📚 Electrophilic Addition in A-Level CIE Chemistry | A-Level CIE 化学：亲电加成 考点精讲
Electrophilic addition is the most characteristic reaction of alkenes and a cornerstone of organic chemistry in the Cambridge International A-Level syllabus. Understanding how electron-rich π bonds attract electrophiles and generate carbocation intermediates is essential for predicting products, explaining reaction mechanisms, and applying Markovnikov’s rule. This article systematically breaks down every CIE examinable aspect of electrophilic addition, from stepwise mechanisms to stereochemical outcomes.
亲电加成是烯烃最具代表性的反应，也是剑桥国际A-Level化学有机部分的基石。理解富电子的π键如何吸引亲电试剂、生成碳正离子中间体，对于预测产物、阐述反应机理和应用马氏规则至关重要。本文系统梳理亲电加成在CIE考试中每一个可考查的要点，从分步机理到立体化学结果，助你彻底掌握。
1. Why alkenes undergo electrophilic addition | 烯烃为何发生亲电加成
Alkenes contain a carbon–carbon double bond consisting of a strong σ bond and a weaker π bond. The π electrons lie above and below the plane of the molecule, forming a region of high electron density. This electron-rich zone is readily attacked by electrophiles – species that are electron-deficient or carry a positive charge. Electrophilic addition results in the conversion of the π bond into two new σ bonds, saturating the carbon atoms.
烯烃含有一个碳碳双键，由一个牢固的σ键和一个较弱的π键组成。π电子云分布在分子平面的上下方，形成一个高电子密度区域。这个富电子区域容易被亲电试剂（缺电子或带正电荷的物种）进攻。亲电加成使π键转化为两个新的σ键，碳原子达到饱和。
2. Common electrophiles and how they are generated | 常见亲电试剂及其生成方式
Many electrophiles are neutral molecules that become polarised or produce positively charged intermediates in situ. For CIE, you must know the following electrophiles and their sources: Br⁺ or Brᵟ⁺–Brᵟ⁻ from Br₂; H⁺ from H–X (e.g. HBr, HCl) or from H₂SO₄; and the carbocation-like sulfur trioxide in fuming sulfuric acid. In aqueous or alcoholic media, H⁺ can also catalyse the addition of water or alcohols.
许多亲电试剂是中性分子，在反应中被极化或原位生成带正电的中间体。CIE要求掌握以下亲电试剂及其来源：Br₂产生Br⁺或Brᵟ⁺–Brᵟ⁻；H–X（如HBr、HCl）或H₂SO₄提供H⁺；发烟硫酸中的三氧化硫可视为亲电物种。在水或醇介质中，H⁺还能催化水或醇的加成。
3. The general two-step mechanism (electrophilic addition) | 通用两步机理（亲电加成）
The mechanism of electrophilic addition to an alkene proceeds via two key steps. Step 1: The π electrons attack the electrophile, forming a covalent bond to one carbon of the double bond and generating a carbocation on the other carbon. This is the slow, rate-determining step. Step 2: A nucleophile (often the negatively charged fragment left from the electrophile, such as Br⁻ or HSO₄⁻) rapidly attacks the carbocation to form the final saturated product.
烯烃亲电加成的机理分两步进行。第一步：π电子进攻亲电试剂，与双键的一个碳原子形成共价键，另一个碳上生成碳正离子。这是慢速的决速步骤。第二步：亲核试剂（通常是亲电试剂留下的带负电碎片，如Br⁻或HSO₄⁻）快速进攻碳正离子，形成最终饱和产物。
4. Bromination of ethene – a detailed walkthrough | 溴与乙烯加成——详细步骤拆解
When ethene gas is bubbled through bromine water or liquid bromine, the red-brown colour disappears. The π bond in ethene induces a dipole in the Br₂ molecule, making the nearer bromine electrophilic. The mechanism proceeds: (i) π electrons attack Brᵟ⁺, Br–Br breaks heterolytically, forming a cyclic bromonium ion (or an acyclic carbocation, depending on the discussion level – CIE accepts the bromonium ion intermediate); (ii) The bromide ion, Br⁻, attacks from the opposite face of the three-membered ring, giving 1,2-dibromoethane with anti stereochemistry.
乙烯气体通入溴水或液溴时，红棕色褪去。乙烯的π键使Br₂分子极化，靠近的溴原子成为亲电中心。机理为：(i) π电子进攻Brᵟ⁺，Br–Br异裂，生成环状溴鎓离子（根据讨论深度，CIE接受溴鎓离子中间体）；(ii) 溴离子Br⁻从三元环反面进攻，生成1,2-二溴乙烷，加成产物为反式构型。
5. Addition of hydrogen halides and Markovnikov’s rule | 卤化氢的加成与马氏规则
When HX adds to an unsymmetrical alkene, two structural isomers are theoretically possible. Markovnikov’s rule states that the hydrogen atom attaches to the carbon that already has more hydrogen atoms (the less substituted carbon), while the halogen attaches to the more substituted carbon. This is because the more stable carbocation intermediate is formed preferentially. For propene with HBr, the major product is 2-bromopropane, not 1-bromopropane.
当HX与不对称烯烃加成时，理论上可能生成两种结构异构体。马氏规则指出：氢原子加在含氢较多的碳（取代度较低的碳）上，卤素加在取代度较高的碳上。这是因为反应优先经过更稳定的碳正离子中间体。以丙烯与HBr为例，主产物是2-溴丙烷，而非1-溴丙烷。
6. Carbocation stability and its impact on regioselectivity | 碳正离子稳定性及其对区域选择性的影响
Carbocation stability increases with alkyl substitution: tertiary (3°) > secondary (2°) > primary (1°) > methyl. Alkyl groups are electron-donating through hyperconjugation and inductive effects, which help disperse the positive charge. In electrophilic addition, the regiochemistry is governed by the formation of the most stable carbocation. If rearrangement can yield a more stable carbocation (e.g. 1,2-hydride shift), the product distribution may change – a classic CIE pitfall.
碳正离子稳定性随烷基取代增多而增强：叔(3°) > 仲(2°) > 伯(1°) > 甲基。烷基通过超共轭效应和诱导效应给电子，帮助分散正电荷。亲电加成中，区域选择性由生成最稳定碳正离子的路径决定。若重排能产生更稳定的碳正离子（如1,2-氢负离子转移），产物分布可能改变——这是CIE经典的陷阱。
7. Addition of concentrated sulfuric acid | 浓硫酸的加成
Alkenes react with cold concentrated sulfuric acid to form alkyl hydrogensulfates. The electrophile is H⁺ from the acid, and the nucleophile is HSO₄⁻. For ethene: CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₃H. This product can be hydrolysed by warming with water to give ethanol, making it an indirect hydration route that avoids direct steam/phosphoric acid conditions. Markovnikov’s rule applies to unsymmetrical alkenes.
烯烃与冷的浓硫酸反应生成硫酸氢烷基酯。亲电试剂是酸中的H⁺，亲核试剂为HSO₄⁻。以乙烯为例：CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₃H。该产物经与水共热水解可得乙醇，这是一种间接水合法，避免了直接水蒸气/磷酸法的苛刻条件。马氏规则适用于不对称烯烃。
8. Acid-catalysed hydration of alkenes | 酸催化烯烃水合
Water can add across the double bond of an alkene in the presence of an acid catalyst, typically phosphoric acid absorbed on silica at 300°C and 60–70 atm. The electrophile is H⁺; the nucleophile is H₂O. The mechanism: protonation gives the more stable carbocation, water attacks, then deprotonation yields the alcohol. This follows Markovnikov’s rule, so propan-2-ol is the main product from propene.
在酸催化剂存在下，水可加成到烯烃双键上。工业条件通常为磷酸吸附在硅胶上，300°C、60–70个atm。亲电试剂为H⁺，亲核试剂为H₂O。机理：质子化形成更稳定碳正离子，水进攻，随后去质子化得到醇。反应遵循马氏规则，丙烯水合主产物为丙-2-醇。
9. Stereochemistry of electrophilic addition | 亲电加成的立体化学
When bromine adds to cyclohexene, the only product is trans-1,2-dibromocyclohexane. This anti addition occurs because the bromide ion attacks the bromonium ion from the side opposite to the bridging bromine, ensuring the two bromine atoms end up on opposite faces of the ring. For straight-chain alkenes, anti addition can lead to a pair of enantiomers if a chiral centre is generated. CIE questions often ask you to draw the three-dimensional outcome.
溴与环己烯加成时，唯一产物是反-1,2-二溴环己烷。这种反式加成是由于溴离子从桥连溴的背面进攻溴鎓离子，确保两个溴原子最终位于环平面的异侧。对于直链烯烃，反式加成若产生手性中心，则可能得到一对对映体。CIE常要求画出立体化学结果。
10. Testing for unsaturation: the bromine water test | 不饱和度的检验：溴水试验
The disappearance of the orange–brown colour when an organic liquid is shaken with bromine water indicates the presence of a carbon–carbon double bond. The electrophilic addition decolorises the bromine rapidly at room temperature without the need for UV light. This test distinguishes alkenes from alkanes and benzene, which do not react under these conditions. Ensure you can write the equation and describe the colour change clearly in the exam.
有机液体与溴水振荡后橙棕色褪去，表明存在碳碳双键。亲电加成在室温、无紫外光下即可使溴水迅速脱色。此试验可区分烯烃与烷烃及苯，后者在该条件下不反应。考试中需能书写方程式并清晰描述颜色变化。
11. Common mistakes and CIE examiner’s tips | 常见错误与考官提示
Students often confuse electrophilic addition with free-radical substitution or electrophilic substitution. Remember: alkenes undergo electrophilic addition, while alkanes with halogens in UV prefer free-radical substitution, and arenes undergo electrophilic substitution. Another error is carelessly applying Markovnikov’s rule without considering carbocation rearrangements or the possibility of a bromonium ion for halogens. Always draw the intermediate explicitly – if a bromonium ion forms, the mechanism changes slightly and dictates anti stereochemistry. Also, do not write ‘HBr adds to give 1-bromopropane’ for unsymmetrical alkenes under normal conditions.
考生常将亲电加成与自由基取代或亲电取代混淆。牢记：烯烃发生亲电加成，烷烃与卤素在紫外光下为自由基取代，芳烃为亲电取代。另一个错误是机械套用马氏规则却不考虑碳正离子重排或卤素加成时的溴鎓离子。务必画出中间体——若生成溴鎓离子，机理略有不同且强制反式加成。此外，一般情况下不对称烯烃与HBr加成，切勿写出生成1-溴丙烷。
12. Summary – the big picture for CIE exams | 总结——CIE考试全局视野
Electrophilic addition of alkenes is examined across multiple papers, from the core theory to practical applications. Master the stepwise mechanism, the concept of electrophile and nucleophile, Markovnikov’s rule grounded in carbocation stability, the special stereochemistry of halogen additions, and the hydration pathways. Being able to draw curly arrows precisely, explain the bromine water test, and predict major products for unsymmetrical alkenes will secure high marks on A-Level CIE Chemistry papers.
烯烃的亲电加成在CIE多份试卷中均有考查，从核心理论到实际应用。掌握分步机理、亲电试剂与亲核试剂概念、基于碳正离子稳定性的马氏规则、卤素加成的特殊立体化学以及水合路径。能够精确画出弯箭头、解释溴水试验、预测不对称烯烃的主产物，将确保你在CIE A-Level化学考试中拿下高分。
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