📚 5.6.5 Addition Reactions of Alkenes | 5.6.5 烯烃的加成反应
Alkenes are a family of hydrocarbons that contain at least one carbon-carbon double bond (C=C). This functional group makes them unsaturated and far more reactive than alkanes. For IGCSE Edexcel Science, the addition reactions of alkenes are a key topic under Section 5.6.5, covering how the double bond can open up to allow atoms to join across it. Mastering these reactions, along with their conditions and equations, is essential for understanding organic chemistry and its real-world applications.
烯烃是一类含有至少一个碳碳双键（C=C）的碳氢化合物。这个官能团使它们变得不饱和，且比烷烃活泼得多。在IGCSE Edexcel科学课程中，烯烃的加成反应是5.6.5节下的核心主题，主要涉及双键如何打开并让其他原子加成上去。掌握这些反应、反应条件和化学方程式，对于理解有机化学及其在现实世界中的应用至关重要。
1. What Are Alkenes? | 什么是烯烃？
Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ. They contain at least one C=C double bond, which is their functional group. The simplest alkene is ethene (C₂H₄), followed by propene (C₃H₆), butene (C₄H₈) and so on. Because they have fewer hydrogen atoms than the corresponding alkanes, they are described as unsaturated and can participate in addition reactions where the double bond breaks and new atoms are added.
烯烃是不饱和碳氢化合物，通式为 CₙH₂ₙ。它们至少含有一个 C=C 双键，这是它们的官能团。最简单的烯烃是乙烯（C₂H₄），其次是丙烯（C₃H₆）、丁烯（C₄H₈）等。由于氢原子数目比相应的烷烃少，它们被称为不饱和烃，可以发生加成反应——双键断裂并加上新的原子。
2. Understanding Unsaturation | 理解不饱和度的概念
Unsaturation refers to the presence of a carbon-carbon double or triple bond within a molecule. In alkenes, the C=C bond consists of one sigma bond and one pi bond. The pi bond is relatively weak and can easily be broken, which allows alkenes to act as a site for addition reactions. This contrasts with alkanes, which are saturated and only undergo substitution reactions under extreme conditions.
不饱和度指的是分子中含有碳碳双键或三键。在烯烃中，C=C 键由一个 σ 键和一个 π 键组成。π 键相对较弱，容易被打破，从而使烯烃成为加成反应的位点。这与饱和的烷烃形成对比，后者只在极端条件下发生取代反应。
3. General Equation for Addition Reactions | 加成反应的通式
The typical pattern of an alkene addition reaction can be written as: C₂H₄ + X–Y → CH₂X–CH₂Y. Here, the attacking molecule X–Y adds across the double bond, with the pi bond breaking and two new sigma bonds forming. This type of reaction is called electrophilic addition because the electron-rich double bond attracts electron-deficient species (electrophiles).
烯烃加成反应的典型模式可表示为：C₂H₄ + X–Y → CH₂X–CH₂Y。在这个反应中，进攻分子 X–Y 加在双键两端，π 键断裂，形成两个新的 σ 键。这类反应称为亲电加成，因为富电子的双键会吸引缺电子的物种（亲电试剂）。
4. Testing for Alkenes: Bromine Water | 检验烯烃：溴水试验
When orange-brown bromine water (Br₂ dissolved in water) is added to an alkene, the colour disappears immediately, turning colourless. This happens because bromine adds across the double bond to form a dibromoalkane. For example, ethene reacts with bromine: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane). This decolourisation is the standard chemical test for unsaturation. Alkanes do not decolourise bromine water under normal conditions.
将橙棕色的溴水（Br₂ 溶于水中）加入烯烃中时，颜色会立即褪去并变为无色。这是因为溴加成到了双键上，生成二溴代烷。例如，乙烯与溴反应：C₂H₄ + Br₂ → C₂H₄Br₂（1,2-二溴乙烷）。这种褪色现象是检验不饱和度的标准化学方法。烷烃在通常条件下不能使溴水褪色。
| Test | Alkene | Alkane |
|---|---|---|
| Bromine water (orange-brown) | Decolourises immediately | No change (remains orange-brown) |
5. Addition of Hydrogen: Hydrogenation | 加氢反应（氢化）
Hydrogen adds across the double bond of an alkene in the presence of a nickel catalyst at about 150 °C. This process is called hydrogenation and converts unsaturated alkenes into saturated alkanes. For example, ethene reacts with hydrogen to form ethane: C₂H₄ + H₂ → C₂H₆. This reaction is used industrially to harden vegetable oils, turning liquid unsaturated fats into solid saturated fats for margarine production.
在约150 °C和镍催化剂存在的条件下，氢气可以加成到烯烃的双键上。这一过程称为氢化，能将不饱和烯烃转化为饱和烷烃。例如，乙烯与氢气反应生成乙烷：C₂H₄ + H₂ → C₂H₆。工业上利用这一反应来硬化植物油，将液态不饱和脂肪转变为固态饱和脂肪，用于生产人造黄油。
- Condition: Nickel catalyst, 150 °C
- Product: Corresponding alkane
- Industrial use: Margarine manufacture
条件：镍催化剂，150 °C；产物：相应的烷烃；工业用途：人造黄油制造。
6. Addition of Halogens (Cl₂, Br₂) | 卤素加成（Cl₂、Br₂）
Halogens such as chlorine and bromine add rapidly to alkenes at room temperature without the need for a catalyst. The reaction with chlorine produces a dichloroalkane: C₂H₄ + Cl₂ → C₂H₄Cl₂ (1,2-dichloroethane). This product is useful as a solvent and as an intermediate in the manufacture of PVC. Similarly, bromine addition gives a dibromoalkane, as seen in the bromine water test.
卤素（如氯和溴）在室温下即可迅速与烯烃加成，无需催化剂。与氯反应生成二氯代烷：C₂H₄ + Cl₂ → C₂H₄Cl₂（1,2-二氯乙烷）。该产物可用作溶剂以及制造聚氯乙烯（PVC）的中间体。同样，与溴加成得到二溴代烷，正如溴水试验中所见。
7. Addition of Hydrogen Halides (HCl, HBr) | 卤化氢加成（HCl、HBr）
Hydrogen halides, such as hydrogen chloride (HCl) or hydrogen bromide (HBr), add across the double bond to give a monohaloalkane. For example, ethene + HCl → chloroethane (C₂H₅Cl). This reaction is important in the production of chloroalkanes used as solvents and pharmaceuticals. The addition follows Markovnikov’s rule when the alkene is unsymmetrical, but with ethene, only one product is formed regardless.
卤化氢（如氯化氢 HCl 或溴化氢 HBr）能与双键加成，生成一卤代烷。例如，乙烯 + HCl → 氯乙烷（C₂H₅Cl）。该反应在生产用作溶剂和药物的氯代烷时十分重要。当烯烃不对称时，加成遵循马氏规则；但对于乙烯这样的对称烯烃，无论如何都只生成一种产物。
8. Addition of Steam: Hydration of Alkenes | 水蒸汽加成：烯烃的水化反应
Steam adds to an alkene in the presence of an acid catalyst (phosphoric acid, H₃PO₄) at high temperature (about 300 °C) and pressure (60–70 atm). This hydration reaction directly produces an alcohol. For ethene, the reaction is: C₂H₄ + H₂O ⇌ C₂H₅OH (ethanol). This industrial method is the main way ethanol is manufactured for use as a fuel and solvent. The reaction is reversible, so conditions are chosen to maximise yield.
在磷酸（H₃PO₄）催化剂、高温（约300 °C）和高压（60–70 atm）条件下，水蒸汽可与烯烃加成。这一水化反应直接生成醇。乙烯的反应为：C₂H₄ + H₂O ⇌ C₂H₅OH（乙醇）。该工业方法是生产燃料和溶剂用乙醇的主要途径。反应是可逆的，因此选择的条件要使产量最大化。
Condition Summary: C₂H₄ + H₂O ⇌ C₂H₅OH
- Catalyst: Phosphoric acid (H₃PO₄)
- Temperature: ≈300 °C
- Pressure: 60–70 atm
催化剂：磷酸；温度：约300 °C；压力：60–70 atm。
9. Addition Polymerisation | 加成聚合反应
Alkenes can also undergo addition polymerisation, where many small alkene monomers join together by breaking their double bonds to form long polymer chains. For example, poly(ethene) is produced from ethene monomers: n C₂H₄ → (–CH₂–CH₂–)ₙ. This type of reaction is used to make a huge range of plastics, including poly(propene), PVC, and PTFE. No other product is formed, making it an addition polymerisation in the true sense.
烯烃还可以发生加成聚合反应：许多烯烃单体通过打开双键而连接起来，形成长链聚合物。例如，由乙烯单体制得聚乙烯：n C₂H₄ → (–CH₂–CH₂–)ₙ。这类反应用来制造种类繁多的塑料，包括聚丙烯、PVC 和特氟龙。反应中不产生其他副产物，是真正的加成聚合。
10. Comparing Addition Reactions Table | 加成反应对比表
| Reactant added | Product type | Key conditions | Example equation |
|---|---|---|---|
| Hydrogen (H₂) | Alkane | Ni catalyst, 150 °C | C₂H₄ + H₂ → C₂H₆ |
| Halogen (Br₂, Cl₂) | Dihaloalkane | Room temp, no catalyst | C₂H₄ + Br₂ → C₂H₄Br₂ |
| Hydrogen halide (HCl) | Monohaloalkane | Room temp | C₂H₄ + HCl → C₂H₅Cl |
| Steam (H₂O) | Alcohol | H₃PO₄, 300 °C, 60–70 atm | C₂H₄ + H₂O ⇌ C₂H₅OH |
This table summarises the key addition reactions of alkenes for quick revision. Notice that all these reactions involve breaking the C=C pi bond and forming two new sigma bonds, which is the hallmark of addition chemistry.
该表总结了烯烃的关键加成反应，方便快速复习。请注意，所有这些反应都涉及断裂 C=C π 键并形成两个新的 σ 键，这正是加成化学的标志。
11. Common Pitfalls and Exam Tips | 常见错误和应试技巧
Many students confuse addition reactions of alkenes with substitution reactions of alkanes. Remember: alkanes react with halogens in UV light via substitution; alkenes react with halogens in the dark via addition. Another common mistake is writing incorrect molecular formulae: when an alkene reacts with Br₂, the product has both bromine atoms attached to the original double-bonded carbons. Also, be sure to indicate the catalyst and conditions for hydrogenation and hydration, as examiners frequently award marks for specifying these details.
许多学生容易混淆烯烃的加成反应和烷烃的取代反应。请记住：烷烃在紫外光下与卤素发生取代反应；烯烃在暗处与卤素发生加成反应。另一个常见错误是写错分子式：当烯烃与 Br₂ 反应时，两个溴原子都连接在原本双键的两个碳原子上。此外，务必标明氢化和水化反应的催化剂与条件，因为考官常常在这类细节上设置得分点。
- Don’t write ‘ethane’ when you mean ‘ethene’.
- Use structural formulae if required: CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br.
- Highlight the colour change from orange-brown to colourless as the test for unsaturation.
不要把“ethene”写成“ethane”；如果题目要求，要会写结构式：CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br；强调从不饱和键检验的溴水由橙棕色变为无色这一颜色变化。
12. Real-World Context and Summary | 实际应用与总结
Understanding addition reactions of alkenes helps to explain how chemists manufacture a wide array of everyday products: ethanol for fuels and hand sanitisers, polyethylene for packaging, and margarine from hydrogenated oils. The reactivity of the C=C bond lies at the heart of these transformations. For Edexcel IGCSE Science, fluency in writing balanced equations, recalling reaction conditions, and linking these reactions to their industrial uses is a strong indicator of high achievement.
理解烯烃的加成反应有助于解释化学家如何制造各种各样的日常产品：作为燃料和洗手液的乙醇、用于包装的聚乙烯，以及通过氢化油脂生产的人造黄油。C=C 键的活泼性是这些转变的核心。对Edexcel IGCSE科学考试来说，能熟练书写配平的化学方程式、记住反应条件并能将这些反应与工业用途联系起来，是取得高分的明显标志。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导