GCSE WJEC Chemistry: Mastering Alkenes | 烯烃考点精讲

📚 GCSE WJEC Chemistry: Mastering Alkenes | 烯烃考点精讲

Alkenes are a fascinating family of hydrocarbons that introduce us to the concept of unsaturation and the reactive carbon‑carbon double bond. In the WJEC GCSE Chemistry specification, mastering alkenes means understanding their structure, naming, characteristic reactions, and how they differ from alkanes. This article will guide you through every key point you need to succeed.

烯烃是一类迷人的碳氢化合物，向我们展示了不饱和以及活泼碳碳双键的概念。在 WJEC GCSE 化学大纲中，掌握烯烃意味着要理解它们的结构、命名、特征反应，以及它们与烷烃的区别。本文将带你逐一攻克所有关键考点。

1. What Are Alkenes? The Homologous Series | 什么是烯烃？同系列

Alkenes are a homologous series of hydrocarbons that contain at least one carbon‑carbon double bond (C=C). Their general formula is CₙH₂ₙ, where n is the number of carbon atoms. They are described as unsaturated because they have fewer hydrogen atoms than the corresponding alkane (CₙH₂ₙ₊₂).

烯烃是一类含有至少一个碳碳双键（C=C）的碳氢化合物同系列。其通式为 CₙH₂ₙ，其中 n 是碳原子数。它们被描述为“不饱和”，因为与相应的烷烃（CₙH₂ₙ₊₂）相比，它们拥有的氢原子更少。

The simplest alkene is ethene (C₂H₄), which has two carbon atoms and one double bond. As the carbon chain increases, each successive member differs by a CH₂ unit, with a gradual change in physical properties like boiling point.

最简单的烯烃是乙烯（C₂H₄），它有两个碳原子和一个双键。随着碳链增长，每个连续同系物相差一个 CH₂ 单元，沸点等物理性质逐渐变化。

2. Naming Alkenes: Straight‑Chain Rules | 烯烃命名：直链规则

Naming alkenes follows a systematic approach. The suffix ‘‑ene’ is used to indicate the presence of a C=C double bond. For a straight‑chain alkene, the chain is numbered from the end that gives the double bond the lowest possible number. The position of the double bond is placed before the stem name.

烯烃命名遵循系统性方法。词尾用“‑ene”表示存在 C=C 双键。对于直链烯烃，从靠近双键的一端给碳链编号，使双键编号尽可能小。双键位置标在母体名称前面。

For example: CH₂=CH‑CH₂‑CH₃ is but‑1‑ene, not but‑3‑ene, because the double bond starts at carbon 1. CH₃‑CH=CH‑CH₃ is but‑2‑ene. Always include the number even if the double bond could only be in one place, unless it is ethene or propene (where no ambiguity exists).

例如：CH₂=CH‑CH₂‑CH₃ 是 1‑丁烯，而不是 3‑丁烯，因为双键从 1 号碳开始。CH₃‑CH=CH‑CH₃ 是 2‑丁烯。即使双键只有一种可能位置（如乙烯、丙烯），也要写上数字，除非没有歧义。

3. Drawing and Recognising the C=C Bond | 绘制与识别 C=C 双键

Each carbon atom in the C=C double bond is sp² hybridised, giving a planar arrangement around the double bond with bond angles of approximately 120°. The double bond consists of one sigma (σ) bond and one pi (π) bond. The π bond is weaker and more exposed, making alkenes much more reactive than alkanes.

碳碳双键中的每个碳原子为 sp² 杂化，使双键周围呈平面排列，键角约为 120°。双键由一个 σ 键和一个 π 键组成。π 键较弱且裸露在外，因此烯烃的反应活性远高于烷烃。

In displayed formula diagrams, a C=C bond is shown as two parallel lines between two carbon atoms. You must be able to draw the structures of ethene, propene, but‑1‑ene and but‑2‑ene, and recognise that only the shared electron pair in the π bond is free to take part in addition reactions.

在结构式中，C=C 双键用两个碳原子之间的两条平行线表示。你必须能画出乙烯、丙烯、1‑丁烯和 2‑丁烯的结构，并认识到只有 π 键中的共用电子对能自由参与加成反应。

4. The Bromine Test for Unsaturation | 溴水检验不饱和性

The classic test for an alkene (or any unsaturated compound) is to shake a few drops of orange‑brown bromine water with the sample. If an alkene is present, the bromine water rapidly turns colourless. This happens because bromine adds across the C=C double bond in an addition reaction, forming a dibromoalkane.

检验烯烃（或任何不饱和化合物）的经典方法是将几滴橙棕色的溴水与样品一起振荡。如果存在烯烃，溴水会迅速褪为无色。这是因为溴发生了横跨 C=C 双键的加成反应，生成二溴代烷。

The reaction for ethene: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2‑dibromoethane). Unlike alkanes, which only react with bromine under UV light via substitution, alkenes decolourise bromine water without any need for light or heat. This test is a key distinguishing feature.

乙烯的反应为：C₂H₄ + Br₂ → C₂H₄Br₂（1,2‑二溴乙烷）。烷烃与溴只有在紫外光下才发生取代反应，而烯烃不需要光或加热就能使溴水褪色。这一检验是关键的辨别特征。

5. Addition Reactions with Hydrogen: Hydrogenation | 与氢气的加成反应：加氢

Alkenes react with hydrogen gas (H₂) in the presence of a nickel catalyst at around 150 °C to form the corresponding alkane. This is an addition reaction known as hydrogenation. The double bond opens and two hydrogen atoms add across it, saturating the molecule.

烯烃在镍催化剂存在下，约 150°C 时与氢气（H₂）反应，生成对应的烷烃。这是一种加成反应，称为加氢。双键打开，两个氢原子加在上面，使分子变为饱和。

For example: ethene + hydrogen → ethane, C₂H₄ + H₂ → C₂H₆. Hydrogenation is used industrially to convert liquid unsaturated vegetable oils into solid margarine by removing some double bonds in polyunsaturated fats.

例如：乙烯 + 氢气 → 乙烷，C₂H₄ + H₂ → C₂H₆。加氢在工业上用于通过去除多不饱和脂肪中的一些双键，将液态不饱和植物油转化为固态人造黄油。

The equation highlights the general principle of addition: the alkene reacts with a small molecule, and only one product is formed.

该方程式突显了加成的一般原理：烯烃与小分子反应，且只生成一种产物。

6. Addition of Steam: Hydration to Alcohols | 与水的加成：水合生成醇

Alkenes react with steam (H₂O in the gaseous state) to produce alcohols. This hydration reaction requires a phosphoric acid (H₃PO₄) catalyst, a temperature of around 300 °C and high pressure (60–70 atm). It is a direct route to ethanol from ethene, and is used industrially alongside fermentation.

烯烃与水蒸气（气态 H₂O）反应生成醇。这种水合反应需要磷酸（H₃PO₄）催化剂、约 300°C 的温度和高压（60–70 个大气压）。这是从乙烯直接制乙醇的路线，在工业上与发酵法并行使用。

For ethene: C₂H₄ + H₂O ⇌ C₂H₅OH. The double arrow indicates that the reaction is reversible. Only one product is formed: ethanol. The reaction obeys Markovnikov’s rule when propene is used, but for GCSE WJEC, recognising that water adds across the double bond to give an alcohol is sufficient.

以乙烯为例：C₂H₄ + H₂O ⇌ C₂H₅OH。可逆箭号表示该反应可逆。只生成一种产物：乙醇。当使用丙烯时，反应遵循马氏规则，但对于 GCSE WJEC 而言，只需识别水横跨双键加成生成醇即可。

7. Addition Polymerisation: Making Plastics | 加成聚合：制造塑料

One of the most important reactions of alkenes is addition polymerisation. Many small alkene molecules (monomers) join together under high pressure and in the presence of a catalyst, by opening their C=C double bonds, to form a very long saturated chain called a polymer. No other product is formed – it is 100% atom economy.

烯烃最重要的反应之一是加成聚合。许多小的烯烃分子（单体）在高压和催化剂存在下，通过打开 C=C 双键连接起来，形成一条很长的饱和链，称为聚合物。该反应不生成其他产物——原子经济性达 100%。

Poly(ethene) (polythene) is made from ethene monomers: n CH₂=CH₂ → (‑CH₂‑CH₂‑)ₙ. Similarly, poly(propene) is made from propene. In the equation, the repeating unit should be drawn with the single bond extending through the brackets. You must be able to deduce the monomer from a given polymer segment and vice versa.

聚（乙烯）（聚乙烯）由乙烯单体制成：n CH₂=CH₂ → (‑CH₂‑CH₂‑)ₙ。类似地，聚（丙烯）由丙烯制成。写方程式时，重复单元应画出括号内两端的单键延伸出去。你必须能够从给定的聚合物链段推导出单体，反之亦然。

8. Comparing Alkenes with Alkanes | 比较烯烃与烷烃

Feature	Alkanes	Alkenes
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Saturation	Saturated	Unsaturated (C=C present)
Bromine water test	No immediate change (requires UV)	Decolourises rapidly, no UV needed
Typical reaction	Substitution (with halogens)	Addition (halogens, H₂, steam, polymerisation)

Alkenes are far more reactive than alkanes because the electron‑rich π bond is readily attacked by electrophiles such as Br⁺. Alkanes possess only strong sigma bonds and require UV light to break bonds homolytically in radical substitution. Understanding this difference is essential for predicting chemical behaviour.

烯烃比烷烃活泼得多，因为富电子的 π 键容易被亲电试剂（如 Br⁺）进攻。烷烃只有牢固的 σ 键，需要紫外光才能使键均裂发生自由基取代。理解这一区别对于预测化学行为至关重要。

9. Electrophilic Addition Mechanism (WJEC Depth) | 亲电加成机理（WJEC 深入理解）

Although the full curly‑arrow mechanism is often examined at A level, WJEC GCSE students may be expected to appreciate that the C=C bond is a centre of high electron density and that addition involves an electrophile (electron‑pair acceptor) attacking the double bond. The bromine molecule becomes polarised as it approaches the double bond, and a pair of electrons from the π bond forms a new bond to a bromine atom.

虽然完整弯箭机理通常在 A Level 考查，但 WJEC GCSE 学生应能体会到 C=C 键是电子密度高的中心，加成涉及亲电试剂（电子对接受体）进攻双键。溴分子在靠近双键时被极化，π 键的一对电子与一个溴原子形成新键。

This explanation can be simplified: the alkene “opens up” its double bond so that two atoms (or groups of atoms) can add to the carbon atoms. The product is saturated at that part of the chain. No atom is lost, hence addition.

可以简化为：烯烃“打开”它的双键，使两个原子（或原子团）加到碳原子上。产物在链的该部分变为饱和。没有原子丢失，因此称为加成。

10. Isomers of Alkenes (Position and Chain) | 烯烃的同分异构（位置与碳链）

Alkenes with four or more carbon atoms exhibit positional isomerism: the double bond can be placed between different C‑C bonds. For example, but‑1‑ene and but‑2‑ene are position isomers. Additionally, chain isomerism is possible: but‑1‑ene and 2‑methylpropene both have the molecular formula C₄H₈.

含有四个及以上碳原子的烯烃表现出位置异构：双键可以位于不同的 C‑C 键之间。例如，1‑丁烯和 2‑丁烯是位置异构体。此外，碳链异构也是可能的：1‑丁烯和 2‑甲基丙烯的分子式均为 C₄H₈。

When asked to draw all possible isomers for a given molecular formula, remember to vary the position of the double bond and the branching of the carbon skeleton. You do not need to consider geometric (E/Z) isomerism at GCSE, except perhaps to note that but‑2‑ene can exist as cis‑trans isomers; WJEC may mention it as challenge material.

当要求画出给定分子式的所有可能异构体时，记得改变双键的位置和碳骨架的分支。在 GCSE 阶段你不需要考虑几何（E/Z）异构，除非提到 2‑丁烯可以存在顺反异构；WJEC 可能将其作为拓展材料提及。

11. Common Errors and How to Avoid Them | 常见错误与对策

Many students write ethene as C₂H₂ or confuse the formula of propene. Remember: ethene is C₂H₄, propene is C₃H₆. Another frequent mistake is forgetting to number the double bond in but‑1‑ene, writing ‘butene’ without indicating the position, which is insufficient. Also, when drawing poly(ethene), ensure the single bonds extend through the brackets: (‑CH₂‑CH₂‑)ₙ.

许多学生把乙烯写成 C₂H₂，或混淆丙烯的分子式。请记住：乙烯是 C₂H₄，丙烯是 C₃H₆。另一个常见错误是忘记给 1‑丁烯的双键编号，只写“丁烯”而未标明位置，这是不充分的。另外，在画聚乙烯时，要确保单键贯穿括号：(‑CH₂‑CH₂‑)ₙ。

In bromination equations, always show the dibromo product, not bromoethane. For hydration, remember the catalyst (phosphoric acid) and that it is a reversible reaction. Practise deducing monomers from repeat units: remove the bond extensions, put back the double bond, and check the monomer name.

在溴代方程式中，一定要写二溴产物，而不是溴乙烷。对于水合反应，记住催化剂（磷酸）以及这是一个可逆反应。反复练习从重复单元推导单体：去掉延伸出来的键，恢复双键，检查单体名称。

12. Summary Checklist for Revision | 复习检查清单

Use this checklist to ensure you feel confident with alkenes for your WJEC GCSE examination: can you recall the general formula CₙH₂ₙ? Can you draw displayed structures for the first four straight‑chain alkenes? Can you give word and balanced symbol equations for addition of bromine, hydrogen, steam, and for polymerisation of ethene and propene? Can you describe and explain the bromine water test? Can you identify the monomer from a polymer segment? Do you understand why alkenes are more reactive than alkanes?

使用这份检查清单，确保你对 WJEC GCSE 烯烃考点充满信心：你能记住通式 CₙH₂ₙ 吗？你能画出前四种直链烯烃的结构式吗？你能写出加成溴、氢气、水蒸气以及乙烯和丙烯聚合的文字方程式和配平符号方程式吗？你能描述并解释溴水试验吗？你能从聚合物链段识别单体吗？你理解为什么烯烃比烷烃更活泼吗？

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GCSE WJEC Chemistry: Mastering Alkenes | 烯烃 考点精讲