📚 IGCSE Chemistry: Alkenes – Key Exam Points | IGCSE 化学：烯烃 考点精讲
Alkenes are a family of unsaturated hydrocarbons that appear frequently in IGCSE Chemistry. Mastering their structure, bonding, reactions and real‑world uses is essential for top marks. This article walks you through every key concept in a clear, exam‑focused way.
烯烃是不饱和烃的一个家族，在IGCSE化学考试中经常出现。掌握它们的结构、键合、反应和实际用途是拿高分的关键。这篇文章将以清晰、紧扣考点的方式，带你梳理每一个重要概念。
1. General Formula and Structure | 通式与结构
Alkenes have the general formula CnH2n, where n is the number of carbon atoms (n ≥ 2). Each molecule contains at least one carbon–carbon double bond (C=C). The two carbon atoms of the double bond and the four atoms directly attached to them lie in the same plane, giving a trigonal planar arrangement around each C=C carbon. The double bond is rigid and does not allow free rotation, unlike the single bonds in alkanes.
烯烃的通式为 CnH2n，其中 n 是碳原子数（n ≥ 2）。每个分子至少含有一个碳碳双键（C=C）。双键上的两个碳原子以及与之直接相连的四个原子共平面，每个双键碳原子周围呈平面三角形。双键是刚性的，不能像烷烃中的单键那样自由旋转。
2. Naming Alkenes | 命名烯烃
IGCSE nomenclature follows these rules: select the longest continuous carbon chain that contains the C=C bond. Number the chain so that the double bond gets the lowest possible locant. The name ends with ‘-ene’, and the position of the double bond is indicated by the lower‑numbered carbon atom of the two. For example, CH2=CH2 is ethene; CH3CH=CH2 is propene (no number needed because the double bond must start at carbon 1 in a three‑carbon chain); CH3CH2CH=CH2 is but‑1‑ene, while CH3CH=CHCH3 is but‑2‑ene.
IGCSE 的命名规则如下：选取包含 C=C 双键的最长连续碳链。给碳链编号，使双键的位次最小。名称以“-ene”结尾，双键的位置用两个碳原子中编号较小的那一个表示。例如，CH2=CH2 命名为 ethene；CH3CH=CH2 是 propene（三碳链中双键只能从碳1开始，故省略位置号）；CH3CH2CH=CH2 为 but‑1‑ene，而 CH3CH=CHCH3 是 but‑2‑ene。
3. Isomerism in Alkenes | 烯烃的同分异构现象
Alkenes show both position isomerism and chain isomerism. Position isomers arise when the C=C bond can be placed at different locations in the chain, as with but‑1‑ene and but‑2‑ene. Chain isomers have different carbon skeletons; for C4H8, 2‑methylpropene (CH2=C(CH3)2) is a branched isomer. Recognising isomers helps when predicting products or drawing structural formulae.
烯烃存在位置异构和碳链异构。位置异构是指 C=C 双键可以在碳链的不同位置上，例如 but‑1‑ene 和 but‑2‑ene。碳链异构则具有不同的碳骨架；对于 C4H8，2‑methylpropene（CH2=C(CH3)2）是一个支链异构体。识别异构体有助于预测产物和绘制结构式。
4. Physical Properties | 物理性质
Short‑chain alkenes (up to C4) are colourless gases at room temperature; longer ones are liquids or waxy solids. Like alkanes, they are insoluble in water but dissolve in organic solvents. Melting and boiling points increase with chain length because of stronger London dispersion forces. Boiling points are typically slightly lower than those of the corresponding alkanes due to the less efficient packing of the double bond geometry.
短链烯烃（C4 以下）在室温下为无色气体，较长链的为液体或蜡状固体。与烷烃一样，它们不溶于水，但可溶于有机溶剂。随着碳链增长，熔点和沸点因伦敦色散力的增强而升高。由于双键几何结构导致分子堆积效率稍低，烯烃的沸点通常略低于相应的烷烃。
5. Chemical Properties: Addition Reactions | 化学性质：加成反应
The C=C double bond is an electron‑rich region, making alkenes much more reactive than alkanes. Their characteristic reactions are addition reactions, in which the pi bond breaks and two new sigma bonds form, so the double bond becomes a single bond. The general pattern is:
Alkene + X–Y → Alkane derivative (saturated product)
This is why alkenes are described as unsaturated – they can take on extra atoms by addition. The main addition reactions covered in IGCSE are hydrogenation, halogenation, addition of hydrogen halides, hydration and polymerisation.
C=C 双键是电子密集区，使烯烃远比烷烃活泼。其特征反应是加成反应：π键断裂，形成两个新的σ键，双键变为单键。反应通式如下：
烯烃 + X–Y → 饱和产物
这也就是为什么烯烃被称为不饱和烃——它们可以通过加成接纳额外的原子。IGCSE 涉及的主要加成反应包括氢化、卤化、卤化氢加成、水合和聚合。
6. Hydrogenation | 氢化反应
Alkenes react with hydrogen gas (H2) in the presence of a nickel catalyst at about 150 °C. The double bond opens and two hydrogen atoms add across it, converting the alkene into the corresponding alkane. For example:
C2H4 + H2 → C2H6
This reaction is used industrially to harden unsaturated vegetable oils into margarine by partially hydrogenating the oils. Hydrogenation is also a useful test‑tube demonstration of addition, though it is less common than the bromine water test.
烯烃与氢气在镍催化剂、约 150 °C 的条件下反应。双键打开，两个氢原子加到双键碳上，烯烃转变为相应的烷烃。例如：
C2H4 + H2 → C2H6
该反应在工业上用于将不饱和植物油部分氢化制成人造黄油。氢化反应也可以作为加成反应的演示实验，不过不如溴水试验常用。
7. Halogenation | 卤化反应
Alkenes add halogens (Cl2 or Br2) rapidly at room temperature without a catalyst. The product is a dihaloalkane. With ethene and bromine:
C2H4 + Br2 → C2H4Br2 (1,2‑dibromoethane)
The red‑brown colour of bromine disappears instantly, providing a convenient visual indication of an addition reaction. Chlorine reacts similarly, but bromine is preferred for testing because the colour change is easier to observe.
烯烃与卤素（Cl2 或 Br2）在室温下迅速发生加成反应，无需催化剂，生成二卤代烷。乙烯与溴的反应：
C2H4 + Br2 → C2H4Br2 (1,2‑二溴乙烷)
红棕色的溴立即褪去，为加成反应提供了一个很方便的视觉指示。氯气反应类似，但溴更常用于检验，因为颜色变化更易观察。
8. Test for Unsaturation (Bromine Water) | 不饱和检验（溴水）
Shaking an alkene with bromine water causes the orange‑brown solution to turn colourless. This is the standard IGCSE test for an unsaturated compound. If the liquid is an alkane, the bromine water stays orange‑brown. The chemical reaction is the same as the halogenation described above, but bromine water is used for safety and convenience. The test works for any compound containing a C=C bond, such as alkenes or unsaturated oils.
将烯烃与溴水一起振荡，橙黄色的溶液会变为无色。这是 IGCSE 检验不饱和化合物的标准方法。如果是烷烃，溴水保持橙黄色。其反应原理与前面描述的卤化反应相同，但使用溴水更安全方便。该检验适用于任何含有 C=C 键的化合物，如烯烃或不饱和油脂。
9. Addition of Hydrogen Halides | 卤化氢加成
Alkenes react with hydrogen halides (HCl, HBr, HI) to form haloalkanes. The reaction is often carried out at room temperature. For ethene with hydrogen bromide:
C2H4 + HBr → C2H5Br (bromoethane)
This is a straightforward way to introduce a halogen atom into an organic molecule. At IGCSE level, you only need to write equations for symmetric alkenes (like ethene) where only one product is formed. In unsymmetrical alkenes the addition can give two possible products, but that is beyond the scope of the IGCSE core syllabus.
烯烃与卤化氢（HCl、HBr、HI）反应生成卤代烷，反应通常在室温下进行。以乙烯与溴化氢为例：
C2H4 + HBr → C2H5Br (溴乙烷)
这是向有机分子中引入卤原子的直接方法。在 IGCSE 阶段，你只需会写对称烯烃（如乙烯）的方程式，它们只生成一种产物。对于不对称烯烃，加成可能生成两种可能的产物，但这已超出 IGCSE 核心大纲范围。
10. Hydration (Addition of Water) | 水合反应
Alkenes can add water (steam) across the double bond to produce alcohols. The reaction requires a phosphoric acid catalyst, high temperature (about 300 °C) and high pressure (60–70 atm). Ethene hydrates to ethanol:
C2H4 + H2O ⇌ C2H5OH
The reaction is reversible, so unreacted ethene and steam are recycled to improve yield. Hydration is the main industrial method for manufacturing ethanol, competing with fermentation. You must be able to recall the catalyst and conditions for this important addition reaction.
烯烃可以与水（蒸汽）在双键处发生加成反应生成醇。该反应需要磷酸催化剂、高温（约 300 °C）和高压（60–70 atm）。乙烯水合生成乙醇：
C2H4 + H2O ⇌ C2H5OH
此反应是可逆的，因此需将未反应的乙烯和蒸汽循环使用以提高产率。水合是工业上制备乙醇的主要方法，与发酵法竞争。你必须能够记住这个重要加成反应的催化剂和条件。
11. Polymerisation | 聚合反应
Addition polymerisation is the process in which many small alkene molecules (monomers) join together by opening their C=C bonds, forming a long‑chain polymer. The polymer contains only single bonds in its backbone. The reaction is initiated by high pressure and a suitable catalyst. Poly(ethene) is made from ethene:
n C2H4 → (-CH2-CH2-)n
Common addition polymers include:
| Monomer | Polymer | Uses |
|---|---|---|
| Ethene | Poly(ethene) (PE) | Plastic bags, bottles |
| Propene | Poly(propene) (PP) | Ropes
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