδ⁺–Brδ⁻ dipole. The Brδ⁺ acts as the electrophile and accepts a pair of π electrons, leading to the formation of a cyclic bromonium ion intermediate. In the second step, a bromide ion (Br⁻) attacks from the opposite face (anti addition) to give a dihalogenoalkane.
烯烃与溴或氯在室温避光条件下反应。当Br₂分子靠近富电子双键时，Br–Br键被极化，产生Brδ⁺–Brδ⁻偶极。Brδ⁺作为亲电试剂接受π电子，形成环状溴鎓离子中间体。第二步，溴负离子从背面进攻（反式加成），生成邻二卤代烷。
The overall equation for ethene is: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br. For an unsymmetrical alkene, stereochemistry should be considered, although Edexcel mainly focuses on the electrophilic addition aspect rather than detailed stereochemical outcomes. Nevertheless, mentioning the bromonium ion helps explain why addition is anti.
乙烯与溴的总反应式为：CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br。对于不对称烯烃，需考虑立体化学，但Edexcel主要考察亲电加成本身，对立体化学细节要求不深。不过提及溴鎓离子有助于解释反式加成的原因。
7. Addition of Sulfuric Acid and Hydration | 硫酸加成与水合反应
Cold concentrated sulfuric acid adds across the double bond via an electrophilic mechanism. The electrophile is H⁺ (from the acid), and the nucleophile is HSO₄⁻, forming an alkyl hydrogensulfate. For example, ethene gives CH₃CH₂OSO₂OH. Subsequent heating with water hydrolyses this ester to ethanol, regenerating sulfuric acid.
冷浓硫酸通过亲电机理加成到双键上：亲电试剂是H⁺（来自硫酸），亲核试剂是HSO₄⁻，生成烷基硫酸氢酯。例如乙烯生成CH₃CH₂OSO₂OH，然后加水加热水解得乙醇，同时再生硫酸。
Direct hydration of alkenes (e.g., ethene + steam) uses a phosphoric acid (H₃PO₄) catalyst at high temperature (~300°C) and pressure (~60 atm). The mechanism still involves protonation to form a carbocation, followed by nucleophilic attack by water. For unsymmetrical alkenes, Markovnikov’s rule applies, so propene gives propan-2-ol as the major product.
工业上乙烯直接水合采用磷酸（H₃PO₄）催化剂，在高温（约300°C）高压（约60 atm）下进行。机理仍包括质子化形成碳正离子以及水分子进攻。不对称烯烃遵循马氏规则，因此丙烯水合主产物为2-丙醇。
8. Anti-Markovnikov Addition (Peroxide Effect) | 反马氏加成（过氧化物效应）
Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导