IGCSE WJEC Chemistry: Alkenes – Key Points | IGCSE WJEC 化学：烯烃考点精讲

📚 IGCSE WJEC Chemistry: Alkenes – Key Points | IGCSE WJEC 化学：烯烃考点精讲

Alkenes are one of the most important hydrocarbon families in IGCSE WJEC Chemistry. They introduce the concept of unsaturation and form the basis for understanding organic reaction mechanisms, polymerisation, and industrial processes. This article condenses all essential alkene knowledge you need to master for your examination.

烯烃是 IGCSE WJEC 化学中最重要的烃类家族之一。它们引入了不饱和度的概念，并为理解有机反应机理、聚合反应和工业过程奠定了基础。本文浓缩了你需要掌握的烯烃全部核心知识，助你冲刺高分。

1. Structure and General Formula of Alkenes | 烯烃的结构与通式

Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). Their general formula is CₙH₂ₙ, meaning they have two fewer hydrogen atoms than the corresponding alkane (CₙH₂ₙ₊₂). The simplest alkene is ethene, C₂H₄.

烯烃是含有至少一个碳碳双键 (C=C) 的不饱和烃。它们的通式为 CₙH₂ₙ，意味着比相应的烷烃 (CₙH₂ₙ₊₂) 少两个氢原子。最简单的烯烃是乙烯，化学式为 C₂H₄。

Each carbon atom in the C=C bond is sp² hybridised. The double bond consists of one sigma (σ) bond formed by head-on overlap of orbitals and one pi (π) bond formed by sideways overlap of p orbitals. The π bond is weaker and more reactive, which explains why alkenes readily undergo addition reactions.

碳碳双键中的每个碳原子都是 sp² 杂化的。双键由一个轨道端对端重叠形成的 σ 键和一个 p 轨道肩并肩重叠形成的 π 键组成。π 键较弱且更活泼，这解释了为什么烯烃容易发生加成反应。

2. Naming Alkenes and Positional Isomerism | 烯烃的命名与位置异构

IUPAC names for alkenes follow the pattern: prefix (indicating number of carbons) + ‘-ene’. The position of the double bond is shown by the lowest possible number before the ‘-ene’. For example, but-1-ene (CH₂=CHCH₂CH₃) and but-2-ene (CH₃CH=CHCH₃).

烯烃的 IUPAC 命名遵循：词头（表示碳原子数）+ ‘-烯’。双键的位置用 ‘-烯’ 前尽可能小的数字表示。例如，丁-1-烯 (CH₂=CHCH₂CH₃) 和丁-2-烯 (CH₃CH=CHCH₃)。

If the double bond can be placed in different positions, positional isomers arise. These molecules have the same molecular formula but different structural arrangements. Furthermore, branching in the carbon chain produces chain isomers, as long as the position of the C=C may change.

如果双键可以位于不同位置，就会产生位置异构体。这些分子具有相同的分子式但结构排列不同。此外，碳链支链化会产生链异构体，同时双键位置也可能发生变化。

3. Physical Properties of Alkenes | 烯烃的物理性质

Alkenes are non-polar molecules, so they are insoluble in water but soluble in non-polar solvents such as cyclohexane. They have relatively low melting and boiling points, which increase with molecular size due to stronger London dispersion forces.

烯烃是非极性分子，因此不溶于水，但溶于环己烷等非极性溶剂。它们的熔点和沸点相对较低，随着分子增大，色散力增强，熔沸点升高。

Ethene, propene, and butenes are gases at room temperature; alkenes with 5–16 carbons are liquids, and higher alkenes are waxy solids. The boiling points of isomeric alkenes decrease with increased branching because surface contact area between molecules is reduced.

乙烯、丙烯和丁烯在室温下为气体；含 5–16 个碳原子的烯烃为液体，更高碳数的烯烃为蜡状固体。异构烯烃的沸点随支链增加而下降，因为分子间表面接触面积减小。

4. Combustion of Alkenes | 烯烃的燃烧

Like alkanes, alkenes burn in plenty of oxygen to produce carbon dioxide and water. However, their combustion is usually more incomplete, producing a smokier, yellow flame because of the higher carbon-to-hydrogen ratio. Incomplete combustion may generate poisonous carbon monoxide and soot (carbon).

与烷烃类似，烯烃在充足氧气中燃烧生成二氧化碳和水。但它们的燃烧通常更不完全，由于碳氢比更高，会产生更浓的黑烟和黄色火焰。不完全燃烧可能产生有毒的一氧化碳和碳烟（碳）。

The balanced equation for complete combustion of ethene is: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O.

乙烯完全燃烧的方程式为：C₂H₄ + 3O₂ → 2CO₂ + 2H₂O。

You are not expected to write equations for incomplete combustion, but you must be able to identify the products and environmental consequences.

你不需要写出不完全燃烧的方程式，但必须能够识别产物及其对环境的影响。

5. Addition Reactions – The Characteristic Reaction of Alkenes | 加成反应——烯烃的特征反应

Alkenes undergo electrophilic addition reactions because the C=C double bond is an area of high electron density. The π bond breaks, and new atoms or groups add across the two carbon atoms, converting the unsaturated molecule into a saturated compound.

烯烃容易发生亲电加成反应，因为 C=C 双键是电子密度高的区域。π 键断裂，新的原子或基团加在两个碳原子上，使不饱和分子转化为饱和化合物。

Four key addition reactions are tested: (1) hydrogenation – addition of H₂ with a nickel catalyst to form an alkane; (2) halogenation – addition of Br₂ or Cl₂ to form a dihalogenoalkane; (3) addition of hydrogen halides (e.g. HBr) to produce a halogenoalkane; (4) hydration – addition of steam (H₂O) with an acid catalyst (H₃PO₄) to produce an alcohol.

四种核心加成反应是考点：(1) 加氢——在镍催化剂下加 H₂ 生成烷烃；(2) 卤化——加 Br₂ 或 Cl₂ 生成二卤代烷；(3) 加卤化氢（如 HBr）生成卤代烷；(4) 水合——在酸催化剂（磷酸）下加 H₂O 生成醇。

The general equation for hydration: CₙH₂ₙ + H₂O → CₙH₂ₙ₊₁OH. For example, ethene + steam → ethanol. This is an important industrial process.

水合的通用方程式：CₙH₂ₙ + H₂O → CₙH₂ₙ₊₁OH。例如，乙烯 + 水蒸气 → 乙醇。这是一个重要的工业过程。

6. The Bromine Water Test for Unsaturation | 溴水试验检测不饱和度

When orange bromine water (Br₂ dissolved in water) is shaken with an alkene, the solution rapidly decolourises. This is because bromine adds across the double bond to form a colourless dibromoalkane. This test distinguishes alkenes from alkanes, which do not react with bromine water unless exposed to UV light.

将橙色的溴水（Br₂ 溶于水）与烯烃一起振荡，溶液迅速褪色。这是因为溴加成到双键上，生成无色的二溴代烷。这个试验可以区分烯烃和烷烃，后者除非在紫外光照射下，否则不与溴水反应。

The chemical equation for ethene is: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane).

乙烯的化学方程式为：C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-二溴乙烷)。

During this test, bromine acts as an electrophile. The polarisation of the Br–Br bond is induced by the electron-rich double bond, forming a cyclic bromonium ion intermediate.

该测试中，溴充当亲电试剂。富电子的双键诱导 Br–Br 键极化，形成一个环状溴鎓离子中间体。

7. Addition Polymerisation | 加成聚合

Addition polymerisation is the process in which many small unsaturated monomer molecules (alkenes) join together to form a long-chain saturated polymer. The double bonds of the monomers break open and the monomers link by single covalent bonds.

加成聚合是指许多不饱和单体小分子（烯烃）连接形成长链饱和聚合物的过程。单体的双键打开，单体通过共价单键连接起来。

The repeating unit of a polymer is derived from the monomer. For example, poly(ethene) (polythene) is formed from ethene: n CH₂=CH₂ → –(CH₂–CH₂)–ₙ. Poly(propene) from propene: n CH₂=CHCH₃ → –(CH₂–CH(CH₃))–ₙ.

聚合物的重复单元来源于单体。例如，聚乙烯由乙烯制得：n CH₂=CH₂ → –(CH₂–CH₂)–ₙ。聚丙烯由丙烯制得：n CH₂=CHCH₃ → –(CH₂–CH(CH₃))–ₙ。

You need to be able to draw the repeating unit given the monomer, and identify the monomer given a section of the polymer chain. The polymer name is simply ‘poly(monomer name)’ except for common names like polythene.

你需要能够根据单体画出重复单元，并根据聚合物链段推断单体。聚合物的名称就是“聚(单体名称)”，除了像聚乙烯这样的通用名称。

8. Common Polymers and Their Uses | 常见聚合物及其用途

Poly(ethene) (polythene) is used for plastic bags, bottles, and cling film. It can be low-density (LDPE) or high-density (HDPE) depending on reaction conditions. Poly(propene) is used for ropes, carpets, and food containers due to its strength and flexibility.

聚乙烯用于塑料袋、瓶子和保鲜膜。根据反应条件不同，可分为低密度 (LDPE) 和高密度 (HDPE)。聚丙烯强度高、柔韧性好，用于绳索、地毯和食品容器。

Poly(chloroethene) or PVC is made from chloroethene (CH₂=CHCl) and is widely used for window frames, pipes, and electrical insulation. Poly(tetrafluoroethene) (PTFE) is a non-stick coating for pans. These uses are often linked to the polymer’s properties, such as resistance to chemicals or electrical insulation.

聚氯乙烯或 PVC 由氯乙烯 (CH₂=CHCl) 制得，广泛用于窗框、管道和电绝缘。聚四氟乙烯 (PTFE) 用作不粘锅涂层。这些用途通常与聚合物的性质相关，如耐化学腐蚀或电绝缘性。

Monomer	Polymer	Uses
Ethene	Poly(ethene)	Bags, bottles
Propene	Poly(propene)	Ropes, containers
Chloroethene	PVC	Pipes, insulation
Tetrafluoroethene	PTFE	Non-stick coating

9. Stereoisomerism – Geometric Isomers (E/Z or cis-trans) | 立体异构——几何异构 (E/Z 或顺反异构)

Alkenes with two different groups attached to each carbon of the double bond can exhibit geometric isomerism. This occurs because rotation around the C=C bond is restricted. If each carbon of the double bond has two different substituents, cis and trans isomers exist.

在双键每个碳原子上连有两个不同基团的烯烃可以表现出几何异构。这是因为绕 C=C 键的旋转受到限制。如果双键上的每个碳都有两个不同的取代基，就存在顺反异构体。

Cis isomer (Z) has the higher priority groups on the same side; trans isomer (E) has them on opposite sides. For IGCSE, you may be asked to recognise and draw the isomers of but-2-ene: cis-but-2-ene and trans-but-2-ene. They have different physical properties, e.g., trans has a lower boiling point due to better packing.

顺式异构体 (Z) 中较高优先级的基团在同一侧；反式异构体 (E) 中它们在相反侧。在 IGCSE 中，你可能需要识别和画出丁-2-烯的异构体：顺-丁-2-烯和反-丁-2-烯。它们有不同的物理性质，例如反式沸点较低，因为分子排列更紧密。

The Cahn–Ingold–Prelog (CIP) priority rules assign Z/E labels based on atomic number. For simple cases, cis/trans nomenclature is enough.

根据 Cahn–Ingold–Prelog (CIP) 优先规则，基于原子序数分配 Z/E 标记。对于简单情况，顺反命名就足够了。

10. Production of Alkenes – Cracking | 烯烃的生产——裂化

Alkenes are produced mainly by the cracking of larger alkane molecules from petroleum fractions. Cracking involves breaking long-chain hydrocarbons into shorter, more useful alkanes and alkenes. There are two methods: thermal cracking (high temperature and pressure) and catalytic cracking (using a zeolite catalyst at a slightly lower temperature).

烯烃主要通过裂化石油馏分中的大烷烃分子来生产。裂化将长链烃断裂成更短、更有用的烷烃和烯烃。有两种方法：热裂化（高温高压）和催化裂化（使用沸石催化剂，温度略低）。

For example, decane (C₁₀H₂₂) can be cracked to produce pentane and pentene: C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀. The high demand for ethene and propene for polymerisation drives the importance of cracking.

例如，癸烷 (C₁₀H₂₂) 可以裂化生成戊烷和戊烯：C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀。乙烯和丙烯用于聚合的需求很高，因此裂化至关重要。

Catalytic cracking also produces branched alkanes and aromatics, which improve the quality of petrol (higher octane number).

催化裂化还产生支链烷烃和芳香烃，这些物质可以提高汽油品质（更高辛烷值）。

11. Comparing Alkanes and Alkenes – Summary | 烷烃与烯烃对比——总结

Alkanes (CₙH₂ₙ₊₂) are saturated, containing only single bonds. They react slowly with halogens via free radical substitution in UV light. Alkenes (CₙH₂ₙ) are unsaturated, react instantly with bromine water via addition, and can polymerise.

烷烃 (CₙH₂ₙ₊₂) 是饱和的，只含单键。它们在紫外光下通过自由基取代与卤素缓慢反应。烯烃 (CₙH₂ₙ) 是不饱和的，通过加成与溴水立即反应，并可发生聚合。

Table: Differences between alkanes and alkenes.

Property	Alkanes	Alkenes
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Saturation	Saturated	Unsaturated
Reaction with Br₂ (aqueous)	No reaction in dark; slow substitution in light	Immediate decolourisation (addition)
Polymerisation	Not possible	Addition polymerisation

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

Always show displayed or structural formulas clearly. For addition reactions, draw the breaking of the π bond and show the new groups attached. In polymerisation, ensure the repeating unit has no double bond (saturated backbone). Avoid confusing ‘monomer’ and ‘polymer’ – the monomer’s double bond opens.

务必清晰地展示结构式或显示式。对于加成反应，画出 π 键的断裂并显示连接的新基团。在聚合反应中，确保重复单元不含双键（饱和主链）。避免混淆“单体”和“聚合物”——单体的双键打开。

When writing equations, balance carbons and hydrogens. For cracked products, the total atoms must match the reactant. Also, remember that the bromine test works only with bromine water, not bromine gas dissolved in an organic solvent where no colour change is observed?

书写方程式时，要平衡碳和氢。对于裂化产物，原子总数必须与反应物匹配。还要记住，溴试验仅适用于溴水，而不是溴溶于有机溶剂的溶液，后者可能观察不到颜色变化？

In geometric isomerism, check that each carbon of the double bond has two different groups – essential condition. Practice drawing both cis/trans isomers and E/Z notation when required.

在几何异构中，检查双键上的每个碳是否都有两个不同的基团——这是必要条件。练习绘制顺反异构体以及所需的 E/Z 标记法。

Finally, relate polymer uses to their properties: PVC is rigid and durable; PTFE is slippery and heat-resistant; poly(ethene) is flexible and cheap. This link often appears in structured questions.

最后，将聚合物的用途与其性质联系起来：PVC 坚硬耐用；PTFE 光滑耐热；聚乙烯柔韧便宜。这种联系经常出现在结构化题目中。

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IGCSE WJEC Chemistry: Alkenes – Key Points | IGCSE WJEC 化学：烯烃 考点精讲