📚 Electrophilic Addition Reactions of Alkenes (IAL Combined Science 1.8.8) | 烯烃的亲电加成反应（IAL 组合科学 1.8.8）
Alkenes are a homologous series of unsaturated hydrocarbons containing at least one carbon– . Their reactivity is dominated by electrophilic addition reactions, in which the electron‐ iles and leads to the formation of saturated products. Mastery of the mechanisms, regioselectivity, and key reagents is essential for Edexcel IAL Combined Science Unit 1, particularly topic 1.8.8.
烯烃是含有至少一个碳碳双键（C=C）的不饱和烃同系物。它们的反应活性以亲电加成反应为主，富电子的双键吸引亲电试剂并形成饱和产物。掌握反应机理、区域选择性和关键试剂对 Edexcel IAL 组合科学第一单元，尤其是主题 1.8.8 至关重要。
1. Structure of Alkenes and Nature of the C=C Bond | 烯烃的结构与 C=C 键的性质
The carbon atoms in a C=C bond are sp² hybridised. Each carbon forms three sigma (σ) bonds – two with other atoms and one with the other carbon – nd above and below the plane of the molecule. This π bond is weaker and more exposed than the σ bond, making alkenes highly susceptible to attack by electrophiles.
C=C 键中的碳原子是 sp² 杂化的。每个碳形成三个 σ 键——两个与其他原子，一个与另一个碳——呈三角平面排列，键角约 120°。每个碳上剩余的 p 轨道侧面重叠，在分子平面的上下方形成一个 π 键。这一 π 键比 σ 键弱且更暴露，使烯烃极易受到亲电试剂的进攻。
The presence of restricted rotation about the double bond gives rise to geometrical isomerism (E/Z isomerism) when each doubly bonded carbon bears two different groups. This structural feature must be considered when predicting products of addition reactions.
双键的限制旋转导致了当每个双键碳上连接两个不同基团时会产生几何异构（E/Z 异构）。在预测加成反应的产物时必须考虑这一结构特征。
2. Definition of Electrophiles and the General Mechanism | 亲电试剂的定义与通用机理
An electrophile is an electron‐deficient species that can accept a pair of electrons. Common electrophiles in alkene reactions include H⁺ (from H–X), Br₂ (polarised as Brᵟ⁺–Brᵟ⁻), and H₂SO₄. The general mechanism of electrophilic addition proceeds in two stages: (1) the π electrons of the alkene attack the electrophile, forming a carbocation intermediate and a leaving group (or partial bond breaking); (2) a nucleophile (often the conjugate base of the electrophile) attacks the carbocation to form the final saturated product.
亲电试剂是缺电子的物种，可以接受一对电子。烯烃反应中常见的亲电试剂包括 H⁺（来自 H–X）、Br₂（极化为 Brᵟ⁺–Brᵟ⁻）和 H₂SO₄。亲电加成的通用机理分两步：(1) 烯烃的 π 电子进攻亲电试剂，形成碳正离子中间体和离去基团（或部分断键）；(2) 亲核试剂（通常是亲电试剂的共轭碱）进攻碳正离子，生成最终的饱和产物。
This pathway is described by curly‐arrow notation. The arrow always starts from a lone pair or a bond and moves to an electron‐deficient atom. Correct use of curly arrows is a core skill examined in this topic.
该路径用弯箭头表示。箭头总是从孤对电子或一根键出发，指向缺电子的原子。正确使用弯箭头是本主题考查的核心技能。
3. Addition of Hydrogen Halides (HBr, HCl, HI) | 卤化氢（HBr、HCl、HI）的加成
When an alkene reacts with a hydrogen halide such as HBr, the H⁺ acts as the electrophile. The π bond attacks the H, leading to the heterolytic fission of the H–Br bond. This generates a carbocation and a Br⁻ ion. The bromide ion, acting as a nucleophile, then attacks the carbocation to form a halogenoalkane.
当烯烃与氢卤酸（如 HBr）反应时，H⁺ 充当亲电试剂。π 键进攻 H，导致 H–Br 键异裂。这产生一个碳正离子和一个 Br⁻ 离子。然后溴离子作为亲核试剂进攻碳正离子，生成卤代烷。
For example:
CH₂=CH₂ + HBr → CH₃CH₂Br
例如：
CH₂=CH₂ + HBr → CH₃CH₂Br
4. Markovnikov's Rule and Carbocation Stability | 马氏规则与碳正离子稳定性
With unsymmetrical alkenes, the major product of H–X addition follows Markovnikov's rule: the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already present, while the halide attaches to the more substituted carbon. This results from the formation of the more stable carbocation intermediate. Carbocation stability follows the order: tertiary (3°) > secondary (2°) > primary (1°) > methyl, due to the positive inductive effect and hyperconjugation provided by alkyl groups.
对于不对称烯烃，H–X 加成的主要产物遵循马氏规则：氢原子加在原本氢原子较多的碳上，而卤素加在取代较多的碳上。这是因为反应过程中形成了更稳定的碳正离子中间体。碳正离子稳定性顺序为：叔（3°）> 仲（2°）> 伯（1°）> 甲基，原因是烷基提供的正诱导效应和超共轭作用。
Thus, propene with HBr yields mainly 2‐bromopropane rather than 1‐bromopropane. The carbocation path via the secondary carbocation (CH₃–ᵟC⁺H–CH₃) is energetically favoured over the primary one (CH₃–CH₂–C⁺H₂).
因此，丙烯与 HBr 反应主要得到 2-溴丙烷而非 1-溴丙烷。经由仲碳正离子（CH₃–ᵟC⁺H–CH₃）的路径在能量上比伯碳正离子（CH₃–CH₂–C⁺H₂）更有利。
| Carbocation type | Relative stability | Example |
|---|---|---|
| 3° (tertiary) | Most stable | (CH₃)₃C⁺ |
| 2° (secondary) | Moderate | (CH₃)₂CH⁺ |
| 1° (primary) | Less stable | CH₃CH₂CH₂⁺ |
| Methyl | Least stable | CH₃⁺ |
Note: In the presence of peroxides, HBr addition can follow an anti‐Markovnikov radical pathway, but this is not part of Edexcel Combined Science expectations for topic 1.8.8.
注意：在过氧化物存在下，HBr 的加成可遵循反马氏自由基路径，但这不在 Edexcel 组合科学主题 1.8.8 的要求范围内。
5. Addition of Halogens (Br₂, Cl₂) | 卤素（Br₂、Cl₂）的加成
A halogen molecule such as Br₂ is non‐polar overall but becomes polarised as it approaches the electron‐rich double bond. The π electrons induce a dipole, making the nearer bromine atom partially positive (Brᵟ⁺–Brᵟ⁻). The electrophilic Brᵟ⁺ accepts electrons from the alkene, forming a cyclic bromonium ion and a bromide ion (Br⁻). Subsequent nucleophilic attack by Br⁻ on the bromonium ion opens the ring, yielding a vicinal dibromoalkane.
像 Br₂ 这样的卤素分子整体是非极性的，但在接近富电子的双键时被极化。π 电子会诱导一个偶极，使较近的溴原子带部分正电荷（Brᵟ⁺–Brᵟ⁻）。亲电的 Brᵟ⁺ 接受来自烯烃的电子，形成一个环状的溴鎓离子和一个溴离子（Br⁻）。随后 Br⁻ 作为亲核试剂进攻溴鎓离子，使环打开，得到邻二溴代烷。
This reaction proceeds with anti‐stereochemistry: the two bromine atoms add to opposite faces of the original alkene. When carried out in water, the bromonium ion can be intercepted by the solvent, leading to a bromohydrin (bromoalcohol).
该反应以反式立体化学进行：两个溴原子加在原来烯烃平面的两侧。当反应在水中进行时，水分子可以拦截溴鎓离子，生成溴代醇（溴醇）。
CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
6. Addition of Steam (Hydration) – Synthesis of Alcohols | 水蒸气加成（水合）——醇的合成
Alkenes can be hydrated to produce alcohols. Industrially, this is carried out by passing the alkene and steam over a phosphoric acid (H₃PO₄) catalyst supported on silica at 300 °C and high pressure (60–70 atm). This heterogeneously catalysed addition is an example of electrophilic addition where the catalyst provides a surface proton for the initial attack.
烯烃可以通过水合生成醇。工业上，在硅胶载体上的磷酸（H₃PO₄）催化剂作用下，于 300 °C 和高压（60–70 atm）下将烯烃与水蒸气通过催化剂进行反应。这种多相催化加成是亲电加成的一个例子，催化剂提供初始攻击所需的表面质子。
For unsymmetrical alkenes, the reaction follows Markovnikov's rule, producing secondary or tertiary alcohols as the major product.
对于不对称烯烃，该反应遵循马氏规则，主产物为仲醇或叔醇。
CH₂=CH₂ + H₂O → CH₃CH₂OH
This reversible exothermic reaction is favoured by high pressure and moderate temperature. Ethanol produced by this method is a major feedstock for the chemical industry.
这一可逆放热反应在高压和中等温度下有利。此法生产的乙醇是化学工业的重要原料。
7. Hydrogenation – Addition of H₂ | 加氢——H₂ 的加成
Hydrogenation is the addition of hydrogen across a double bond, converting an alkene into an alkane. This process requires a metal catalyst such as nickel, palladium, or platinum to adsorb the H₂ and the alkene and weaken the H–H bond. The reaction is an example of heterogeneous catalysis and is widely used to harden unsaturated vegetable oils into margarine.
加氢是将氢气加至双键上，将烯烃转化为烷烃。这一过程需要金属催化剂（如镍、钯或铂）来吸附 H₂ 和烯烃并削弱 H–H 键。该反应是多相催化的一个例子，广泛用于将不饱和植物油氢化硬化制成人造黄油。
CH₂=CH₂ + H₂ → CH₃CH₃
The mechanism on the surface is not required in detail for Combined Science, but it is important to recognise that this is an addition reaction that reduces unsaturation.
组合科学不要求表面的详细机理，但重要的是认识到这是一步减少不饱和度的加成反应。
8. Test for Unsaturation Using Bromine Water | 用溴水检验不饱和性
ple chemical test for the presence of a C=C double bond is to shake the sample with orange– rs and the colourless dibromoalkane forms. This test is used to distinguish alkenes from alkanes, as alkanes do not react under these conditions (they require UV light for substitution).
检验 C=C 双键存在的简单化学测试是在无紫外线照射下，将样品与橙黄色的溴水一起振荡。如果存在烯烃，溴水会迅速褪色，因为发生了亲电加成并生成了无色的二溴代烷。该方法用于区分烯烃和烷烃，因为烷烃在此条件下不反应（它们需要紫外光才能发生取代反应）。
The safety note: bromine water is toxic and corrosive; it must be handled in a well‐ventilated area using appropriate protective equipment.
安全提示：溴水有毒且具腐蚀性；必须在通风良好的地方并使用适当的防护设备进行操作。
9. Carbocation Rearrangements During Addition | 加成过程中的碳正离子重排
Occasionally, the initially formed carbocation may rearrange to a more stable carbocation before nucleophilic attack. A 1,2‐hydride shift or a 1,2‐alkyl shift can occur if a more stable 3° or 2° carbocation can be generated from an adjacent carbon. This can lead to unexpected products, especially in reactions with H–X where the solvent can trap the rearranged cation. However, for typical Edexcel Combined Science questions, recognising the conditions for rearrangement is a higher‐order skill.
有时，最初生成的碳正离子在亲核进攻之前可能会重排为更稳定的碳正离子。如果相邻碳上能生成一个更稳定的 3° 或 2° 碳正离子，就可能发生 1,2-氢迁移或 1,2-烷基迁移。这可能导致生成意外产物，特别是在使用 H–X 的反应中，溶剂可以捕获重排后的阳离子。不过，对于典型的 Edexcel 组合科学题目，识别重排条件属于较高层次的技能。
10. Comparison of Reactivity and Conditions | 反应活性和条件的比较
| Reaction | Electrophile | Conditions | Product type |
|---|---|---|---|
| + HBr | H⁺ | Room temp, gas or conc. aq. | Bromoalkane |
| + Br₂ | Brᵟ⁺ | Room temp, in inert solvent | Dibromoalkane |
| + H₂O (steam) | H⁺ (from cat.) | H₃PO₄ catalyst, 300 °C, 60–70 atm | Alcohol |
| + H₂ | (catalyst surface) | Ni/Pt/Pd, 150 °C, moderate pressure | Alkane |
The rate of electrophilic addition depends on the electron density of the double bond; alkyl‐substituted alkenes react faster than ethene because the alkyl groups release electron density through the inductive effect, stabilising the transition state and the carbocation.
亲电加成的速率取决于双键的电子密度；烷基取代的烯烃反应快于乙烯，因为烷基通过诱导效应释放电子密度，稳定了过渡态和碳正离子。
11. Industrial and Biological Relevance | 工业与生物学联系
Electrophilic addition reactions are not limited to the laboratory. The industrial production of ethanol via hydration of ethene, the hardening of vegetable oils to make spreads, and the synthesis of important polymer precursors all rely on alkene addition chemistry. In biological systems, enzymes sometimes catalyse similar addition reactions, although via different mechanisms, during the metabolism of unsaturated fatty acids.
亲电加成反应不仅限于实验室。工业上通过乙烯水合生产乙醇、植物油氢化制造涂抹酱、以及重要聚合物前体的合成都依赖于烯烃的加成化学。在生物系统中，酶有时催化类似的加成反应（尽管机理不同），参与不饱和脂肪酸的代谢。
Understanding how to control regioselectivity and stereochemistry has had a huge impact on designing efficient synthetic routes in the pharmaceutical and agricultural sectors.
理解如何控制区域选择性和立体化学对制药和农业领域设计高效合成路线产生了巨大影响。
12. Key Summary Points for Revision | 复习要点总结
- Electrophile: An electron‐pair acceptor that attacks the alkene π bond.
亲电试剂：攻击烯烃 π 键的电子对接受体。 - Mechanism: Two‐step process – carbocation/bromonium ion formation, then nucleophilic attack.
机理：两步过程——碳正离子/溴鎓离子形成，随后亲核进攻。 - Regioselectivity: Follow Markovnikov's rule for H–X and H₂O; anti‐addition for Br₂.
区域选择性：H–X 和 H₂O 遵循马氏规则；Br₂ 为反式加成。 - Carbocation stability: 3° > 2° > 1° – governs major product.
碳正离子稳定性：3° > 2° > 1°——决定主产物。 - Bromine water test: Decolourisation confirms unsaturation (C=C).
溴水试验：褪色即证实不饱和（C=C）。 - Curly arrows: Must show electron movement from bond/lone pair to electron‐deficient site.
弯箭头：必须表示电子从键/孤对电子向缺电子位的移动。
Master these concepts and practise drawing mechanisms with curly arrows to ensure full marks on the electrophilic addition section of the exam.
掌握这些概念并练习用弯箭头绘制机理，确保在考试亲电加成部分获得满分。
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