GCSE AQA Chemistry: Alkenes Revision Focus | GCSE AQA 化学：烯烃 考点精讲

📚 GCSE AQA Chemistry: Alkenes Revision Focus | GCSE AQA 化学：烯烃 考点精讲

Alkenes are a vital family of hydrocarbons in GCSE AQA Chemistry, introduced as the first unsaturated compounds students encounter. Their carbon–carbon double bond defines a whole new set of reactions, from decolourising bromine water to forming long‑chain polymers. Mastering alkenes means understanding structure, naming, addition reactions and the ways they differ from alkanes – exactly the material that appears year after year in the AQA exam papers.

烯烃是 GCSE AQA 化学中极为重要的烃类化合物，也是学生最早接触的不饱和有机物。碳碳双键的存在赋予烯烃一系列独特的反应，比如使溴水褪色、通过加成形成长链聚合物等。要掌握烯烃，就必须透彻理解其结构、命名、加成反应，以及它们与烷烃的区别——这些正是 AQA 试卷中反复出现的考点。

1. What Are Alkenes? | 什么是烯烃？

Alkenes are a homologous series of hydrocarbons that contain at least one carbon–carbon double bond. Because they have two fewer hydrogen atoms than the corresponding alkanes, they are described as unsaturated. The simplest alkene is ethene (C₂H₄), followed by propene (C₃H₆) and butene (C₄H₈).

烯烃是含有至少一个碳碳双键的烃类同系物。由于它们比对应的烷烃少两个氢原子，因此被称为不饱和烃。最简单的烯烃是乙烯 (C₂H₄)，其次是丙烯 (C₃H₆) 和丁烯 (C₄H₈)。

2. General Formula and Naming | 通式与命名

The general molecular formula for alkenes with one double bond is CₙH₂ₙ. You name alkenes by replacing the ‘‑ane’ ending of the parent alkane with ‘‑ene’, and for molecules with four or more carbon atoms you must use a number to show the position of the double bond. For example, but‑1‑ene has the double bond between carbons 1 and 2, while but‑2‑ene has it between carbons 2 and 3.

含一个双键的烯烃通式为 CₙH₂ₙ。命名时，将对应烷烃词尾的“烷”改为“烯”；对于碳原子数不少于 4 的烯烃，还需用数字标出双键的位置。例如，丁‑1‑烯的双键在 1 号和 2 号碳之间，而丁‑2‑烯的双键则在 2 号和 3 号碳之间。

• The prefix tells you the number of carbon atoms: meth‑ (1), eth‑ (2), prop‑ (3), but‑ (4).
• 前面缀表示碳原子数：甲‑ (1)、乙‑ (2)、丙‑ (3)、丁‑ (4)。
• The number between the prefix and ‘‑ene’ indicates the lower‑numbered carbon of the double bond.
• 前缀和“烯”之间的数字代表双键上编号较小的碳原子位置。

Always count the longest chain that includes the double bond. This is a common exam pitfall: students sometimes count the wrong chain and lose marks on naming.

务必将包含双键的最长碳链作为主链。这是考试中的常见失分点：许多学生选错了主链，导致命名错误而丢分。

3. The Carbon–Carbon Double Bond | 碳碳双键

The C=C double bond consists of two pairs of shared electrons, making it much more reactive than a single C–C bond. One of these bonds is relatively weak and can break open to allow atoms to add across the double bond. This high electron density is what makes alkenes vulnerable to attack by electrophiles.

碳碳双键由两对共用电子组成，反应活性远高于单键。其中一个键相对较弱，容易断裂，使其他原子能够加成到双键两端。这种高电子密度正是烯烃易受亲电试剂进攻的原因。

In the exam, you may be asked to draw the displayed structure, showing all atoms and bonds. Always draw the double bond as two parallel lines and make sure each carbon atom forms four bonds in total.

考试中你可能需要画出烯烃的完整结构式，展示所有原子与化学键。双键一定要画成两条平行线，并确保每个碳原子总共形成四个共价键。

4. Unsaturated Nature | 不饱和性质

A molecule is unsaturated if it contains at least one double bond between carbon atoms. Since alkenes have a double bond, they are unsaturated, in contrast to alkanes which are saturated because every carbon atom forms four single bonds. The unsaturation gives alkenes two key properties: they can undergo addition reactions, and they can decolourise bromine water – a classic test for unsaturation.

如果分子中含有至少一个碳碳双键，就被视为不饱和化合物。烯烃含有双键，因此是不饱和的；而烷烃每个碳原子都形成四个单键，是饱和的。不饱和度赋予烯烃两个关键性质：它们能发生加成反应，并且能使溴水褪色——这是检验不饱和性的经典方法。

5. Addition Reactions – An Overview | 加成反应概述

Alkenes take part in addition reactions, where the double bond opens up and two new atoms or groups attach to the carbon atoms that previously formed the double bond. Only one product is formed. This contrasts with substitution reactions of alkanes, where two products are usually made.

烯烃参与加成反应，反应时双键打开，两个新的原子或原子团分别加到原来形成双键的碳原子上，只生成一种产物。这与烷烃的取代反应截然不同——取代反应通常生成两种产物。

The general pattern can be summarised as:

加成反应的通式可概括为：

C=C + X–Y → X–C–C–Y

This is a favourite AQA topic because it links structure to chemical behaviour.

这是 AQA 考试的热点，因为它将结构与化学行为有机地联系了起来。

6. Reaction with Halogens and the Bromine Test | 与卤素反应及溴水测试

When an alkene is shaken with bromine water (orange), the water turns colourless. This is an addition reaction where bromine adds across the double bond to form a dibromoalkane. The equation for ethene and bromine is:

将烯烃与溴水（橙黄色）一起振荡，溴水会褪为无色。这是一个加成反应，溴分子加成到双键上，生成二溴代烷。乙烯与溴反应的方程式为：

C₂H₄ + Br₂ → C₂H₄Br₂

The same principle applies with chlorine or iodine, although the bromine test is the standard laboratory method for detecting an unsaturated compound. The rapid decolourisation without UV light distinguishes alkenes from alkanes, which only react with bromine under ultraviolet light via substitution.

氯或碘也遵循同样的原理，但溴水试验是实验室检测不饱和化合物的标准方法。快速褪色且无需紫外线照射，这一点将烯烃与烷烃区分开来——烷烃只能在紫外光下与溴发生取代反应。

Make sure you can write both the word and the balanced symbol equation for this reaction.

请务必既能写出化学方程式，也能用文字表述该反应。

7. Hydrogenation | 加氢反应

Alkenes react with hydrogen gas in the presence of a nickel catalyst at around 150 °C. This addition reaction converts an unsaturated alkene into a saturated alkane. For example, ethene becomes ethane:

在镍催化剂、约 150 °C 的条件下，烯烃与氢气发生加成反应，不饱和的烯烃转变为饱和的烷烃。例如，乙烯转变为乙烷：

C₂H₄ + H₂ → C₂H₆

Hydrogenation is used in industry to harden vegetable oils, turning liquid oils into solid spreads. The double bonds in the unsaturated fat molecules are hydrogenated to raise the melting point.

加氢在工业上用于硬化植物油，使液态油转变为固态涂抹脂。不饱和脂肪分子中的双键通过加氢反应被饱和，熔点随之提高。

8. Hydration: Addition of Water | 水合反应

Alkenes can react with steam to produce alcohols. This is an industrially important method for making ethanol from ethene. The conditions required are: a phosphoric acid catalyst, a temperature of about 300 °C and a pressure of around 60–70 atmospheres. The reaction is reversible, so only a portion of the ethene is converted in each pass.

烯烃能与水蒸气反应生成醇。这是从乙烯制备乙醇的重要工业方法。反应条件为：磷酸作催化剂，温度约 300 °C，压力约 60–70 个大气压。该反应可逆，因此每次只有部分乙烯转化为乙醇。

C₂H₄ + H₂O ⇌ C₂H₅OH

Notice the reversible arrow (⇌) in the equation – examiners often credit this explicitly.

注意方程式中的可逆号 (⇌) ——评卷人经常专门给这一点赋分。

Hydration of ethene is preferred over fermentation when a pure, continuous supply of ethanol is required, though it relies on crude oil as a raw material. You should be able to compare the two methods of ethanol production.

在需要纯度高、可连续供应乙醇时，乙烯水合法优于发酵法，但它以原油为原料。学生应能比较这两种乙醇生产方法。

9. Reaction with Hydrogen Halides | 与卤化氢反应

Alkenes add hydrogen halides (HCl, HBr, HI) to form halogenoalkanes. For example, ethene reacts with hydrogen bromide to give bromoethane:

烯烃与卤化氢（HCl, HBr, HI）发生加成反应，生成卤代烷。例如，乙烯与溴化氢反应生成溴乙烷：

C₂H₄ + HBr → C₂H₅Br

These reactions are fast and occur at room temperature. The hydrogen halide molecule adds across the double bond, with the hydrogen atom attaching to one carbon and the halide to the other.

这些反应在室温下即可快速进行。卤化氢分子加成到双键上，氢原子加到一个碳原子上，卤素原子加到另一个碳原子上。

For unsymmetrical alkenes like propene, Markovnikov’s rule is not required at GCSE level, but AQA may expect you to recognise that more than one product is possible. Stick to symmetrical alkenes in most written questions unless told otherwise.

对于丙烯等不对称烯烃，GCSE 阶段不要求掌握马氏规则，但 AQA 可能希望你知道存在生成多种产物的可能。除非题目特别说明，一般以对称烯烃书写方程式为主。

10. Addition Polymerisation | 加成聚合

Alkenes are the monomers used to make addition polymers. Under high pressure and with a suitable catalyst, many small alkene molecules (monomers) join together by opening their double bonds, forming a long saturated chain called a polymer. The process is called addition polymerisation.

烯烃是制备加成聚合物的单体。在高压和适当催化剂作用下，许多烯烃小分子（单体）通过打开双键连接在一起，形成一条长链饱和分子，即聚合物。该过程称为加成聚合。

For example, poly(ethene) – commonly known as polythene – is made from ethene monomers:

例如，聚乙烯（俗称“塑料”）由乙烯单体制得：

n C₂H₄ → [–CH₂–CH₂–]ₙ

Poly(propene) is formed from propene, and poly(chloroethene) (PVC) from chloroethene. You should be able to represent the repeating unit of a polymer given the monomer, and vice versa.

聚丙烯由丙烯单体聚合而成，聚氯乙烯 (PVC) 则由氯乙烯制得。考试中你要能从单体画出聚合物的重复单元，也能从重复单元反推单体。

AQA commonly provides the displayed structure of the monomer and asks you to draw a section of the polymer chain. Remember to show the single bond in the backbone and the side groups attached.

AQA 通常会给出单体的完整结构式，要求你画出部分聚合物链。记得画出主链上的单键，并正确连接侧基。

11. Comparing Alkanes and Alkenes | 烷烃与烯烃的对比

Understanding the differences between alkanes and alkenes is a core part of the GCSE organic chemistry specification. A summary table is often the best way to secure these marks in the exam:

理解烷烃与烯烃的区别是 GCSE 有机化学的核心内容。用表格总结往往是考试中拿到这部分分数的有效方法：

Feature	Alkanes	Alkenes
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Functional group	None (C–C single bonds)	C=C double bond
Saturation	Saturated	Unsaturated
Reaction type	Substitution (with halogens, UV light)	Addition (with H₂, H₂O, halogens, hydrogen halides)
Bromine water test	No change without UV (stays orange)	Turns colourless immediately
Combustion	Cleaner burn (blue flame)	Smokier flame (incomplete combustion more likely due to higher C:H ratio)

Use the acronym ‘SUAS – Saturated, Unreactive, Alkanes, Substitution’ to remember alkane behaviour, and ‘UAA – Unsaturated, Addition, Alkenes’ for alkenes.

可以用缩写“SUAS——饱和、不活泼、烷烃、取代”来记忆烷烃行为，用“UAA——不饱和、加成、烯烃”来记忆烯烃行为。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

When writing equations for addition reactions, always check that only one product is shown. Do not include an extra small molecule like water or hydrogen as a second product – that would describe a substitution or elimination, not addition.

书写加成反应方程式时，务必确认只生成了一种产物。不要把水或氢气等小分子写成第二种产物——那会写成取代反应或消除反应，而不是加成反应。

For naming, practice numbering the carbon chain from the end closest to the double bond. If the double bond is exactly in the middle, choose the end that gives the lowest number for any other side group present.

在命名时，要从离双键较近的一端开始给碳链编号。如果双键恰好在正中央，则选择能使其他取代基编号较小的那端进行编号。

Many students lose marks on the bromine water test by stating that ‘bromine water goes clear’ rather than ‘colourless’. ‘Clear’ is not a colour; use ‘colourless’.

很多学生在描述溴水试验时失分，因为他们写的是“溴水变清澈”，而不是“褪为无色”。“清澈”不是一种颜色；一定要写“无色”。

Finally, when drawing polymers, ensure the repeating unit has two carbon atoms in the backbone for ethene‑based polymers, and that all atoms satisfy the bonding rules.

最后，在绘制聚合物结构时，要确保以乙烯为单体的聚合物其重复单元主链上有两个碳原子，并且所有原子都满足化合价规则。

Published by TutorHao | Chemistry Revision Series | aleveler.com

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