📚 IGCSE OCR Chemistry: Alkenes Key Points Explained | IGCSE OCR 化学：烯烃 考点精讲
Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond, C=C. They are vital feedstocks in the petrochemical industry, used to manufacture plastics, alcohols, detergents and many other materials. For OCR IGCSE Chemistry, you must be confident with their general formula, naming, unique addition reactions, the bromine water test for unsaturation, addition polymerisation, and how they are produced by cracking. This article covers all the essential points in a clear bilingual format.
烯烃是一类不饱和烃，分子中至少含有一个碳碳双键 (C=C)。它们是石化工业的重要原料，用于制造塑料、醇类、洗涤剂等。在 OCR IGCSE 化学中，你需要熟练掌握烯烃的通式、命名、特征加成反应、检验不饱和性的溴水实验、加成聚合反应，以及通过裂解获得烯烃的方法。本文以清晰的中英双语形式梳理所有核心考点。
1. General Formula and Introduction | 通式和简介
Alkenes have the general formula CnH2n, where n is the number of carbon atoms (n ≥ 2). They are unsaturated because they contain fewer hydrogen atoms than the corresponding alkane (CnH2n+2). The simplest alkene is ethene, C2H4, followed by propene, C3H6. The functional group is the carbon-carbon double bond.
烯烃的通式为 CnH2n (n 为碳原子数，n ≥ 2)。因为它们所含的氢原子数少于相应的烷烃 (CnH2n+2)，所以称为不饱和烃。最简单的烯烃是乙烯 (C2H4)，其次是丙烯 (C3H6)。官能团是碳碳双键。
2. Naming and Structure of Alkenes | 烯烃的命名与结构
Alkenes are named using the suffix ‘-ene’. The longest carbon chain containing the double bond is chosen. The chain is numbered from the end nearest the double bond, and the position of the double bond is indicated by a number. For example, but-1-ene (CH2=CHCH2CH3) has the double bond starting at carbon 1, while but-2-ene (CH3CH=CHCH3) starts at carbon 2. Positional isomers occur when the double bond can be placed in different locations.
烯烃以“-烯”为词尾命名。选择含有双键的最长碳链作为主链，从靠近双键的一端开始编号，并用数字标出双键的位置。例如，1-丁烯 (CH2=CHCH2CH3) 的双键从第 1 个碳开始，而 2-丁烯 (CH3CH=CHCH3) 从第 2 个碳开始。当双键可以处在不同位置时，会产生位置异构体。
3. The C=C Double Bond and Its Reactivity | C=C 双键及其反应性
The double bond consists of a sigma (σ) bond and a pi (π) bond. The π bond is formed by sideways overlap of p orbitals and is weaker than the σ bond. This makes the π electrons more exposed and accessible to attacking reagents. As a result, alkenes are much more reactive than alkanes and readily undergo addition reactions, breaking the weaker π bond while the stronger σ bond remains intact.
双键由一个 σ 键和一个 π 键组成。π 键由 p 轨道的侧面重叠形成，比 σ 键弱，因此 π 电子更加暴露，容易被进攻试剂攻击。所以烯烃比烷烃活泼得多，容易发生加成反应——较弱的 π 键断裂，而较强的 σ 键保持完整。
4. Addition Reaction: Hydrogenation | 加成反应：氢化
Hydrogenation is the addition of hydrogen (H2) across the double bond to form an alkane. The reaction requires a nickel catalyst and a temperature of around 150 °C. It is an important industrial process used to convert unsaturated vegetable oils into solid margarine.
氢化是将氢气 (H2) 加成到双键上，生成烷烃。反应需要镍催化剂和约 150 °C 的温度。这是一个重要的工业过程，用于将不饱和植物油转化为固态人造黄油。
C2H4 + H2 → C2H6 (ethene to ethane, Ni catalyst, 150 °C)
C2H4 + H2 → C2H6 (乙烯到乙烷，Ni 催化剂，150 °C)
5. Addition Reaction: Halogenation | 加成反应：卤化
Halogenation is the addition of a halogen such as bromine (Br2) or chlorine (Cl2) to the double bond. The reaction occurs rapidly at room temperature without a catalyst. Ethene reacts with bromine to form 1,2-dibromoethane, a colourless liquid. This reaction is the basis of the bromine water test for unsaturation.
卤化是指溴 (Br2) 或氯 (Cl2) 等卤素与双键发生加成。反应在室温下无需催化剂即可迅速进行。乙烯与溴反应生成 1,2-二溴乙烷（无色液体）。该反应是检验不饱和性的溴水实验的基础。
C2H4 + Br2 → CH2BrCH2Br (1,2-dibromoethane)
C2H4 + Br2 → CH2BrCH2Br (1,2-二溴乙烷)
6. Addition Reaction: Hydrohalogenation | 加成反应：与卤化氢
Hydrogen halides (e.g. HBr, HCl) also add across the double bond. The hydrogen atom attaches to one carbon of the double bond and the halogen to the other, forming a halogenoalkane. With unsymmetrical alkenes like propene, two products are possible: the major product follows Markovnikov’s rule, where the hydrogen attaches to the carbon with more hydrogen atoms already attached.
卤化氢（如 HBr, HCl）也可与双键加成。氢原子加到双键的一个碳上，卤原子加到另一个碳上，生成卤代烷。对于不对称烯烃（如丙烯），可能生成两种产物：主要产物遵循马氏规则，即氢加在含氢较多的双键碳上。
CH2=CH2 + HBr → CH3CH2Br (bromoethane)
CH2=CH2 + HBr → CH3CH2Br (溴乙烷)
7. Addition Reaction: Hydration | 加成反应：水合
Alkenes react with steam (water) to form alcohols. This is called hydration. The reaction requires a phosphoric acid (H3PO4) catalyst, high temperature (300 °C) and high pressure (60–70 atm). Ethene is hydrated to produce ethanol, a vital industrial solvent and biofuel.
烯烃与水蒸气发生反应生成醇，称为水合。反应需要磷酸 (H3PO4) 催化剂、高温 (300 °C) 和高压 (60–70 大气压)。乙烯水合生成乙醇，乙醇是一种重要的工业溶剂和生物燃料。
C2H4 + H2O → C2H5OH (H3PO4, 300 °C, 60 atm)
C2H4 + H2O → C2H5OH (磷酸，300 °C，60 大气压)
8. Bromine Water Test for Unsaturation | 溴水检验不饱和性
The most straightforward test for an alkene (or any unsaturated compound) is to shake it with orange-brown bromine water. If unsaturation is present, the bromine adds across the double bond, and the solution turns colourless immediately. This colour change is quick and unmistakable. Alkanes do not react with bromine water under the same conditions, so the test distinguishes alkenes from alkanes.
检验烯烃（或任何不饱和化合物）最简单的方法是与橙棕色的溴水一起振荡。如果不饱和键存在，溴会加成到双键上，溶液立即变为无色。这一颜色变化迅速且明显。烷烃在相同条件下不与溴水反应，因此该实验可以区分烯烃和烷烃。
9. Addition Polymerisation | 加成聚合
Alkenes can undergo addition polymerisation, where many small monomer molecules join together by opening their double bonds to form a long polymer chain. No other products are formed. The repeating unit of the polymer has the same atoms as the monomer, but with single bonds. For example, ethene polymerises to poly(ethene), commonly known as polythene.
烯烃可以发生加成聚合反应：许多小分子单体通过打开双键连接成长链聚合物，没有其他副产物。聚合物重复单元与单体含有相同的原子，只是变成了单键。例如，乙烯聚合成聚（乙烯），即常见的聚乙烯。
n CH2=CH2 → –[CH2–CH2]–n (poly(ethene))
n CH2=CH2 → –[CH2–CH2]–n (聚(乙烯))
Propene forms poly(propene). When drawing the repeating unit, it is essential to show the correct backbone and side groups, along with extension bonds through brackets.
丙烯形成聚（丙烯）。绘制重复单元时，必须正确展示主链和侧基，并用括号和延长线表示延展。
10. Uses and Environmental Impact of Polymers | 聚合物用途与环境影响
Addition polymers are widespread: poly(ethene) is used for plastic bags and bottles; poly(propene) for ropes and carpets; poly(chloroethene) (PVC) for window frames and pipes. However, most addition polymers are non-biodegradable, accumulating in landfills and oceans. Incineration can generate toxic gases, such as hydrogen chloride from PVC. Recycling and developing bioplastics help mitigate these problems.
加成聚合物用途广泛：聚乙烯用于塑料袋和瓶子；聚丙烯用于绳索和地毯；聚氯乙烯 (PVC) 用于窗框和管道。然而，大多数加成聚合物不可生物降解，会在填埋场和海洋中累积。焚化可能产生有毒气体，如 PVC 产生氯化氢。回收和开发生物塑料有助于缓解这些问题。
11. Cracking: Source of Alkenes | 裂解：烯烃的来源
Alkenes are produced industrially by cracking long-chain alkanes obtained from petroleum. In thermal cracking, high temperatures (up to 900 °C) and pressure break carbon-carbon bonds to produce a mixture of shorter alkanes and alkenes. Catalytic cracking uses a zeolite catalyst at around 500 °C to achieve more control and yield more branched and aromatic hydrocarbons. Ethene and propene are the most desired products because of their role in polymer production.
烯烃在工业上通过裂解石油中获取的长链烷烃来生产。热裂解在高温（可达 900 °C）和高压下使碳碳键断裂，生成短链烷烃和烯烃的混合物。催化裂解则使用沸石催化剂在约 500 °C 下进行，能更好地控制产物，获得更多支链和芳香烃。乙烯和丙烯因其在聚合物生产中的作用而需求最大。
12. Comparison with Alkanes | 与烷烃的比较
Alkanes have the general formula CnH2n+2, only single bonds, and are saturated. Alkenes have CnH2n, contain a C=C double bond, and are unsaturated. Alkanes undergo substitution reactions (e.g. with halogens in UV light) and do not decolourise bromine water. Alkenes undergo electrophilic addition reactions and rapidly decolourise bromine water. Both can be used as fuels, but alkenes are much more chemically useful as building blocks for synthesis.
烷烃通式为 CnH2n+2，只有单键，是饱和烃。烯烃通式 CnH2n，含有 C=C 双键，属不饱和烃。烷烃发生取代反应（如在紫外光下与卤素反应），不能使溴水褪色。烯烃发生亲电加成反应，能使溴水迅速褪色。两者都可用作燃料，但烯烃作为合成的“积木”更具化工价值。
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