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

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

Alkenes are a fundamental homologous series in organic chemistry that you must master for the IGCSE Edexcel Chemistry examination. Their distinct carbon–carbon double bond leads to a rich variety of addition reactions and industrial applications, from making plastics to producing alcohols. This article condenses every essential concept, equation and examiner tip into a clear, bilingual revision guide that will help you tackle multiple‑choice questions, structured theory and practical‑based assessments with confidence.

烯烃是 IGCSE Edexcel 化学考试中必须掌握的基础同系物。其独特的碳碳双键带来丰富的加成反应和工业应用，从制造塑料到生产醇类。本文将所有核心概念、方程式和考官提示浓缩为一份清晰的双语复习指南，帮助你自信应对选择题、结构化理论题和基于实验的评估。

1. Homologous Series and General Formula | 同系物与通式

Alkenes form a homologous series of unsaturated hydrocarbons with the general formula CₙH₂ₙ, where n is the number of carbon atoms (n ≥ 2). Each member differs from the next by a CH₂ unit. Because they contain fewer hydrogen atoms than the corresponding alkanes, they are classified as unsaturated.

烯烃是一类不饱和碳氢化合物的同系物，通式为 CₙH₂ₙ（n 为碳原子数，n ≥ 2）。每相邻两个成员相差一个 CH₂ 单元。由于它们所含的氢原子比相应的烷烃少，因此被归类为不饱和烃。

The first three alkenes you need to know are ethene (C₂H₄), propene (C₃H₆) and butene (C₄H₈). For butene, two structural isomers exist: but‑1‑ene and but‑2‑ene, differing in the position of the double bond. You may be asked to draw or recognise these isomers.

你需要掌握的前三种烯烃是乙烯 (C₂H₄)、丙烯 (C₃H₆) 和丁烯 (C₄H₈)。丁烯存在两种结构异构体：丁‑1‑烯和丁‑2‑烯，区别在于双键的位置。你可能会被要求画出或辨认这些异构体。

Name	Molecular Formula	Structure (condensed)
Ethene	C₂H₄	CH₂=CH₂
Propene	C₃H₆	CH₃CH=CH₂
But‑1‑ene	C₄H₈	CH₂=CHCH₂CH₃
But‑2‑ene	C₄H₈	CH₃CH=CHCH₃

2. The Functional Group: Carbon–Carbon Double Bond | 官能团：碳碳双键

The reactive site in all alkenes is the C=C double bond. One of the bonds is a strong sigma (σ) bond; the other is a weaker pi (π) bond. The electron‑rich π bond makes alkenes much more reactive than alkanes, readily taking part in addition reactions.

所有烯烃的反应活性中心都是 C=C 双键。其中一个键是较强的 σ 键，另一个是较弱的 π 键。富电子的 π 键使烯烃的反应活性远高于烷烃，容易发生加成反应。

Examiners often ask why alkenes are more reactive than alkanes. The answer lies in the presence of the π bond, which has lower bond energy than a σ bond and can easily break to form new bonds with attacking species.

考官常问为什么烯烃比烷烃更具反应活性。答案在于 π 键的存在，其键能低于 σ 键，容易断裂并与进攻物种形成新的化学键。

3. Naming Alkenes | 烯烃的命名

The IUPAC name of an alkene ends in ‑ene. Number the longest carbon chain containing the double bond to give the double bond the lowest possible locant. If the double bond is between carbon 1 and 2, the name is but‑1‑ene, not but‑2‑ene. You must be able to name and draw alkenes with up to four carbon atoms and simple branched isomers.

烯烃的 IUPAC 名称以 ‑ene 结尾。选择含双键的最长碳链进行编号，使双键的位次尽可能小。如果双键在碳原子 1 和 2 之间，则命名为丁‑1‑烯，而不是丁‑2‑烯。你必须能够命名和画出最多含四个碳原子及简单支链的烯烃异构体。

Common mistake: forgetting to indicate the position of the double bond when numbering is required. For ethene and propene the position is not needed because only one location is possible.

常见错误：当需要编号时忘记标明双键的位置。乙烯和丙烯无需标明位置，因为只有一种可能的位置。

4. Preparation of Ethene in the Laboratory | 乙烯的实验室制备

Ethene is prepared by the dehydration of ethanol using an acid catalyst, such as concentrated sulfuric acid or aluminium oxide. The reaction is an elimination reaction where a water molecule is removed. The equation is:

乙烯通过乙醇脱水制备，使用浓硫酸或氧化铝作为催化剂。该反应是消去反应，脱去一分子水。方程式如下：

CH₃CH₂OH → CH₂=CH₂ + H₂O

In the laboratory, ethanol vapour is passed over hot aluminium oxide powder or heated with concentrated sulfuric acid. The ethene gas is collected over water because it is insoluble in water. You must be able to describe the apparatus and safety precautions.

在实验室中，乙醇蒸气通过热的氧化铝粉末，或与浓硫酸共热。乙烯气体通过排水集气法收集，因为它不溶于水。你必须能够描述装置和安全注意事项。

5. Addition Reactions – The Key Reaction Type | 加成反应——关键反应类型

Alkenes undergo addition reactions in which the C=C double bond opens up and two atoms or groups attach to the carbon atoms. The product is a single, saturated compound. This is in contrast to substitution, typical of alkanes.

烯烃发生加成反应，C=C 双键打开，两个原子或原子团分别加到两个碳原子上。产物是一个饱和化合物。这与烷烃典型的取代反应不同。

The general pattern is: Alkene + X–Y → saturated product. You must be able to give the conditions, equations and observations for the addition of hydrogen, halogens, hydrogen halides and water.

一般模式为：烯烃 + X–Y → 饱和产物。你必须能够给出与氢气、卤素、卤化氢和水加成反应的条件、方程式和实验现象。

6. Hydrogenation (Addition of Hydrogen) | 氢化（加氢反应）

Alkenes react with hydrogen in the presence of a nickel catalyst at about 150 °C to form the corresponding alkane. For example:

烯烃在镍催化剂存在下约 150 °C 与氢气反应，生成相应的烷烃。例如：

C₂H₄ + H₂ → C₂H₆

This reaction is used industrially in the hardening of vegetable oils, where unsaturated liquid oils become saturated solid fats (margarine). The exam may ask you to link this to the test for unsaturation or to the concept of saturation.

该反应用于工业上的植物油硬化，不饱和的液态油变成饱和的固体脂肪（人造黄油）。考试可能会要求你将此与不饱和度测试或饱和概念联系起来。

7. Halogenation (Addition of Halogens) | 卤化（与卤素加成）

Alkenes react rapidly with halogens (Cl₂, Br₂) at room temperature without a catalyst. This reaction is used as the chemical test for unsaturation. When orange bromine water is added to an alkene, it is decolourised immediately.

烯烃在室温下无需催化剂即可与卤素（Cl₂、Br₂）迅速反应。该反应用作不饱和性（碳碳双键）的化学检验。当向烯烃中加入橙色溴水时，溴水立即褪色。

The equation for ethene with bromine is:

乙烯与溴的方程式为：

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

A common exam question asks you to describe the colour change: orange/brown to colourless. The product is a dibromoalkane. Note that alkanes require UV light for a substitution reaction with bromine, which is a useful distinction.

常见的考题要求描述颜色变化：由橙色 / 红棕色变为无色。产物是二溴代烷。注意烷烃与溴反应需要紫外光并以取代反应进行，这是一个有用的区分方法。

8. Hydrohalogenation (Addition of Hydrogen Halides) | 与卤化氢加成

Alkenes add hydrogen halides (HCl, HBr, HI) to form halogenoalkanes. The reaction is fast at room temperature with dry hydrogen halide gas or concentrated acid. For example:

烯烃与卤化氢（HCl、HBr、HI）加成，生成卤代烷烃。反应在室温下使用干燥的卤化氢气体或浓酸即可迅速进行。例如：

C₂H₄ + HCl → CH₃CH₂Cl

With unsymmetrical alkenes like propene, you must apply Markovnikov’s rule: the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already attached, leading to the more stable carbocation intermediate. Thus, propene + HBr gives mainly 2‑bromopropane, not 1‑bromopropane.

对于丙烯等不对称烯烃，你必须运用马氏规则：氢原子加到含氢较多的碳原子上，生成更稳定的碳正离子中间体。因此，丙烯 + HBr 主要生成 2‑溴丙烷，而不是 1‑溴丙烷。

9. Hydration (Addition of Water) | 水合（与水加成）

Alkenes react with steam in the presence of a phosphoric(V) acid catalyst at about 300 °C and 60‑70 atm to produce alcohols. This is the direct industrial route to ethanol from ethene:

烯烃在磷酸(V)催化剂存在下，于约 300 °C、60‑70 大气压的条件下与水蒸气反应生成醇。这是工业上由乙烯直接生产乙醇的路线：

CH₂=CH₂ + H₂O(g) ⇌ CH₃CH₂OH

The reaction is reversible, so conditions are chosen to give a good yield. You should compare this method with fermentation, considering raw materials, rate, purity and energy requirements.

该反应是可逆的，因此选择适当条件以获得良好的产率。你应将该法与发酵法进行比较，从原料、速率、纯度和能耗等方面考虑。

10. Oxidation Reactions | 氧化反应

Alkenes burn with a smokier flame than alkanes due to a higher carbon‑to‑hydrogen ratio. Complete combustion produces carbon dioxide and water:

由于碳氢比例较高，烯烃燃烧时的火焰比烷烃更明亮并带黑烟。完全燃烧产生二氧化碳和水：

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

More importantly, alkenes can be oxidised by cold, dilute, acidified potassium manganate(VII). The purple solution turns colourless, and a diol is formed. This reaction is also used as a test for unsaturation; be clear that the decolourisation with KMnO₄ indicates the presence of a C=C double bond.

更重要的是，烯烃可被冷的、酸化的稀高锰酸钾溶液氧化。紫色的溶液变为无色，同时生成二醇。该反应也用作不饱和性检验；要清楚地指出，KMnO₄ 溶液的褪色表明存在 C=C 双键。

11. Addition Polymerisation | 加成聚合反应

Many alkene molecules (monomers) can join together under high pressure and with a catalyst to form long‑chain polymers. This is addition polymerisation. The monomer must have a C=C double bond. For example, the formation of poly(ethene):

许多烯烃分子（单体）在高压和催化剂的作用下可以连接成长链聚合物，这就是加成聚合反应。单体必须含有 C=C 双键。例如，聚乙烯的生成：

n CH₂=CH₂ → [–CH₂–CH₂–]ₙ

You need to be able to draw the repeating unit of polymers such as poly(propene), poly(chloroethene) (PVC) and poly(tetrafluoroethene) (PTFE). Examiners may ask you to deduce the monomer from a given polymer section. The repeating unit must show the open bonds extending through the brackets.

你需要能够画出聚丙烯、聚氯乙烯 (PVC) 和聚四氟乙烯 (PTFE) 等聚合物的重复单元。考官可能会要求你根据给定的聚合物片段推导单体。重复单元必须显示通过方括号延伸的开放键。

The disposal of addition polymers poses environmental problems because they are non‑biodegradable. You should know the advantages and disadvantages of incineration, landfill and recycling.

加成聚合物的处置带来环境问题，因为它们不可生物降解。你应了解焚烧、填埋和回收的优缺点。

12. Testing for Unsaturation and Comparison with Alkanes | 不饱和性检验及与烷烃的比较

Two main chemical tests distinguish alkenes from alkanes:

区分烯烃与烷烃的两种主要化学测试：

Bromine water test: shake orange bromine water with the sample; alkenes decolourise it immediately, alkanes only decolourise in UV light after a delay.
溴水试验：将橙色溴水与样品振荡；烯烃立刻使其褪色，烷烃仅在紫外光照下延迟褪色。

Acidified potassium manganate(VII) test: purple solution turns colourless with alkenes at room temperature; alkanes show no change.
酸化高锰酸钾试验：室温下紫色溶液遇到烯烃变为无色；烷烃无变化。

A summary table helps you compare alkanes and alkenes directly, which is a common exam requirement:

下面的总结表有助于直接比较烷烃和烯烃，这是考试中的常见要求：

Property	Alkanes	Alkenes
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Functional group	None (only C–C, C–H)	C=C double bond
Saturation	Saturated	Unsaturated
Reaction type	Substitution (with halogens, UV)	Addition (with H₂, halogens, HX, H₂O)
Bromine water test	Slow decolourisation in UV light	Immediate decolourisation
Combustion	Cleaner, less smoky flame	Smokier flame (higher C:H ratio)

Remember to link the test results back to the presence or absence of the C=C double bond and always quote the colour changes precisely as the IGCSE mark scheme demands.

记住要将测试结果与是否存在 C=C 双键联系起来，并始终按照 IGCSE 评分方案的要求精确描述颜色变化。

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