📚 Alkenes: Key Points for GCSE Edexcel Chemistry | 烯烃考点精讲
Alkenes are a fascinating and essential family of hydrocarbons in organic chemistry. For GCSE Edexcel Chemistry, you need to master their structure, bonding, reactions and uses. This guide breaks down every key concept, providing clear explanations and comparing alkenes with alkanes to build deep understanding.
烯烃是有机化学中引人入胜且必不可少的烃类家族。针对GCSE Edexcel化学，你需要掌握它们的结构、键合、反应和用途。本指南将分解每一个关键概念，提供清晰的解释，并将烯烃与烷烃进行比较以建立深刻的理解。
1. What are Alkenes? Structure and General Formula | 什么是烯烃？结构与通式
Alkenes are unsaturated hydrocarbons. This means they contain at least one carbon– ecules. Unlike alkanes, which are saturated, the presence of the double bond gives alkenes a distinctive reactivity.
烯烃是不饱和烃。这意味着它们的分子中至少含有一个碳碳双键 (C=C)。与饱和的烷烃不同，双键的存在赋予了烯烃独特的反应活性。
The general formula for alkenes with one double bond is CₙH₂ₙ, where n is the number of carbon atoms. For example, ethene is C₂H₄ and propene is C₃H₆. Alkenes form a homologous series – a family of compounds with the same functional group, similar chemical properties, and each member differing by a CH₂ unit.
含有一个双键的烯烃通式为 CₙH₂ₙ，其中 n 是碳原子的数目。例如，乙烯是 C₂H₄，丙烯是 C₃H₆。烯烃形成一个同系列——具有相同的官能团、相似的化学性质，且相邻成员间相差一个 CH₂ 单元的化合物家族。
e bond, which is the site of most chemical reactions. Because alkenes have two fewer hydrogen atoms than the corresponding alkane (CₙH₂ₙ₊₂), they are described as unsaturated.
官能团是 C=C 双键，这是大多数化学反应的发生部位。因为烯烃比相应的烷烃 (CₙH₂ₙ₊₂) 少两个氢原子，它们被描述为不饱和的。
2. Naming Alkenes | 烯烃命名
Naming alkenes follows the same principles as alkanes, but with the suffix '-ene' instead of '-ane'. The stem indicates the number of carbon atoms (meth-, eth-, prop-, but-, pent-, etc.). The position of the double bond is shown by a number between the stem and the suffix, counting from the end of the carbon chain that gives the double bond the lowest possible number.
烯烃的命名遵循与烷烃相同的原则，但后缀用“-ene”代替“-ane”。词干表示碳原子数（甲、乙、丙、丁、戊等）。双键的位置用词干和后缀之间的数字表示，从使双键编号最小的碳链末端开始计数。
H₃) has it between carbon 2 and 3. Ethene and propene do not need a number because the double bond can only be in one position in these short chains.
例如，丁-1-烯 (CH₂=CHCH₂CH₃) 的双键位于碳1和碳2之间，而丁-2-烯 (CH₃CH=CHCH₃) 的双键位于碳2和碳3之间。乙烯和丙烯不需要数字，因为在这些短链中双键只能处于一个位置。
Remember that when drawing displayed formulae, all atoms and bonds must be shown. Each carbon atom must always have four bonds.
记住，绘制显示式时必须显示所有原子和键。每个碳原子始终必须有四个键。
3. Unsaturation and the Bromine Test | 不饱和性与溴水测试
le chemical test can detect unsaturation: shaking the substance with bromine water (an orange-brown solution). For an alkene, the bromine water rapidly decolorises, turning from orange-brown to colourless.
有机化学中的不饱和性是指存在多个键，如 C=C 双键。一个简单的化学测试可以检测不饱和性：将物质与溴水（橙棕色溶液）一起振荡。对于烯烃，溴水会迅速褪色，由橙棕色变为无色。
acts with bromine: C₂H₄ + Br₂ → C₂H₄Br₂. Alkanes do not react with bromine water unless exposed to UV light, so the test distinguishes alkenes from alkanes.
这种褪色是因为溴在加成反应中加到了双键上，形成二溴代烷。例如，乙烯与溴反应：C₂H₄ + Br₂ → C₂H₄Br₂。除非在紫外光照射下，烷烃不会与溴水反应，因此该测试可以区分烯烃和烷烃。
ght; it works instantly at room temperature, indicating the presence of a C=C double bond.
你必须记住，溴水测试不需要催化剂或紫外光；它在室温下立即反应，表明存在 C=C 双键。
4. Addition Reactions – An Overview | 加成反应概述
The characteristic reaction of alkenes is addition. In an addition reaction, the double bond opens up and two new atoms or groups attach to the carbon atoms, turning the unsaturated molecule into a saturated one. Only one product is formed, unlike substitution reactions with alkanes.
烯烃的特征反应是加成反应。在加成反应中，双键打开，两个新的原子或基团连接到碳原子上，将不饱和分子转化为饱和分子。与烷烃的取代反应不同，加成反应只生成一种产物。
General equation for addition of a reagent X–Y across the C=C bond: C=C + X–Y → X–C–C– e bond is replaced by a single bond, and the product belongs to another homologous series.
试剂 X–Y 在 C=C 双键上加成的通式：C=C + X–Y → X–C–C–Y。双键被单键取代，产物属于另一个同系列。
The main addition reactions covered in GCSE Edexcel are: hydrogenation (adding H₂), halogenation (adding Br₂ or Cl₂), hydrogen halide addition (adding HBr or HCl), and hydration (adding steam/water, H₂O).
GCSE Edexcel 涵盖的主要加成反应有：加氢（加 H₂）、加卤素（加 Br₂ 或 Cl₂）、加卤化氢（加 HBr 或 HCl）和水合（加水蒸气/水，H₂O）。
5. Addition of Hydrogen (Hydrogenation) | 加氢反应（氢化）
corresponding alkane. This process is called hydrogenation. For example: C₂H₄ + H₂ → C₂H₆.
烯烃与氢气在镍催化剂存在下，约150°C时反应生成相应的烷烃。这个过程称为氢化。例如：C₂H₄ + H₂ → C₂H₆。
Hydrogenation is used industrially to convert liquid unsaturated vegetable oils into solid saturated fats for margarine production. The reaction increases the melting point by removing double bonds, making the oil more solid at room temperature.
氢化在工业上用于将液态不饱和植物油转化为固态饱和脂肪，用于生产人造黄油。该反应通过消除双键提高了熔点，使油在室温下更加固态。
Essential conditions: finely divided nickel catalyst and a temperature of about 150°C. Hydrogen gas must be bubbled through the alkene. This is an addition reaction, not a reduction in the traditional sense, but hydrogen is added.
必要条件：细微的镍催化剂和约150°C的温度。氢气必须通过烯烃鼓泡。这是一个加成反应，不是传统意义上的还原，但氢被加了上去。
6. Addition of Halogens | 加卤素反应
ogen atom adds to each carbon. With ethene and bromine: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane).
烯烃在室温下无需催化剂即可与溴 (Br₂) 和氯 (Cl₂) 等卤素迅速反应。双键打开，每个碳原子上加一个卤素原子。乙烯与溴反应：C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-二溴乙烷)。
The product is a dihaloalkane. This reaction is the basis of the bromine water test. During the test, bromine adds to the alkene, and the colour of bromine disappears. The reaction is very quick and easy to observe.
产物是二卤代烷。这个反应是溴水测试的基础。在测试中，溴加成到烯烃上，溴的颜色消失。反应非常快，易于观察。
Chlorine also adds in a similar way, forming dichloroalkanes. These reactions are used to distinguish unsaturated from saturated hydrocarbons and to produce useful solvents and intermediates.
氯也以类似方式加成，生成二氯代烷。这些反应用于区分不饱和烃与饱和烃，并用于生产有用的溶剂和中间体。
7. Addition of Hydrogen Halides | 加卤化氢反应
the double bond, and the halogen to the other. For example, ethene with HBr: C₂H₄ + HBr → C₂H₅Br (bromoethane).
卤化氢如溴化氢 (HBr) 和氯化氢 (HCl) 可以加成到双键上。氢原子连接到双键的一个碳上，卤素连接到另一个碳上。例如，乙烯与 HBr 反应：C₂H₄ + HBr → C₂H₅Br (溴乙烷)。
If the alkene is unsymmetrical (e.g. propene), two possible products can form. In GCSE, you are not required to apply Markovnikov's rule in depth, but you should know that one product is usually favoured. For propene with HBr, the major product is 2-bromopropane (CH₃CHBrCH₃), not 1-bromopropane.
如果烯烃是不对称的（如丙烯），可能生成两种产物。在GCSE中，你不需要深入应用马尔科夫尼科夫规则，但应该知道通常一种产物是主要的。对于丙烯与HBr，主要产物是2-溴丙烷 (CH₃CHBrCH₃)，而不是1-溴丙烷。
Conditions: The reaction occurs at room temperature when the gases are mixed, or sometimes with an acid catalyst. It produces haloalkanes, which are useful in synthesis.
条件：气体混合时在室温下反应，或有时用酸催化剂。它生成卤代烷，这在合成中很有用。
8. Addition of Water (Hydration) | 加水反应（水合）
Alkenes can react with steam (water in the gas phase) to produce alcohols. This is called hydration. For example, ethene reacts with steam to form ethanol: C₂H₄ + H₂O ⇌ C₂H₅OH. The reaction is reversible, so conditions are chosen to favour the forward reaction.
烯烃可以与水蒸气（气相水）反应生成醇。这称为水合。例如，乙烯与水蒸气反应生成乙醇：C₂H₄ + H₂O ⇌ C₂H₅OH。该反应是可逆的，因此选择的条件要有利于正向反应。
Industrial conditions: a phosphoric acid (H₃PO₄) catalyst, temperature around 300°C, and high pressure (about 60–70 atm). The high pressure shifts the equilibrium towards fewer gas molecules (the product side). The alcohol produced is an important solvent and fuel.
工业条件：磷酸 (H₃PO₄) 催化剂，温度约300°C，高压（约60–70个大气压）。高压使平衡向气体分子数较少的方向（产物侧）移动。生成的醇是重要的溶剂和燃料。
This is an example of an addition reaction that does not result in a saturated hydrocarbon, but the product is a saturated compound (ethanol, C₂H₅OH, has only single bonds). Water adds as H–OH across the double bond.
这是一个加成反应不生成饱和烃的例子，但产物是饱和化合物（乙醇 C₂H₅OH 只有单键）。水以 H–OH 形式加在双键上。
9. Addition Polymerisation | 加成聚合
Addition polymerisation is a very important reaction in which many small alkene molecules (monomers) join together to form a long-chain polymer. The double bond of each monomer opens, and the monomers link to form a saturated backbone. No other product is formed, hence 'addition' polymerisation.
加成聚合是一个非常重要的反应，许多小分子烯烃（单体）连接在一起形成长链聚合物。每个单体的双键打开，单体连接形成饱和的主链。没有其他产物生成，因此称为“加成”聚合。
For example, poly(ethene) is made from ethene monomers: n CH₂=CH₂ → (–CH₂–CH₂–)ₙ, where n is a very large number. Poly(propene) is formed from propene: n CH₂=CHCH₃ → (–CH₂–CH(CH₃)–)ₙ.
例如，聚乙烯由乙烯单体制成：n CH₂=CH₂ → (–CH₂–CH₂–)ₙ，其中 n 是一个非常大的数字。聚丙烯由丙烯制成：n CH₂=CHCH₃ → (–CH₂–CH(CH₃)–)ₙ。
The repeating unit must be drawn with brackets and extending bonds. In displayed formulae, carbon atoms in the backbone are shown with single bonds, and any side groups are attached. The polymer is saturated along its main chain.
重复单元必须用括号和延伸键画出。在显示式中，主链上的碳原子用单键表示，任何侧基都连接在上面。聚合物沿其主链是饱和的。
10. Poly(ethene), Poly(propene) and Their Uses | 聚乙烯、聚丙烯及其用途
Poly(ethene), commonly known as polythene, is one of the most widely used plastics. It is lightweight, flexible, and a good electrical insulator. Low-density poly(ethene) (LDPE) is used for plastic bags and film wrap, while high-density poly(ethene) (HDPE) is used for bottles and pipes.
聚乙烯，通常称为塑料，是使用最广泛的塑料之一。它质轻、柔韧且是良好的电绝缘体。低密度聚乙烯 (LDPE) 用于塑料袋和保鲜膜，而高密度聚乙烯 (HDPE) 用于瓶子和管道。
Poly(propene) is stronger and more rigid, making it suitable for ropes, carpets, and food containers. Both polymers are produced from alkene monomers obtained from crude oil cracking. They are non-biodegradable, which raises environmental concerns.
聚丙烯更坚固、更硬，适用于绳索、地毯和食品容器。这两种聚合物均由来自原油裂解的烯烃单体制成。它们不可生物降解，这引发了环境问题。
The properties of a polymer depend on the monomer and the conditions of polymerisation. The ability to produce plastics with varied properties makes alkenes extremely valuable feedstocks in the chemical industry.
聚合物的性质取决于单体和聚合条件。生产具有不同性质的塑料的能力使烯烃成为化学工业中极有价值的原料。
11. Comparing Alkenes and Alkanes | 烯烃与烷烃的比较
It is crucial to compare these two families for the exam:
考试中比较这两个家族至关重要：
- General formula: Alkanes: CₙH₂ₙ₊₂; Alkenes: CₙH₂ₙ.
- 通式: 烷烃：CₙH₂ₙ₊₂；烯烃：CₙH₂ₙ。
- Bonding: Alkanes contain only single C–C bonds (saturated). Alkenes contain at least one C=C double bond (unsaturated).
- 键合: 烷烃仅含 C–C 单键（饱和）。烯烃至少含一个 C=C 双键（不饱和）。
- Bromine water test: Alkenes decolorise bromine water immediately; alkanes do not react in the dark.
- 溴水测试: 烯烃使溴水立即褪色；烷烃在黑暗中不反应。
- Reactivity: Alkenes are more reactive due to the double bond; they mainly undergo addition reactions. Alkanes undergo substitution (e.g. with halogens under UV light) and combustion.
- 反应活性: 烯烃由于双键更活泼；主要发生加成反应。烷烃发生取代反应（如在紫外光下与卤素）和燃烧。
Understanding these differences helps explain why alkenes are so useful for making polymers and other chemicals, while alkanes are primarily used as fuels.
了解这些差异有助于解释为什么烯烃对于制造聚合物和其他化学品如此有用，而烷烃主要用作燃料。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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