📚 GCSE CCEA Chemistry: Alkenes Exam Revision | GCSE CCEA 化学：烯烃 考点精讲
Alkenes are a fascinating and essential topic in GCSE CCEA Chemistry. As unsaturated hydrocarbons containing a carbon-carbon double bond, their distinctive reactivity makes them a central pillar of organic chemistry. In this revision guide, we will systematically cover everything you need to know: the general formula and nomenclature, structural isomerism, the key addition reactions (including hydrogenation, halogenation and hydration), the classic bromine water test for unsaturation, and the principles of addition polymerisation. We will also compare alkenes with alkanes to highlight their differences. Use this article to consolidate your understanding and build confidence for the exam.
烯烃是 GCSE CCEA 化学中既迷人又重要的专题。作为含有碳碳双键的不饱和烃，它们独特的反应活性使其成为有机化学的核心支柱。在本篇复习精讲中，我们将系统梳理所有你需要掌握的考点：通式与命名、结构异构、关键的加成反应（包括氢化、卤化与水合）、经典的不饱和性检验——溴水测试，以及加成聚合反应的原理。我们还会对比烷烃与烯烃，突出它们的差异。请利用本文巩固理解，建立考试信心。
1. What are Alkenes? | 什么是烯烃？
on double bond (C=C), which is the functional group that governs their chemical behaviour.
烯烃是一类不饱和烃的同系物。每一分子烯烃都至少含有一个碳碳双键（C=C），该双键是决定其化学行为的官能团。
The general formula for the alkene series is CnH2n ). This formula highlights that alkenes have two fewer hydrogen atoms than the corresponding alkanes (CnH2n+2), making them unsaturated.
烯烃同系物的通式为 CnH2n（n 代表碳原子数，且 n ≥ 2）。该通式突显了烯烃比对应的烷烃（通式 CnH2n+2）少两个氢原子，因而它们是不饱和的。
The simplest alkenes are ethene (C2H4), propene (C3H6) and butene (C4H8). Their physical properties – such as melting point, boiling point and viscosity – increase with molecular size, just like alkanes.
最简单的烯烃有乙烯（C2H4）、丙烯（C3H6）和丁烯（C4H8）。它们的物理性质——如熔点、沸点和黏度——随分子尺寸增大而升高，这一点与烷烃类似。
2. Naming and Structural Isomerism | 命名与结构异构
Alkenes are named using standard IUPAC rules: choose the longest carbon chain that includes the double bond, number the chain from the end nearest the double bond, and indicate the position of the double bond with the lowest possible number. The suffix '-ene' replaces '-ane'.
烯烃的命名遵循标准 IUPAC 规则：选取含有双键的最长碳链为主链，从靠近双键的一端开始编号，并用尽可能小的数字标明双键的位置。词尾用“-烯”替换烷烃的“-烷”。
For example, but-1-ene has the double bond between carbon 1 and 2 (CH2=CHCH2CH3), while but-2-ene has the double bond between carbon 2 and 3 (CH3CH=CHCH3). These two molecules are structural isomers: they share the same molecular formula C4H8 but have different arrangements of atoms.
例如，丁-1-烯的双键位于碳1和碳2之间（CH2=CHCH2CH3），而丁-2-烯的双键位于碳2和碳3之间（CH3CH=CHCH3）。这两种分子互为结构异构体：它们的分子式同为 C4H8，但原子的排列方式不同。
CCEA candidates should be able to draw and name alkenes with up to four carbon atoms and recognise that moving the double bond creates a different isomer. Branched alkenes like methylpropene are also covered.
CCEA 考生应能绘制并命名含有多达四个碳原子的烯烃，并认识到双键位置的改变会产生不同的异构体。带有支链的烯烃（如甲基丙烯）也在考查范围之内。
3. The C=C Double Bond and Unsaturation | C=C 双键与不饱和性
The carbon atoms in the C=C double bond are sp2 hybridised, giving a planar geometry. The double bond consists of a strong sigma (σ) bond and a weaker pi (π) bond. The pi bond is a region of high electron density and makes alkenes significantly more reactive than alkanes.
C=C 双键中的碳原子采取 sp2 杂化，呈平面几何构型。双键由一个较强的 σ 键和一个较弱的 π 键组成。π 键区域电子密度很高，这使烯烃的反应活性远高于烷烃。
Because alkenes are unsaturated, they are able to undergo addition reactions: atoms or groups can add across the double bond, converting it into a single bond and saturating the molecule. This is the cornerstone of alkene chemistry.
由于烯烃是不饱和的，它们能够发生加成反应：原子或基团可以加成到双键的两端，将双键转变为单键，使分子饱和。这是烯烃化学的基石。
4. Testing for Alkenes: Bromine Water | 溴水检验烯烃
The classic test for an alkene (or any unsaturated organic compound) is the addition of bromine water. Bromine water is an orange-brown solution. When it is shaken with an alkene, the orange colour rapidly disappears and the solution turns colourless.
检验烯烃（或任何不饱和有机化合物）的经典方法是加入溴水。溴水为橙棕色溶液。将其与烯烃一起振荡，橙棕色会迅速褪去，溶液变为无色。
The chemical reaction is an addition: Br2 adds across the C=C double bond, forming a colourless dibromoalkane. For ethene: CH2=CH2 + Br2 → CH2BrCH2Br (1,2-dibromoethane). No catalyst or UV light is required – this is a rapid electrophilic addition.
该化学反应为加成反应：Br2 加成到 C=C 双键上，生成无色的二溴代烷。以乙烯为例：CH2=CH2 + Br2 → CH2BrCH2Br（1,2-二溴乙烷）。此反应无需催化剂或紫外光——它是一个快速的亲电加成反应。
This test distinguishes alkenes from alkanes, as alkanes do not react with bromine water in the dark. It is the standard test for unsaturation.
该测试可以区分烯烃与烷烃，因为烷烃在暗处不与溴水反应。这是不饱和性的标准检验方法。
5. Addition Reactions – An Overview | 加成反应概论
An addition reaction is a chemical reaction in which two or more molecules combine to form a single, larger product. In alkenes, the pi bond breaks and new sigma bonds form with the incoming atoms, leaving only single bonds between the original carbon atoms.
加成反应是指两个或多个分子结合形成一个更大的单分子的化学反应。在烯烃中，π 键断裂，并与进入的原子形成新的 σ 键，使得原来碳原子之间只留下单键。
and water (hydration). All these reactions are characteristic of the C=C functional group.
烯烃可以与氢气（氢化）、卤素（卤化）、卤化氢（加卤化氢）和水（水合）发生加成反应。所有这些反应都是 C=C 官能团的特征反应。
6. Hydrogenation (Addition of Hydrogen) | 氢化（与氢气加成）
When an alkene is mixed with hydrogen gas and passed over a hot nickel catalyst (typically at 150 °C), the hydrogen adds across the double bond to produce the corresponding alkane.
将烯烃与氢气混合，并通过热的镍催化剂（通常在 150 °C），氢气便会加成到双键上生成对应的烷烃。
The general equation for hydrogenation is: CnH2n + H2 → CnH2n+2. For example, ethene is converted into ethane: C2H4 + H2 → C2H6 onverting some of their C=C bonds into single bonds, raising the melting point of the oil.
氢化反应的通式为：CnH2n + H2 → CnH2n+2。例如，乙烯转化为乙烷：C2H4 + H2 → C2H6。该工艺在工业上用于固化植物油，通过将部分 C=C 键转化为单键，从而提高油脂的熔点。
7. Halogenation (Addition of Halogens) | 卤化（与卤素加成）
Alkenes react rapidly with halogens such as chlorine (Cl2) or bromine (Br2) at room temperature without any catalyst. The halogen adds across the double bond to form a dihaloalkane.
烯烃在室温下无需催化剂即可与氯（Cl2）、溴（Br2）等卤素快速反应。卤素加成到双键上生成二卤代烷。
Using bromine as an example: CH2=CH2 + Br2 → CH2BrCH2Br. If propene is used, the product is 1,2-dibromopropane (CH2BrCHBrCH3). The rapid decolourisation of bromine is the basis of the unsaturation test.
以溴为例：CH2=CH2 + Br2 → CH2BrCH2Br。若使用丙烯，产物为 1,2-二溴丙烷（CH2BrCHBrCH3）。溴的快速褪色正是不饱和性检验的依据。
8. Addition of Hydrogen Halides | 与卤化氢加成
Hydrogen halides such as HCl, HBr and HI add readily to alkenes. The hydrogen atom attaches to one carbon of the double bond, and the halide ion attaches to the other, producing a haloalkane.
卤化氢如 HCl、HBr 和 HI 容易与烯烃发生加成。氢原子连接到双键的一个碳上，卤离子连接到另一个碳上，生成卤代烷。
For ethene: CH2=CH2 + HBr → CH3CH2Br (bromoethane). When an unsymmetrical alkene like propene is used, two products are possible; however, the major product generally follows Markovnikov's rule: the hydrogen atom adds to the carbon that already has more hydrogen atoms. Thus, from propene with HBr, 2-bromopropane (CH3CHBrCH3) is the main product.
以乙烯为例：CH2=CH2 + HBr → CH3CH2Br（溴乙烷）。当使用不对称烯烃如丙烯时，可能生成两种产物；然而主产物通常遵循马尔科夫尼科夫规则：氢原子加成到含氢较多的碳原子上。因此，丙烯与 HBr 反应的主产物是 2-溴丙烷（CH3CHBrCH3）。
9. Hydration (Addition of Water) | 水合（与水的加成）
Alkenes can react with steam to produce alcohols. This reaction requires a phosphoric acid (H3PO4) catalyst, high temperature (around 300 °C) and high pressure (about 60–70 atm).
烯烃可以与水蒸气反应生成醇。该反应需要磷酸（H3PO4）作催化剂，并在高温（约 300 °C）和高压（约 60–70 atm）下进行。
The general equation for hydration is: CnH2n + H₂O ⇌ CₙH2n+2O (an alcohol). For ethene, this produces ethanol: CH2=CH2 + H2O ⇌ C2H5OH. This reaction is reversible, so a high yield of ethanol is favoured by using excess steam and removing the ethanol as it forms.
水合反应的通式为：CnH2n + H2O ⇌ CnH2n+2O（醇）。对乙烯而言，反应生成乙醇：CH2=CH2 + H2O ⇌ C2H5OH。这是一个可逆反应，因此使用过量的水蒸气并及时移走生成的乙醇有利于提高乙醇的产率。
Hydration of ethene is a key industrial route to ethanol, competing with fermentation. It offers the advantage of a continuous process and a purer product, although it relies on non-renewable crude oil fractions.
乙烯的水合是生产乙醇的一条重要工业路线，与发酵法竞争。其优点是连续生产且产品更纯，不过它依赖不可再生的原油馏分。
10. Addition Polymerisation | 加成聚合
ogether to form a long-chain polymer. The double bonds break open, and the monomers link up to form a saturated backbone consisting only of single bonds. Published by TutorHao | GCSE Chemistry Revision Series | aleveler.com
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