📚 IGCSE AQA Chemistry: Alkenes – Key Concepts Explained | IGCSE AQA 化学：烯烃 考点精讲
Alkenes are a homologous series of hydrocarbons that contain at least one carbon-carbon double bond (C=C). This functional group defines their chemical properties, making them unsaturated and far more reactive than alkanes. In the IGCSE AQA Chemistry specification, understanding alkenes involves mastering their bonding, isomerism, characteristic addition reactions, and industrial significance. This article systematically covers all essential exam points, from structure to polymerisation, to support your revision.
烯烃是一类烃的同系物，其分子中至少含有一个碳碳双键 (C=C)。这个官能团决定了它们的化学性质，使烯烃不饱和且比烷烃活泼得多。在 IGCSE AQA 化学考纲中，掌握烯烃需要透彻理解其成键方式、同分异构现象、特征加成反应以及工业意义。本文系统梳理从结构到聚合反应的所有核心考点，助力你的复习备考。
1. General Formula and Homologous Series | 通式与同系物
The general formula of alkenes with one double bond is CₙH₂ₙ. For example, ethene (C₂H₄), propene (C₃H₆), and butene (C₄H₈) all fit this pattern. Each member differs from the next by a CH₂ unit, and they show a gradual trend in physical properties, such as boiling point increasing with chain length.
含一个双键的烯烃通式为 CₙH₂ₙ。如乙烯 (C₂H₄)、丙烯 (C₃H₆) 和丁烯 (C₄H₈) 均符合此通式。同系物之间相差一个 CH₂ 单元，物理性质呈现规律性递变，例如随着碳链增长，沸点逐渐升高。
2. Unsaturation and the Carbon-Carbon Double Bond | 不饱和度与碳碳双键
Each C=C double bond consists of one sigma (σ) bond and one pi (π) bond. The π bond is weaker and more exposed, which makes alkenes much more reactive than alkanes. The term ‘unsaturated’ means they can take up extra atoms through addition reactions, converting the double bond into single bonds.
每个 C=C 双键由一个 σ 键和一个 π 键构成。π 键较脆弱且暴露在外，使得烯烃比烷烃活泼得多。“不饱和”意味着它们可以通过加成反应接纳更多原子，将双键变为单键。
3. Naming Alkenes and Positional Isomers | 烯烃的命名与位置异构
The systematic name of an alkene ends in ‘-ene’. The position of the double bond must be indicated by the lowest possible number in the carbon chain. For example, but-1-ene (double bond between C1 and C2) and but-2-ene (between C2 and C3) are positional isomers. Alkenes with four or more carbon atoms exhibit this type of isomerism, which is tested frequently in exams.
烯烃的系统命名以“-ene”结尾。双键的位置必须用碳链上尽可能小的编号标出。例如丁-1-烯（双键在 C1 与 C2 之间）和丁-2-烯（双键在 C2 与 C3 之间）互为位置异构体。含四个及以上碳原子的烯烃存在此类异构，这是考试中的常见考点。
4. Test for Unsaturation Using Bromine Water | 用溴水检验不饱和度
When an alkene is shaken with orange-brown bromine water, the solution immediately turns from orange-brown to colourless. This is an addition reaction where bromine adds across the double bond, forming a dibromoalkane. Alkanes, being saturated, do not react with bromine water under normal conditions, so this test is used to distinguish between alkanes and alkenes.
将烯烃与橙棕色的溴水一起振荡，溶液会立即从橙棕色变为无色。此加成反应中，溴原子加成到双键上，生成二溴代烷。烷烃是饱和烃，在通常条件下不与溴水反应，因此该实验用于区分烷烃和烯烃。
5. Addition Reactions: General Mechanism | 加成反应：一般机理
In addition reactions, the double bond opens up and two new atoms or groups attach to the carbon atoms. The C=C double bond becomes a C-C single bond, and the molecule becomes saturated. This pattern occurs in all major reactions of alkenes: with hydrogen, halogens, hydrogen halides, and water.
在加成反应中，双键打开，两个新的原子或原子团连接到碳原子上。C=C 双键变成 C-C 单键，分子变为饱和的。这一模式适用于烯烃的所有主要反应：与氢气、卤素、卤化氢以及水的反应。
6. Catalytic Hydrogenation | 催化加氢
Alkenes react with hydrogen (H₂) in the presence of a nickel catalyst at about 150 °C to form the corresponding alkane. For instance, ethene + H₂ → ethane. This reaction is used industrially to harden unsaturated vegetable oils into solid margarine by converting some double bonds to single bonds, raising the melting point.
烯烃在镍催化剂存在下于约 150 °C 与氢气 (H₂) 反应，生成相应的烷烃。例如：乙烯 + H₂ → 乙烷。该反应在工业上用于将不饱和植物油转化为固态人造黄油，通过将部分双键变为单键来提高熔点。
7. Halogenation: Addition of Halogens | 卤素加成反应
Alkenes react rapidly with chlorine, bromine, or iodine at room temperature. For example, ethene + Br₂ → 1,2-dibromoethane. The reaction with bromine is used as the standard test for unsaturation. These reactions proceed without the need for a catalyst or UV light, in contrast to alkane halogenation which requires UV light and is a substitution reaction.
烯烃在室温下与氯、溴或碘迅速反应。例如：乙烯 + Br₂ → 1,2-二溴乙烷。与溴的反应被用作检验不饱和度的标准方法。这些反应无需催化剂或紫外线，而烷烃的卤化则需紫外光且属于取代反应。
8. Addition of Hydrogen Halides | 卤化氢加成
Hydrogen halides such as HBr or HCl add across the double bond to form halogenoalkanes. For example, ethene + HBr → bromoethane. With unsymmetrical alkenes like propene, two products are possible, but the major product follows Markovnikov’s rule: the hydrogen atom attaches to the carbon that already has the most hydrogen atoms. This concept is explored in more depth at A-level, but IGCSE questions may ask for the displayed formula of the addition product without requiring the rule by name.
卤化氢如 HBr 或 HCl 能加成到双键上，生成卤代烷。例如：乙烯 + HBr → 溴乙烷。对于不对称烯烃如丙烯，可能生成两种产物，但主产物遵循马氏规则：氢原子加在原本含氢较多的碳原子上。这一概念在 A-level 中会深入探讨，但 IGCSE 题目可能要求写出加成产物的结构式，而不需要提及规则名称。
9. Hydration: Addition of Steam | 水化反应：水蒸气加成
Alkenes react with steam (H₂O) to produce alcohols. This requires a phosphoric acid catalyst, high temperature (around 300 °C), and high pressure (60–70 atm). For example, ethene + steam ⇌ ethanol. This is a reversible reaction, so conditions are optimised to maximise yield. It is the main industrial route for manufacturing ethanol, competing with fermentation.
烯烃与水蒸气 (H₂O) 反应生成醇。该反应需要磷酸作催化剂、高温（约 300 °C）和高压（60–70 atm）。如：乙烯 + 水蒸气 ⇌ 乙醇。这是一个可逆反应，因此需通过优化条件来获得最高产率。这是工业上制取乙醇的主要途径，与发酵法互为替代。
10. Polymerisation of Alkenes | 烯烃的聚合反应
Addition polymerisation is the process where many small alkene molecules (monomers) join together to form a long-chain polymer. The double bond in each monomer opens up, and the monomers link via single bonds. For poly(ethene), the repeating unit is –CH₂–CH₂–. In the exam, you may be required to draw the polymer structure given the monomer, or identify the monomer from a section of the polymer chain.
加成聚合是指许多小分子烯烃（单体）连接成长链聚合物的过程。每个单体中的双键打开，单体之间通过单键相连。以聚乙烯为例，重复单元为 –CH₂–CH₂–。考试中可能会要求根据单体画出聚合物结构，或根据一段聚合物链反推单体。
11. Common Polymers and Their Uses | 常见聚合物及其用途
| Monomer (单体) | Polymer (聚合物) | Typical Uses (典型用途) |
|---|---|---|
| Ethene (乙烯) | Poly(ethene) (聚乙烯) | Plastic bags, bottles (塑料袋、瓶子) |
| Propene (丙烯) | Poly(propene) (聚丙烯) | Ropes, carpets, medical syringes (绳索、地毯、医用注射器) |
| Chloroethene (氯乙烯) | PVC (聚氯乙烯) | Window frames, pipes (窗框、管道) |
| Tetrafluoroethene (四氟乙烯) | PTFE (聚四氟乙烯) | Non-stick pans, waterproof clothing (不粘锅、防水衣物) |
These addition polymers are unreactive and non-biodegradable, which creates disposal issues. The exam commonly links polymerisation to environmental concerns, so you should be able to discuss both the advantages and problems associated with polymer waste.
这些加成聚合物化学性质稳定且不可生物降解，由此带来了处理难题。考试常将聚合反应与环境问题联系起来，因此你应能讨论聚合物废弃物的利弊。
12. Key Comparisons and Summary | 关键比较与总结
Remember that alkenes undergo addition reactions, whereas alkanes undergo substitution (with halogens under UV light). Alkenes decolourise bromine water instantly; alkanes do not react in the dark. The C=C functional group governs the chemistry: it provides a site of high electron density that attracts electrophiles in addition reactions. Mastering the displayed formulae for alkenes up to butene and the corresponding addition products is essential for exam success.
记住烯烃发生加成反应，而烷烃（在紫外光下与卤素）发生取代反应。烯烃能使溴水立即褪色，而烷烃在黑暗中不反应。C=C 官能团决定了化学性质：它提供了高电子密度位点，在加成反应中吸引亲电试剂。掌握丁烯及之前烯烃的展示式以及对应的加成产物，是考试成功的关键。
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