📚 Alkenes for IGCSE CCEA Chemistry: Key Points Explained | IGCSE CCEA 化学：烯烃 考点精讲
Alkenes are a fascinating and highly important family of hydrocarbons. In the IGCSE CCEA Chemistry specification, understanding alkenes is crucial because they introduce the concept of unsaturation and a wide range of addition reactions. This article will cover all the key points you need to excel, from structure and naming to reactivity and polymerisation.
烯烃是一类既迷人又极为重要的碳氢化合物。在 IGCSE CCEA 化学大纲中，理解烯烃至关重要，因为它们引入了不饱和的概念以及多种加成反应。本文将涵盖你需要掌握的所有关键知识点，从结构和命名到反应活性与聚合反应。
1. What are Alkenes? | 什么是烯烃？
Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). Being unsaturated means they have fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms. The double bond consists of one sigma (σ) bond and one pi (π) bond, which gives the molecule a region of high electron density and makes it much more reactive than alkanes.
烯烃是含有至少一个碳碳双键（C=C）的不饱和碳氢化合物。不饱和意味着与相同碳原子数的相应烷烃相比，氢原子数更少。双键由一个 σ 键和一个 π 键组成，这为分子提供了高电子密度区域，使其比烷烃活泼得多。
The simplest alkene is ethene (C₂H₄), followed by propene (C₃H₆), butene (C₄H₈), and so on. Each member of the alkene homologous series differs from the next by a –CH₂– unit and shares similar chemical properties and a gradual trend in physical properties.
最简单的烯烃是乙烯（C₂H₄），然后是丙烯（C₃H₆）、丁烯（C₄H₈）等。烯烃同系物中每个相邻成员相差一个 –CH₂– 单元，具有相似的化学性质，而物理性质则呈现渐变趋势。
2. General Formula and Homologous Series | 通式与同系物
The general formula for alkenes with one double bond is CₙH₂ₙ. This formula holds true for straight-chain and branched alkenes when only one C=C bond is present. For example, when n = 2, we get C₂H₄ (ethene); n = 3 gives C₃H₆ (propene).
含一个双键的烯烃通式为 CₙH₂ₙ。当分子中只有一个 C=C 双键时，无论是直链烯烃还是支链烯烃都遵循这一通式。例如，n=2 时得到 C₂H₄（乙烯）；n=3 时得到 C₃H₆（丙烯）。
Alkenes form a homologous series: a family of organic compounds with the same functional group (C=C) and general formula, where each successive member differs by CH₂. This leads to predictable gradation in boiling points, melting points, and viscosity as the chain length increases.
烯烃构成一个同系物系列：一系列具有相同官能团（C=C）和通式的有机化合物，相邻成员相差一个 CH₂ 单元。这导致随着碳链增长，沸点、熔点和粘度呈现可预测的渐变规律。
3. Naming Alkenes | 烯烃的命名
IUPAC naming of alkenes follows clear rules. The parent chain must contain the double bond. The suffix is ‘-ene’. The position of the double bond is indicated by the lowest possible number assigned to the first carbon of the C=C bond. If there is more than one double bond, use ‘-diene’, ‘-triene’, etc.
烯烃的 IUPAC 命名遵循明确的规则。主链必须包含双键，词尾为“-ene”。双键的位置用编号最小的双键起点碳原子标出。若存在多个双键，则使用“-二烯”、“-三烯”等。
Example: CH₂=CH–CH₂–CH₃ is but-1-ene, not but-4-ene or but-1-ene? Actually the numbering should give the double bond the lowest number, so it is but-1-ene (double bond starts at C1). CH₃–CH=CH–CH₃ is but-2-ene. Substituents like methyl groups are named with position numbers, e.g. 2-methylpropene.
例如：CH₂=CH–CH₂–CH₃ 是丁-1-烯，而不是丁-4-烯或丁-1-烯？编号应使双键编号最小，因此是丁-1-烯（双键始于C1）。CH₃–CH=CH–CH₃ 是丁-2-烯。有取代基如甲基时，用位置数字标出，例如 2-甲基丙烯。
4. Structural Isomerism in Alkenes | 烯烃的结构异构
Alkenes exhibit structural isomerism from butene (C₄H₈) onwards. Structural isomers have the same molecular formula but different structural arrangements. For C₄H₈, the possible isomers include but-1-ene, but-2-ene, and 2-methylpropene (also called methylpropene). Note that cycloalkanes also have the same general formula (CₙH₂ₙ) and are ring structural isomers of alkenes.
从丁烯（C₄H₈）开始，烯烃出现结构异构现象。结构异构体具有相同的分子式，但原子排列方式不同。对于 C₄H₈，可能的异构体包括丁-1-烯、丁-2-烯和 2-甲基丙烯（也称甲基丙烯）。请注意，环烷烃也具有相同的通式（CₙH₂ₙ），是烯烃的环状结构异构体。
Positional isomerism occurs when the double bond is at a different position, e.g. but-1-ene and but-2-ene. Chain isomerism occurs when the carbon skeleton is branched, e.g. 2-methylpropene vs straight-chain butenes. Recognising different types of isomerism is an essential skill for IGCSE CCEA papers.
当双键位于不同位置时出现位置异构，例如丁-1-烯和丁-2-烯。当碳骨架为支链时出现碳链异构，例如 2-甲基丙烯与直链丁烯。识别不同类型的异构现象是 IGCSE CCEA 考试的重要技能。
5. Geometric (Cis-Trans) Isomerism | 几何(顺反)异构
Geometric isomerism, also known as cis-trans isomerism, occurs in alkenes when each carbon atom of the C=C bond has two different groups attached. The restricted rotation around the double bond locks the groups in fixed positions. If the two identical (or priority) groups are on the same side, it is the cis isomer; if they are on opposite sides, it is the trans isomer.
几何异构，又称顺反异构，发生在双键碳原子各自连接两个不同基团的烯烃中。双键周围的旋转受限使基团固定在特定位置。若两个相同（或优先级高）的基团在双键同侧，则为顺式异构体；若在异侧，则为反式异构体。
For example, but-2-ene (CH₃–CH=CH–CH₃) exists as cis-but-2-ene (both methyl groups on the same side) and trans-but-2-ene (methyl groups on opposite sides). These isomers have different physical properties such as boiling points and dipole moments. IGCSE CCEA expects you to recognise when cis-trans isomerism is possible and to draw the two forms.
例如，丁-2-烯（CH₃–CH=CH–CH₃）存在顺-丁-2-烯（两个甲基在同侧）和反-丁-2-烯（甲基在异侧）。这些异构体具有不同的沸点和偶极矩等物理性质。IGCSE CCEA 要求你能够判断何时可能存在顺反异构，并能画出两种形式。
6. Physical Properties of Alkenes | 烯烃的物理性质
At room temperature, the first three members (ethene, propene, butenes) are colourless gases; alkenes with 5–15 carbon atoms are liquids, and higher alkenes are waxy solids. Alkenes are insoluble in water but dissolve in non-polar organic solvents. Their boiling points increase with molecular mass due to greater van der Waals forces.
室温下，前三个烯烃（乙烯、丙烯、各种丁烯）为无色气体；含5–15个碳原子的烯烃为液体，更高级的烯烃为蜡状固体。烯烃不溶于水，但可溶于非极性有机溶剂。由于分子间范德华力增大，它们的沸点随分子量增加而升高。
Branched alkenes tend to have lower boiling points than their straight-chain isomers because branching reduces surface contact, weakening intermolecular forces. Cis isomers generally have slightly higher boiling points than trans isomers due to a small net dipole moment.
支链烯烃的沸点通常低于其直链异构体，因为支链减少了分子间接触面积，削弱了分子间作用力。顺式异构体的沸点通常略高于反式异构体，因为顺式结构存在微小的净偶极矩。
7. Chemical Reactivity: Why Do Alkenes Undergo Addition Reactions? | 化学活性：烯烃为何发生加成反应？
The C=C double bond is an area of high electron density. The pi bond is weaker and more exposed than the sigma bond, so it breaks relatively easily. This allows alkenes to act as electrophilic centres, readily undergoing addition reactions. In an addition reaction, two reactant molecules combine to form a single product, with the double bond opening up to form two new single bonds.
C=C 双键是一个高电子密度区域。π 键比 σ 键更弱、更暴露，因此相对容易断裂。这使得烯烃可作为亲电中心，容易发生加成反应。在加成反应中，两个反应物分子结合形成一个产物，双键打开并形成两个新的单键。
Typical addition reactions include hydrogenation, halogenation, hydrohalogenation, and hydration. These reactions are characteristic tests for unsaturation and are used industrially to make a vast array of products, from margarine to polymers.
典型的加成反应包括氢化、卤化、与卤化氢加成以及水化。这些反应是检验不饱和性的特征反应，并被工业上用来制造从人造黄油到聚合物的多种产品。
8. Addition of Hydrogen – Hydrogenation | 与氢气加成——氢化
Alkenes react with hydrogen gas (H₂) in the presence of a nickel catalyst at about 150 °C to form alkanes. This is called catalytic hydrogenation. For example:
C₂H₄ + H₂ → C₂H₆
烯烃在镍催化剂存在下于约150 °C与氢气（H₂）反应生成烷烃。这称为催化加氢。例如：
C₂H₄ + H₂ → C₂H₆
This reaction is used industrially to convert liquid unsaturated vegetable oils into solid saturated fats for margarine production. The degree of hydrogenation controls the hardness of the product.
该反应在工业上用于将液态不饱和植物油转化为固态饱和脂肪，以生产人造黄油。氢化的程度控制产品的硬度。
9. Addition of Halogens – Halogenation | 与卤素加成——卤化
Alkenes react quickly with halogens (e.g. bromine, chlorine) at room temperature without the need for a catalyst. The reaction with bromine water is a standard test for unsaturation: orange-brown bromine water is decolourised as the alkene forms a colourless dibromoalkane. For ethene:
C₂H₄ + Br₂ → C₂H₄Br₂
烯烃在室温下迅速与卤素（如溴、氯）反应，无需催化剂。与溴水的反应是检验不饱和性的标准方法：橙黄色的溴水褪色，因为烯烃生成了无色的二溴代烷。以乙烯为例：
C₂H₄ + Br₂ → C₂H₄Br₂
Chlorine addition proceeds similarly, though sometimes with UV light initiation. The mechanism involves electrophilic addition where the pi electrons induce a dipole in the halogen molecule, leading to a bridged or carbocation intermediate.
氯加成反应类似，但有时需紫外光引发。反应机理涉及亲电加成：π 电子诱导卤素分子产生偶极，进而形成桥式或碳正离子中间体。
10. Addition of Hydrogen Halides | 与卤化氢加成
Alkenes add hydrogen halides (HCl, HBr, HI) to form haloalkanes. For symmetrical alkenes such as ethene, only one product is formed. For unsymmetrical alkenes like propene, Markovnikov’s rule predicts the major product: the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already attached, and the halide adds to the more substituted carbon. Thus:
CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (major product)
烯烃与卤化氢（HCl、HBr、HI）加成生成卤代烷。对于对称烯烃如乙烯，只生成一种产物。对于不对称烯烃如丙烯，马氏规则预测主要产物：氢原子加到含氢较多的双键碳上，卤原子加到取代基较多的碳上。因此：
CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (主要产物)
This reaction is important for synthesising specific haloalkanes and is explained by the stability of the carbocation intermediate formed during the reaction.
该反应对于合成特定的卤代烷至关重要，可用反应过程中形成的碳正离子中间体的稳定性来解释。
11. Addition of Water – Hydration | 与水的加成——水合
Alkenes can be hydrated to alcohols in the presence of an acid catalyst, usually concentrated phosphoric acid (H₃PO₄) or sulfuric acid (H₂SO₄), under high temperature and pressure. Ethene reacts with steam to form ethanol:
C₂H₄ + H₂O → C₂H₅OH
烯烃可在酸催化剂（通常为浓磷酸 H₃PO₄ 或硫酸 H₂SO₄）存在下，在高温高压下与水加成生成醇。乙烯与水蒸气反应生成乙醇：
C₂H₄ + H₂O → C₂H₅OH
This is an industrial method for ethanol production. For unsymmetrical alkenes, Markovnikov addition applies, giving the more substituted alcohol as the major product.
这是工业生产乙醇的方法之一。对于不对称烯烃，加成遵循马氏规则，生成取代较多的醇作为主要产物。
12. Polymerisation of Alkenes | 烯烃的聚合反应
Alkenes can undergo addition polymerisation. The double bond opens up, and monomers join together to form long polymer chains. For example, ethene polymerises to poly(ethene) (also called polythene):
n CH₂=CH₂ → –(CH₂–CH₂)–ₙ
烯烃可以发生加聚反应。双键打开，单体彼此连接形成长链聚合物。例如，乙烯聚合成聚乙烯：
n CH₂=CH₂ → –(CH₂–CH₂)–ₙ
Propene forms poly(propene). The reaction requires high pressure, a catalyst, and moderate temperature. Polymers are unreactive, lightweight, and versatile materials used in packaging, fabrics, and containers. IGCSE CCEA often asks you to draw the repeating unit from a given monomer or vice versa.
丙烯则生成聚丙烯。该反应需要高压、催化剂和中等温度。聚合物是不活泼、轻质且多用途的材料，用于包装、织物和容器。IGCSE CCEA 经常要求你根据给定单体画出重复单元，或反之。
13. Test for Unsaturation | 不饱和性检验
The most common test for the presence of a C=C bond is the bromine water test. Shake a few drops of orange-brown bromine water with the sample. If an alkene is present, the bromine water is rapidly decolourised. Alkanes do not decolourise bromine water in the dark (though they may react slowly under UV light via substitution).
检验 C=C 键存在的最常用方法是溴水试验。将几滴橙黄色溴水与样品一起振荡。若样品中含有烯烃，溴水迅速褪色。烷烃在黑暗中不会使溴水褪色（虽然在紫外光下可能通过取代反应缓慢反应）。
This test works because bromine adds across the double bond, forming a colourless dibromo compound. It is a simple, effective way to distinguish between saturated and unsaturated hydrocarbons.
该试验的原理是溴与双键发生加成反应，生成无色的二溴代物。这是区分饱和烃与不饱和烃的一种简单有效的方法。
14. Cracking and the Production of Alkenes | 裂化与烯烃的生产
Alkenes are primarily obtained from petroleum fractions through catalytic cracking or steam cracking. Long-chain alkanes are broken down into smaller alkanes and alkenes at high temperature with a catalyst. This process is vital because it produces valuable short-chain alkenes (like ethene and propene) which are feedstocks for the petrochemical industry.
烯烃主要通过催化裂化或蒸汽裂化从石油馏分中获得。长链烷烃在高温和催化剂作用下分解为更小的烷烃和烯烃。这一过程至关重要，因为它能生产出有价值的短链烯烃（如乙烯和丙烯），作为石化工业的原料。
Cracking also generates hydrogen and branched-chain alkanes, which help meet the demand for fuels and raw materials. Understanding the link between crude oil and alkene chemistry is a key aspect of the IGCSE syllabus.
裂化还会生成氢气和支链烷烃，有助于满足燃料和原料的需求。理解原油与烯烃化学之间的联系是 IGCSE 课程大纲的一个关键方面。
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