📚 Alkenes in GCSE OCR Chemistry | GCSE OCR 化学：烯烃 考点精讲
Alkenes are a fundamental homologous series in organic chemistry. This article summarises the key points required for the GCSE OCR Chemistry specification, covering structure, naming, reactions and polymerisation of alkenes.
烯烃是有机化学中一个基础的同系物。本文梳理了 GCSE OCR 化学考试大纲的核心考点，涵盖烯烃的结构、命名、反应以及聚合内容。
1. What Are Alkenes? | 什么是烯烃？
Alkenes are unsaturated hydrocarbons because they contain at least one carbon‑carbon double bond (C=C). They are made up only of carbon and hydrogen atoms. The presence of the double bond makes them more reactive than alkanes.
烯烃是不饱和烃，因为其分子中至少含有一个碳碳双键（C=C）。它们仅由碳原子和氢原子组成。双键的存在使烯烃比烷烃更活泼。
2. General Formula and Naming | 通式与命名
The general formula for alkenes with one double bond is CₙH₂ₙ. The simplest alkene is ethene (n=2), followed by propene (n=3), butene (n=4) and pentene (n=5). The names all end in ‘‑ene’. When numbering the carbon chain, the double bond gets the lowest possible number, e.g. but‑1‑ene and but‑2‑ene.
含一个双键的烯烃通式为 CₙH₂ₙ。最简单的烯烃是乙烯（n=2），其次是丙烯（n=3）、丁烯（n=4）和戊烯（n=5）。名称均以“‑ene”结尾。给碳链编号时，要让双键获得尽可能小的位置号，例如丁‑1‑烯和丁‑2‑烯。
3. Functional Group: The C=C Double Bond | 官能团：碳碳双键
The functional group responsible for the characteristic reactions of alkenes is the C=C double bond. It consists of one sigma (σ) bond and one pi (π) bond. The pi bond is weaker and can break easily during addition reactions.
赋予烯烃特征反应的官能团是 C=C 双键。它由一个 σ 键和一个 π 键组成。π 键较弱，在加成反应中容易断裂。
4. Structure and Isomerism | 结构与同分异构
Alkenes can exhibit both chain and position isomerism. For example, butene (C₄H₈) has three isomers: but‑1‑ene, but‑2‑ene (with cis‑trans versions possible, though not emphasised at GCSE) and methylpropene. Drawing and naming these isomers is a common exam skill.
烯烃可表现出碳链异构和位置异构。例如，丁烯（C₄H₈）有三种异构体：丁‑1‑烯、丁‑2‑烯（可能存在顺反异构，但 GCSE 不作重点）和甲基丙烯。绘制并命名这些异构体是常见的考试技能。
5. Physical Properties of Alkenes | 烯烃的物理性质
Alkenes have physical properties similar to alkanes. As the number of carbon atoms increases, boiling points and viscosity rise, while flammability decreases. They are insoluble in water but dissolve in non‑polar solvents. At room temperature, ethene, propene and butene are gases.
烯烃的物理性质与烷烃相似。随着碳原子数增加，沸点和黏度升高，而可燃性下降。它们不溶于水，但溶于非极性溶剂。室温下，乙烯、丙烯和丁烯为气体。
6. Chemical Reactions: Combustion | 化学反应：燃烧
Alkenes burn in oxygen to produce carbon dioxide and water. Complete combustion of ethene:
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
However, incomplete combustion can produce carbon monoxide and soot (carbon), which is why alkenes often burn with a smoky flame in air.
烯烃在氧气中燃烧生成二氧化碳和水。乙烯的完全燃烧：
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
而不完全燃烧会生成一氧化碳和炭黑（碳），因此烯烃在空气中燃烧时常伴有黑烟。
7. Addition Reactions with Hydrogen | 与氢气的加成反应
Alkenes react with hydrogen in the presence of a nickel catalyst at around 150°C. This is called hydrogenation, and it converts an alkene into the corresponding alkane. For example, ethene + H₂ → ethane.
烯烃在镍催化剂存在下、约 150°C 时与氢气反应，称为氢化反应，可将烯烃转化为相应的烷烃。例如，乙烯 + H₂ → 乙烷。
C₂H₄ + H₂ → C₂H₆
8. Addition Reactions with Halogens | 与卤素的加成反应
Alkenes react rapidly with halogens (e.g. bromine, chlorine) at room temperature. The double bond opens up and a halogen atom adds to each carbon. Ethene and bromine form 1,2‑dibromoethane. No catalyst is needed.
烯烃在室温下就能与卤素（如溴、氯）迅速反应。双键打开，每个碳原子上加成一个卤原子。乙烯与溴反应生成 1,2‑二溴乙烷，无需催化剂。
C₂H₄ + Br₂ → C₂H₄Br₂
9. Addition Reactions with Water (Steam) | 与水的加成反应（水蒸气）
Alkenes can react with steam to produce alcohols. This hydration reaction requires a phosphoric acid (H₃PO₄) catalyst, high temperature (around 300°C) and high pressure (60–70 atm). Ethene becomes ethanol.
烯烃可与水蒸气反应生成醇。这种水合反应需要磷酸（H₃PO₄）催化剂、高温（约 300°C）和高压（60–70 个大气压）。乙烯转变为乙醇。
C₂H₄ + H₂O ⇌ C₂H₅OH
10. Test for Unsaturation: Bromine Water | 不饱和性检验：溴水试验
Bromine water is orange‑brown. When added to an alkene, it goes colourless immediately. This is the standard test for a C=C double bond. Alkanes, being saturated, do not decolourise bromine water under normal conditions.
溴水呈橙黄色。将其加入烯烃，溴水会立即褪为无色。这是检验 C=C 双键的标准方法。饱和的烷烃在通常条件下不能使溴水褪色。
11. Addition Polymerisation | 加成聚合
Alkenes can join together to form long chains called polymers. This process is addition polymerisation. The double bond in each monomer opens, and thousands of monomers link together. No other products are formed.
烯烃可以彼此连接形成长链，即聚合物。该过程称为加成聚合。每个单体的双键打开，成千上万个单体连接起来，没有其他产物生成。
12. Representing Polymers and Repeating Units | 聚合物与重复单元的表示
To show the repeating unit of a polymer, write the monomer with the double bond opened, and enclose it in brackets with an ‘n’ subscript. For poly(ethene) the repeating unit is –CH₂–CH₂–. The exam often asks you to draw the polymer from a given monomer, or vice versa.
表示聚合物的重复单元时，写出双键打开后的单体结构，并用方括号括起来，右下角标上“n”。聚乙烯的重复单元为 –CH₂–CH₂–。考试中常要求由给定单体画出聚合物，或反过来推断单体。
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