📚 Electrophilic Addition in Alkenes | 亲电加成反应考点精讲
Electrophilic addition is the most characteristic reaction of alkenes, where the π bond acts as a nucleophile and attacks an electrophile, leading to the formation of two new σ bonds. A thorough grasp of this mechanism is essential for predicting products, explaining regioselectivity (Markownikoff's rule), and understanding stereochemical outcomes. This article consolidates all key concepts required for IB and CIE Chemistry examinations, from the fundamental electron-pushing steps to the influence of carbocation stability on reaction pathways.
亲电加成是烯烃最具特征的反应，其中π键充当亲核试剂进攻亲电试剂，最终形成两个新的σ键。透彻掌握该机理对于预测产物、解释区域选择性（马氏规则）以及理解立体化学结果至关重要。本文整合了IB和CIE化学考试所需的所有核心概念，从基础的电子转移步骤到碳正离子稳定性对反应路径的影响，逐一精讲。
1. Introduction to Electrophilic Addition | 亲电加成反应简介
Alkenes contain an electron-rich carbon–carbon double bond composed of one σ bond and one π bond. The π electrons are loosely held and are exposed above and below the plane of the molecule, making them readily available for attack by species that are electron-deficient. These electron-loving reagents are called electrophiles. The typical reaction involves the breaking of the π bond and the addition of two atoms or groups across the former double bond, transforming sp² hybridised carbons into sp³ hybridised centres.
烯烃含有一个富电子的碳碳双键，由一个σ键和一个π键组成。π电子结合松散，分布在分子平面的上方和下方，因而很容易被缺电子物种进攻。这些亲电子的试剂被称为亲电试剂。典型的反应涉及π键断裂，两个原子或基团加成到原双键两端，使sp²杂化碳转变为sp³杂化中心。
2. The Electrophile and Nucleophile Concept | 亲电试剂与亲核试剂概念
An electrophile is a species that accepts an electron pair; it is either positively charged or contains an electron-deficient atom. Common electrophiles in alkene additions include H⁺ (from H–X or H₂SO₄), Br⁺ (generated from Br₂ polarisation), and Hg²⁺. The π bond of the alkene, being a region of high electron density, acts as a nucleophile (electron-pair donor). Understanding this donor–acceptor interaction is the key to drawing correct curly arrow mechanisms.
亲电试剂是接受电子对的物种；它要么带正电荷，要么含有缺电子的原子。烯烃加成中常见的亲电试剂有 H⁺（来自 H–X 或 H₂SO₄）、Br⁺（由 Br₂ 极化产生）和 Hg²⁺。烯烃的π键作为电子密度高的区域，充当亲核试剂（电子对给体）。理解这种给体—受体相互作用是正确绘制弯箭头机理的关键。
3. Mechanism Basics: Electrophilic Attack | 机理基础：亲电进攻
The first step of electrophilic addition involves the π electrons forming a bond to the electrophile (e.g., H⁺ from HBr). This step generates a carbocation intermediate and the expelled leaving group (e.g., Br⁻). The mechanism is drawn using curly arrows: one arrow from the centre of the π bond to the electrophilic atom, and if the electrophile is part of a molecule, a second arrow shows bond cleavage to release the leaving group. This bimolecular step is rate-determining in many additions.
亲电加成的第一步是π电子与亲电试剂（如 HBr 中的 H⁺）成键。这一步产生一个碳正离子中间体，并放出离去基团（如 Br⁻）。反应机理用弯箭头表示：一支箭头从π键中心指向亲电原子；若亲电试剂属于某个分子的一部分，则再用第二支箭头表示键断裂并释放离去基团。在许多加成反应中，这一双分子步骤是速率决定步骤。
4. Carbocation Intermediate Formation | 碳正离子中间体的形成
The addition of H⁺ to an unsymmetrical alkene may occur at either end of the double bond, leading to two possible carbocations. The observed selectivity is governed by carbocation stability: tertiary > secondary > primary > methyl. The more stable carbocation forms faster because the transition state leading to it is lower in energy (Hammond's postulate). This stability trend is due to hyperconjugation and the inductive effect of alkyl groups that donate electron density towards the positively charged carbon.
在不对称烯烃中，H⁺ 可以在双键的任意一端加成，形成两种可能的碳正离子。实际观察到的选择性与碳正离子稳定性有关：叔碳 > 仲碳 > 伯碳 > 甲基碳。更稳定的碳正离子形成得更快，因为通往它的过渡态能量更低（哈蒙德假说）。这一稳定性趋势源于超共轭效应和烷基的诱导效应——烷基向带正电的碳提供电子密度。
5. Markownikoff's Rule | 马氏规则
Markownikoff's rule states that in the addition of H–X to an unsymmetrical alkene, the hydrogen atom attaches to the carbon that already has the greater number of hydrogen substituents, while the X group attaches to the more substituted carbon. In mechanistic terms, this is because the more stable carbocation intermediate is formed preferentially. For example, propene + HBr gives mainly 2-bromopropane, not 1-bromopropane, because the secondary carbocation is much more stable than the primary one.
马氏规则指出，在 H–X 对不对称烯烃的加成中，氢原子加在原本连有较多氢的碳上，而 X 基团加在取代较多的碳上。从机理讲，这是因为反应优先经过更稳定的碳正离子中间体。例如，丙烯与 HBr 主要生成 2-溴丙烷，而非 1-溴丙烷，因为仲碳正离子远比伯碳正离子稳定。
6. Addition of Hydrogen Halides | 卤化氢的加成
Hydrogen halides (HCl, HBr, HI) add rapidly to alkenes at room temperature to form haloalkanes. The reactivity order is HI > HBr > HCl, correlating with bond strength and acidity. In the presence of peroxides, HBr undergoes anti-Markownikoff addition via a free-radical mechanism, but this applies only to HBr due to favourable energetics of the radical chain steps. Students must distinguish between the ionic (Markownikoff) and radical (anti-Markownikoff) pathways for HBr, a common examination pitfall.
卤化氢（HCl、HBr、HI）在室温下与烯烃迅速加成，生成卤代烷。反应活性顺序为 HI > HBr > HCl，这与键能和酸性相关。在过氧化物存在下，HBr 通过自由基机理发生反马氏加成，但这一行为仅限于 HBr，因为其自由基链步骤的能量变化适宜。学生必须区分 HBr 的离子型（马氏）和自由基型（反马氏）路径，这是考试中常见的陷阱。
7. Addition of Halogens (Br₂ and Cl₂) | 卤素的加成
Bromine and chlorine add to alkenes via a cyclic bromonium or chloronium ion intermediate, rather than an open carbocation. The approach of the π electrons polarises the Br–Br bond, generating a δ⁺ Br that acts as the electrophile. The resulting bridged intermediate forces the subsequent nucleophilic attack by Br⁻ to occur from the opposite face, giving exclusively anti addition. The decolourisation of bromine water is a standard test for unsaturation.
溴和氯通过环状溴𬭩离子或氯𬭩离子中间体与烯烃加成，而不是通过敞开的碳正离子。π电子靠近时极化 Br–Br 键，产生 δ⁺ 溴作为亲电试剂。由此形成的桥环中间体迫使后续 Br⁻ 的亲核进攻从背面发生，从而得到专一的 anti 加成。溴水褪色是检验不饱和度的标准方法。
8. Addition of Water: Hydration of Alkenes | 水的加成：烯烃水合
Water can be added across the double bond in the presence of an acid catalyst, typically concentrated sulfuric acid followed by hydrolysis, or directly using dilute sulfuric acid with heat. The reaction proceeds via protonation of the alkene to form the more stable carbocation, followed by nucleophilic attack by water and deprotonation to yield the alcohol. This follows Markownikoff's rule, so ethene gives ethanol, but propene gives propan-2-ol as the major product.
在酸催化下，水可以加成到双键上，通常先与浓硫酸反应再水解，或直接用稀硫酸加热。反应首先通过烯烃质子化生成较稳定的碳正离子，然后水进行亲核进攻，再去质子化得到醇。该反应遵循马氏规则，因此乙烯生成乙醇，而丙烯主要生成 2-丙醇。
| Method | Conditions | 备注 |
|---|---|---|
| Indirect hydration | conc. H₂SO₄ then H₂O, heat | Markownikoff addition |
| Direct hydration | H₃PO₄ catalyst, 300 °C, 60 atm | Industrial ethanol production |
9. Stereochemistry of Addition | 加成反应的立体化学
The stereochemical outcome depends on the intermediate. In hydrogen halide additions, the nucleophile (X⁻) can attack the planar carbocation from either face, leading to a racemic mixture if the carbocation is chiral or becomes chiral upon addition. For halogen addition, the bridged ion ensures anti addition. When both the alkene and reagent are considered, syn addition is observed in catalytic hydrogenation (not electrophilic). Exam questions frequently ask to draw and label stereoisomers formed.
立体化学结果取决于中间体。在卤化氢加成中，亲核试剂（X⁻）可以从平面碳正离子的任一面进攻，如果碳正离子本身手性或加成后产生手性中心，通常会得到外消旋混合物。对于卤素加成，桥环离子保证了 anti 加成。若考虑催化加氢（非亲电加成），则观察到 syn 加成。考试常要求学生画出并标记所形成的立体异构体。
10. Carbocation Rearrangements | 碳正离子重排
When a less stable carbocation is generated adjacent to a more substituted carbon, a 1,2-shift of a hydride (H⁻) or alkyl group can occur to produce a more stable carbocation. This rearrangement leads to products that appear to violate Markownikoff's rule or produce unexpected skeletons. The driving force is the increased stability of the resulting carbocation. Recognising the possibility of rearrangement is crucial for predicting products in reactions of alkenes that form secondary carbocations next to tertiary centres.
当生成的碳正离子较不稳定，而相邻碳上连有更多取代基时，可能会发生 1,2-氢负离子（H⁻）或烷基迁移，生成更稳定的碳正离子。这种重排会导致产物表面上违反马氏规则或产生意料之外的骨架。驱动力是最终碳正离子稳定性的提高。在预测反应产物时，必须识别重排的可能性，尤其当烯烃加成形成的仲碳正离子邻近叔碳中心时。
11. Summary and Exam Tips | 总结与考试技巧
- Always draw the full curly arrow mechanism for electrophilic addition, showing the intermediate and all charges where appropriate. | 务必画出完整的亲电加成弯箭头机理，标出中间体和所有相应的电荷。
- Apply Markownikoff's rule only for ionic additions of H–X or H₂O/H⁺; be prepared to explain it in terms of carbocation stability. | 马氏规则仅适用于 H–X 或 H₂O/H⁺ 的离子型加成；要能从碳正离子稳定性角度进行解释。
- For halogen addition, show the bridged ion and specify anti stereochemistry. | 卤素加成要画出桥环离子，并明确标注 anti 立体化学。
- Watch for carbocation rearrangements when drawing products – check if a more stable carbocation can form via hydride or alkyl shift. | 画产物时注意碳正离子重排——检查是否可通过氢负离子或烷基迁移形成更稳定的碳正离子。
- Remember that HBr with peroxides follows the anti-Markownikoff radical pathway, giving the other regioisomer. | 记住 HBr 在过氧化物存在下遵循反马氏自由基路径，得到另一种区域异构体。
- bromine water decolourisation as a qualitative test for the presence of C=C double bonds. | 利用溴水褪色作为检验 C=C 双键的定性鉴定方法。
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