📚 A-Level Chemistry: Alkenes – Key Concepts and Exam Focus | A-Level 化学：烯烃 考点精讲
Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). They serve as a cornerstone of organic chemistry in the A-Level syllabus, underpinning key ideas about bonding, isomerism, and characteristic reaction mechanisms – especially electrophilic addition. This article provides a thorough revision of alkene structure, nomenclature, isomerism, properties, and their typical reactions, equipping you with the understanding needed to tackle both structured and multiple‑choice questions with confidence.
烯烃是含有至少一个碳碳双键（C=C）的不饱和烃。它们在A-Level有机化学中占据核心地位，涵盖键合、异构现象以及特征反应机理（特别是亲电加成）等关键概念。本文对烯烃的结构、命名、异构、性质及其典型反应进行全面梳理，帮助你建立扎实的理解，自信应对结构化试题和选择题。
1. Introduction to Alkenes | 烯烃简介
Alkenes have the general formula CnH2n for chain alkenes with one double bond. They are more reactive than alkanes because the π-bond is relatively weak and electron‑rich, making the double bond an attractive target for electrophiles. Common examples include ethene (C2H4), propene (C3H6), and butene isomers.
链状单烯烃的通式为 CnH2n。由于π键相对较弱且电子云密度高，烯烃比烷烃更活泼，双键容易受到亲电试剂的进攻。常见的例子包括乙烯（C2H4）、丙烯（C3H6）和丁烯的各种异构体。
2. Structure and Bonding | 结构与键合
The carbon atoms in the C=C double bond are sp² hybridised. Each carbon uses three sp² orbitals to form three σ-bonds (two with other atoms and one with the other carbon). The remaining unhybridised p-orbital on each carbon overlaps sideways to form a π-bond. This π-bond locks the molecule into a planar arrangement around the double bond, with bond angles of approximately 120°, and prevents free rotation – a fact that gives rise to geometrical isomerism.
碳碳双键中的碳原子为sp²杂化。每个碳用三个sp²轨道形成三个σ键（两个与其他原子，一个与另一个碳）。每个碳上剩余的未杂化p轨道通过肩并肩重叠形成一个π键。这个π键使双键周围的分子保持平面构型，键角约为120°，并限制自由旋转——这正是产生几何异构现象的原因。
3. Naming Alkenes | 烯烃命名
The IUPAC name of an alkene is based on the longest continuous carbon chain that contains the double bond. The suffix ‘‑ene’ replaces ‘‑ane’, and the position of the double bond is indicated by the lowest possible number. Substituents are named and numbered as usual. If stereochemistry is specified, E/Z or cis/trans prefixes are placed at the front.
烯烃的IUPAC命名以包含双键的最长连续碳链为基础。后缀‘‑ene’取代‘‑ane’，并用尽可能小的数字标示双键的位置。取代基按常规方法命名和编号。若需指明立体化学，则在名称最前端加上E/Z或顺/反标记。
Example: CH3CH=CHCH3 is but‑2‑ene | 例：CH3CH=CHCH3 为丁‑2‑烯
4. Isomerism in Alkenes (Cis‑Trans / E‑Z) | 烯烃异构现象（顺反 / E‑Z）
Because the double bond cannot rotate, alkenes can exhibit geometrical isomerism when each carbon of the double bond has two different groups attached. The traditional cis/trans system uses ‘cis’ when the highest‑priority groups are on the same side, and ‘trans’ when they are opposite. For more complex molecules, the IUPAC E/Z system is used, based on the Cahn–Ingold–Prelog priority rules. ‘Z’ (from German zusammen) means the two higher‑priority groups are on the same side; ‘E’ (from entgegen) means they are opposite.
由于双键不能旋转，当双键每个碳原子都连接两个不同的基团时，烯烃就能表现出几何异构。传统的顺/反命名法中，当优先基团位于同侧时称为‘顺’，对侧称为‘反’。对于更复杂的分子，则采用基于Cahn–Ingold–Prelog优先规则的E/Z系统。‘Z’表示两个较优先基团在同侧，‘E’表示在异侧。
This type of isomerism is crucial in physical and chemical properties: for instance, trans isomers often have higher melting points (better packing) but lower boiling points (less polar) than their cis counterparts.
这类异构现象对物理和化学性质影响很大：例如，反式异构体通常比顺式异构体熔点更高（分子排列更紧密），但沸点更低（极性较小）。
5. Physical Properties | 物理性质
Alkenes are non‑polar or only slightly polar molecules; their only intermolecular forces are van der Waals (London) forces. As a result, they are insoluble in water but dissolve in non‑polar organic solvents. Boiling points increase with molecular size (chain length) and decrease with branching – just as for alkanes. The double bond introduces a small degree of polarity, but the dominant interactions remain van der Waals forces.
烯烃是非极性或极性很弱的分子，分子间仅存在范德华力。因此它们不溶于水，但可溶于非极性有机溶剂。沸点随分子体积（碳链长度）增加而升高，随支链增多而降低——与烷烃类似。双键会引入微弱极性，但主要相互作用仍是范德华力。
6. Reactivity of Alkenes: Electrophilic Addition | 烯烃反应性：亲电加成
The high electron density of the π-bond makes alkenes susceptible to attack by electrophiles (electron‑pair acceptors). The typical reaction is electrophilic addition, in which the π-bond breaks and two new σ-bonds are formed. The mechanism proceeds via a carbocation intermediate (or a cyclic intermediate, depending on the reagent). This mechanism explains why unsymmetrical alkenes often give a mixture of products, with Markovnikov’s rule predicting the major product.
π键的高电子密度使烯烃容易受到亲电试剂（电子对接受体）的进攻。典型反应为亲电加成，其中π键断裂并形成两个新的σ键。机理通常经由碳正离子中间体（或依据试剂不同经由环状中间体）进行。这一机理也解释了为什么不对称烯烃常常生成混合物，而马氏规则预测主产物。
7. Addition Reactions with Hydrogen and Halogens | 与氢和卤素的加成反应
Hydrogenation: Alkenes react with hydrogen gas in the presence of a metal catalyst (nickel, platinum, or palladium) to form alkanes. This is an addition reaction used in industry to harden vegetable oils.
加氢反应：烯烃在金属催化剂（镍、铂或钯）存在下与氢气反应生成烷烃。工业上利用此反应硬化植物油。
RCH=CH₂ + H₂ → RCH₂CH₃ (Ni catalyst, heat) | 烯烃加氢：RCH=CH₂ + H₂ → RCH₂CH₃（Ni催化，加热）
Halogenation: Alkenes react readily with halogens (e.g. Br₂, Cl₂) at room temperature. The reaction with bromine water is the standard test for unsaturation: the orange bromine water is decolourised as the halogen adds across the double bond to form a dibromoalkane. The mechanism for bromination involves a cyclic bromonium ion, which gives anti addition.
卤素加成：烯烃在室温下极易与卤素（如 Br₂、Cl₂）反应。与溴水的反应是检验不饱和度的标准方法：橙黄色的溴水褪色，表明卤素加成了双键，生成二溴代烷。溴加成的机理涉及环状溴鎓离子，从而得到反式加成产物。
CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br | 乙烯与溴加成：CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
8. Addition of Hydrogen Halides and Markovnikov’s Rule | 卤化氢加成与马氏规则
Alkenes add hydrogen halides (HCl, HBr, HI) to form halogenoalkanes. With unsymmetrical alkenes, two structural isomers are possible. Markovnikov’s rule states that the hydrogen atom of H–X attaches to the carbon with the greater number of hydrogen atoms already attached (the “rich get richer”), while the halogen goes to the more substituted carbon. This outcome is controlled by the stability of the carbocation intermediate: a tertiary carbocation is more stable than secondary, which is more stable than primary.
烯烃与卤化氢（HCl、HBr、HI）加成生成卤代烷。对于不对称烯烃，理论上可得到两种结构异构体。马氏规则指出：H–X 中的氢原子加到含氢较多的双键碳上（‘富者愈富’），而卤素则加到取代较多的碳上。这一结果由碳正离子中间体的稳定性决定：叔碳正离子比仲碳正离子稳定，仲碳正离子比伯碳正离子稳定。
CH3CH=CH2 + HBr → CH3CHBrCH3 (major) + CH3CH2CH2Br (minor) | 丙烯与HBr：CH3CHBrCH3 （主） + CH3CH2CH2Br （次）
9. Hydration of Alkenes | 烯烃的水合反应
Alkenes can be converted to alcohols by direct hydration with steam in the presence of an acid catalyst, typically concentrated phosphoric acid (H3PO4) supported on silica. This is an industrial route to ethanol from ethene. The addition follows Markovnikov’s rule: water adds with the –OH group ending up on the more substituted carbon. The reaction conditions typically require temperatures around 300 °C and pressures of 60–70 atm.
烯烃可在酸催化剂（通常为负载于硅胶上的浓磷酸 H3PO4）存在下，与水蒸气直接水合生成醇。这是工业上由乙烯制乙醇的路线。加成遵循马氏规则：水分子中的 –OH 最终连在取代较多的碳上。反应条件通常需约300 °C和60–70 atm的压力。
CH2=CH2 + H2O ⇌ CH3CH2OH | 乙烯水合：CH2=CH2 + H2O ⇌ CH3CH2OH
10. Oxidation with Acidified KMnO₄ | 酸性高锰酸钾氧化
Alkenes are readily oxidised by cold, dilute, acidified potassium manganate(VII). The purple solution is decolourised, and a diol is formed. This reaction is another useful test for the presence of a C=C double bond. Under harsher conditions (hot, concentrated KMnO₄), oxidative cleavage occurs, breaking the double bond and producing carbonyl compounds or carboxylic acids, which can help deduce the position of the double bond in an unknown alkene.
烯烃易被冷的、稀的酸性高锰酸钾氧化。紫色溶液褪色，同时生成二醇。该反应也是检验C=C双键的另一种有效方法。在更剧烈的条件下（热、浓KMnO₄），会发生氧化断裂，双键断裂生成羰基化合物或羧酸，可用于推断未知烯烃中双键的位置。
CH₂=CH₂ + [O] + H₂O → HOCH₂–CH₂OH (cold KMnO₄) | 冷高锰酸钾氧化：乙烯 → 乙二醇
11. Polymerisation of Alkenes | 烯烃的聚合反应
Under high pressure and with a suitable initiator (or a Ziegler–Natta catalyst), alkenes undergo addition polymerisation to form long‑chain polymers. The π-bond opens up, and monomers link together without elimination of any small molecule. Poly(ethene), poly(propene), and poly(chloroethene) (PVC) are classic examples.
在高压和适当引发剂（或齐格勒–纳塔催化剂）作用下，烯烃发生加聚反应，形成长链聚合物。π键打开，单体连接起来，无需脱去任何小分子。聚乙烯、聚丙烯和聚氯乙烯（PVC）是典型例子。
| Monomer 单体 | Polymer 聚合物 | Uses 用途 |
|---|---|---|
| CH₂=CH₂ (ethene) | Poly(ethene) –(CH₂–CH₂)ₙ– | Plastic bags, bottles |
| CH₂=CHCl (chloroethene) | Poly(chloroethene) PVC | Pipes, window frames |
| CH₂=CHC₆H₅ (styrene) | Poly(styrene) | Packaging, insulation |
The properties of polymers depend on chain length, branching, and the presence of side groups. Both thermoplastics (which soften on heating) and thermosetting plastics can be derived from alkene monomers, but addition polymers are typically thermoplastic.
聚合物的性质取决于链长、支链以及侧基。由烯烃单体既能制得热塑性塑料（加热软化），也能制得热固性塑料，但加聚物通常为热塑性的。
12. Summary and Exam Tips | 总结与应试技巧
Mastering alkene chemistry requires you to connect structure (sp² hybridisation, restricted rotation) with consequences (E/Z isomerism) and reactivity (electrophilic addition, carbocation stability). In the exam, be prepared to draw and label electrophilic addition mechanisms, predict major products using Markovnikov’s rule, interpret colour changes for bromine water and acidified KMnO₄, and relate polymer structures to their monomers. Regularly practise nomenclature, isomer identification, and writing balanced equations to secure top marks.
掌握烯烃化学需要将结构（sp²杂化、受限旋转）与结果（E/Z异构）以及反应性（亲电加成、碳正离子稳定性）紧密联系。在考试中，要能够画出并标注亲电加成机理，运用马氏规则预测主产物，解释溴水和酸性高锰酸钾的变色现象，并关联聚合物与其单体。经常练习命名、异构体鉴别以及配平方程式，确保获得高分。
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