📚 IGCSE CIE Chemistry: Alkenes Exam Tips | IGCSE CIE 化学：烯烃 考点精讲
n the Cambridge IGCSE Chemistry syllabus you must be able to describe their structure and general formula, name them using IUPAC rules, explain their addition reactions, and describe how they form addition polymers. This article summarises the key exam points, worked equations and the mistakes that cost candidates marks, so you can revise efficiently and aim for the top grade.
烯烃是一类重要的不饱和碳氢化合物同系列，分子中含有至少一个碳碳双键（C=C）。在剑桥 IGCSE 化学大纲中，你需要掌握其结构与通式、IUPAC 命名规则、加成反应，以及加聚反应生成聚合物的过程。本文总结关键考点、常用方程式与考生最容易失分的地方，帮助你高效复习、冲刺高分。
1. Introduction to Alkenes | 烯烃简介
Alkenes are unsaturated hydrocarbons, which means they contain at least one carbon-carbon double bond. They belong to the same homologous series: every member differs from the next by one CH₂ unit, they have the same general formula, they show a gradual change in physical properties, and they all react in a similar way because they share the same functional group.
烯烃是不饱和烃，即分子中含有至少一个碳碳双键。它们属于同一同系列：相邻成员相差一个 CH₂ 单元，具有相同的通式，物理性质随碳链增长呈规律性变化，并且因为含有相同的官能团，化学性质相似。
Because of the double bond, alkenes are more reactive than alkanes. The double bond can open up and allow other atoms to add across it, so alkenes undergo addition reactions. This makes them extremely useful as chemical feedstocks for making plastics, ethanol, antifreeze and many other products. Industry obtains most short-chain alkenes by cracking larger alkane fractions from crude oil.
由于双键的存在，烯烃比烷烃更活泼。双键可以打开，让其他原子加到两个碳原子上，因此烯烃能发生加成反应。这使它们成为制造塑料、乙醇、防冻剂等产品的关键化工原料。工业上主要通过裂解原油中较大的烷烃馏分来获得短链烯烃。
The simplest alkene is ethene (C₂H₄), followed by propene (C₃H₆) and butene (C₄H₈). Ethene is the monomer used to make poly(ethene), the most common plastic in the world.
最简单的烯烃是乙烯（C₂H₄），其次是丙烯（C₃H₆）和丁烯（C₄H₈）。乙烯是制造聚乙烯的单体，而聚乙烯是世界上产量最大的塑料。
2. General Formula and Functional Group | 通式与官能团
d, written as >C=C<, and this group is responsible for all their characteristic reactions.
含一个双键的非环状烯烃的通式为 CₙH₂ₙ，其中 n ≥ 2。其官能团是碳碳双键，可写作 >C=C<，烯烃所有的特征反应都源于这个官能团。
Alkene: CₙH₂ₙ | Alkane: CₙH₂ₙ₊₂
For example, ethene is C₂H₄ and propene is C₃H₆. Notice that for every carbon atom in the chain, there are twice as many hydrogen atoms as carbon atoms, so the carbon-to-hydrogen ratio is relatively high and the percentage by mass of carbon is higher than in the corresponding alkane. This is why alkenes burn with a more smoky, luminous flame than alkanes.
例如，乙烯为 C₂H₄，丙烯为 C₃H₆。可以看到氢原子数是碳原子数的两倍，因此碳氢比较高，碳的质量分数也高于对应的烷烃。这解释了为什么烯烃燃烧时的火焰比烷烃更明亮、更易冒黑烟。
One exam trap: cycloalkanes also have the formula CₙH₂ₙ, but they are saturated and do not decolourise bromine water. If a question gives you CₙH₂ₙ, check whether a double bond or a ring is being described.
一个常见陷阱：环烷烃的通式同样是 CₙH₂ₙ，但它们是饱和的，不能使溴水褪色。如果题目给出 CₙH₂ₙ，要判断描述的是双键还是环状结构。
3. Naming Alkenes | 烯烃的命名
IUPAC naming of alkenes follows these rules: choose the longest carbon chain that contains the double bond; number the chain from the end that gives the double bond gets the lowest possible number; use the suffix -ene; and place the number of the first carbon of the double bond immediately before the -ene ending.
烯烃的 IUPAC 命名规则如下：选取包含双键的最长碳链；从离双键最近的一端开始编号，使双键位次最小；词尾使用 -ene（烯）；并把双键起始碳的位次数字写在 -ene 之前。
- CH₂=CH₂ - ethene (乙烯)
- CH₂=CH-CH₃ - propene (丙烯)
- CH₂=CH-CH₂-CH₃ - but-1-ene (丁-1-烯)
- CH₃-CH=CH-CH₃ - but-2-ene (丁-2-烯)
Side chains are named alphabetically in front of the main chain name, and if the same group appears more than once, prefixes such as di- or tri- are used. Remember that the position number is essential: but-1-ene and but-2-ene are different compounds with different boiling points.
支链按字母顺序写在主链名称之前；若同一基团出现多次，使用 di-、tri- 等前缀。要注意位次数字不可省略：丁-1-烯与丁-2-烯是两种不同的化合物，沸点也不相同。
| Structure 结构 | Name 名称 | Formula 分子式 |
| CH₂=CH₂ | ethene 乙烯 | C₂H₄ |
| CH₂=CH-CH₃ | propene 丙烯 | C₃H₆ |
| CH₂=CH-CH₂-CH₃ | but-1-ene 丁-1-烯 | C₄H₈ |
| CH₃-CH=CH-CH₃ | but-2-ene 丁-2-烯 | C₄H₈ |
4. Drawing Structures Correctly | 正确书写结构式
Examiners often ask for a displayed formula rather than a molecular formula. In a displayed formula every atom and every bond is drawn out, and the double bond must be shown as two separate parallel lines between the two carbon atoms.
考官常要求书写展示式（displayed formula）而不是分子式。展示式需要画出每一个原子和每一根键，其中双键必须画成两条平行的短线，连接两个碳原子。
Ethene 乙烯: H₂C=CH₂ (displayed: H-C=C-H with the two C-H bonds on each carbon)
or but-2-ene. A molecular formula only shows the number of each atom, for example C₄H₈. Read the command word carefully: "state the molecular formula" and "draw the displayed formula" require completely different answers.
结构式按顺序写出基团，例如丁-2-烯为 CH₃CH=CHCH₃；分子式只写出各原子的数目，例如 C₄H₈。一定要看清指令词："写出分子式"和"画出展示式"要求的答案是截然不同的。
5. Isomerism in Alkenes | 烯烃的同分异构现象
Isomers are compounds with the same molecular formula but different arrangements of atoms, and therefore different structures and properties. Alkenes show two important types at IGCSE level: position isomerism, where the double bond is in a different place, and chain isomerism, where the carbon skeleton is branched.
同分异构体是指分子式相同但原子排列不同，因而结构和性质也不同的化合物。在 IGCSE 阶段烯烃有两种重要类型：位置异构，即双键位置不同；以及碳链异构，即碳骨架带有支链。
- C₄H₈: but-1-ene (丁-1-烯) and but-2-ene (丁-2-烯) - position isomers 位置异构
- C₄H₈: 2-methylpropene (2-甲基丙烯) - a branched chain isomer 支链异构
- C₅H₁₀: pent-1-ene, pent-2-ene and 2-methylbut-1-ene 等
As an extension, but-2-ene also shows cis-trans isomerism because the two CH₃ groups can be on the same side (cis) or opposite sides (trans) of the double bond. Check your syllabus statement before you use terminology that is not required for your paper.
作为拓展，丁-2-烯还存在顺反异构：两个 CH₃ 基团可以位于双键同侧（顺式）或异侧（反式）。使用大纲未要求的术语前，请先核对你所在考纲的具体表述。
6. Testing for Unsaturation | 检验不饱和性
The standard test for an alkene is bromine water. Bromine water is orange-brown; when it is shaken with an alkene at room temperature it is decolourised and turns colourless. Alkanes produce no change under the same conditions.
检验烯烃的标准方法是用溴水。溴水呈橙棕色，在室温下与烯烃一起振荡后褪色变为无色；相同条件下烷烃没有明显变化。
C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane 1,2-二溴乙烷)
This is an addition reaction, not a substitution. The two bromine atoms add across the double bond, so no hydrogen bromide is produced. In contrast, alkanes react with bromine only in the presence of ultraviolet light, and then by substitution, giving bromoalkane plus hydrogen bromide.
这是加成反应，不是取代反应。两个溴原子加到双键两侧，因此不会生成溴化氢。相比之下，烷烃只有在紫外光照射下才与溴反应，且属于取代反应，生成溴代烷和溴化氢。
Exam tip: always write "orange-brown to colourless" or "decolourised", and never write "clear" - clear only means transparent, not colourless.
答题提示：应写"橙棕色变为无色"或"褪色"，不要写"变清澈"——"clear"仅表示透明，并不表示无色。
7. Addition Reactions: The General Pattern | 加成反应的通式规律
d two new atoms or groups are attached. Only one product is formed; nothing is eliminated.
在加成反应中，双键的两个碳原子各形成一根新的单键，C=C 变为 C-C，同时连接两个新的原子或基团。反应只生成一种产物，没有小分子被脱去。
C=C + X-Y → C(X)-C(Y)
| Reagent 试剂 | Condition 条件 | Product 产物 |
| H₂ 氢气 | nickel catalyst 镍催化 | alkane 烷烃 |
| Br₂ 溴 | room temperature 室温 | dibromoalkane 二溴代烷 |
| HBr 溴化氢 | room temperature 室温 | bromoalkane 溴代烷 |
| steam 水蒸气 | H₃PO₄ catalyst, 300 °C, 60 atm | alcohol 醇 |
8. Hydrogenation and Hydration | 加氢反应与加水反应
Hydrogenation is the addition of hydrogen across the double bond. Ethene reacts with hydrogen in the presence of a nickel catalyst to form ethane. The same idea is used in the food industry to harden vegetable oils, turning liquid oils into semi-solid fats such as margarine.
加氢反应是氢气加到双键两侧的反应。乙烯在镍催化下与氢气反应生成乙烷。食品工业利用同一原理把液态植物油加氢固化，制成人造黄油等半固体脂肪。
C₂H₄ + H₂ → C₂H₆ (nickel catalyst 镍催化)
Hydration is the addition of water in the form of steam. Ethene reacts with steam over a phosphoric acid catalyst at about 300 °C and 60 atmospheres to make ethanol. This is the main industrial route to ethanol and is a reversible reaction, written with the ⇌ symbol in some textbooks.
加水反应是水蒸气加到双键上的反应。乙烯在磷酸催化下、约 300 °C 和 60 个大气压下与水蒸气反应生成乙醇。这是工业制乙醇的主要方法，该反应是可逆反应，部分教材用 ⇌ 表示。
C₂H₄ + H₂O → C₂H₅OH (H₃PO₄, 300 °C, 60 atm)
Compare this with fermentation, which uses glucose and yeast and gives a much lower yield but needs simpler equipment.
可与发酵法比较：发酵法以葡萄糖和酵母为原料，产率低得多，但设备要求简单。
9. Addition of Halogens and Hydrogen Halides | 加卤素与加卤化氢
Halogens add directly across the double bond. Bromine water is both the test for unsaturation and an example of electrophilic addition: ethene gives 1,2-dibromoethane, a colourless liquid. Chlorine behaves in the same way, which is why alkenes are used to remove chlorine from some industrial waste streams.
卤素可以直接加到双键上。溴水既是不饱和性的检验试剂，也是加成反应的实例：乙烯与溴反应生成无色的 1,2-二溴乙烷。氯气也会发生类似反应，因此烯烃可用于除去某些工业废水中的氯。
Hydrogen halides such as HBr also add across the double bond, and a hydrogen atom goes to one carbon while the halogen goes to the other. With ethene, the only product is bromoethane.
溴化氢等卤化氢同样能加成到双键上，氢原子加到一个碳上，卤素加到另一个碳上。乙烯与 HBr 反应只生成一种产物：溴乙烷。
C₂H₄ + HBr → C₂H₅Br (bromoethane 溴乙烷)
Extension: with an unsymmetrical alkene such as propene, the halogen tends to attach to the carbon that already has fewer hydrogen atoms, so propene plus HBr gives mainly 2-bromopropane. Only use this rule if your syllabus or your teacher has covered it.
拓展内容：对于丙烯这类不对称烯烃，卤素倾向于加在含氢较少的碳上，因此丙烯与 HBr 主要生成 2-溴丙烷。只有在考纲或老师讲过时才使用这一规律。
10. Addition Polymerisation | 加聚反应
her to form one very large molecule. The double bonds open and the monomers link carbon to carbon, with no other product formed.
加聚反应是许多含 C=C 的不饱和单体分子相互连接形成一个大分子的过程。反应中双键打开，单体以碳碳单键相连，不生成其他产物。
n CH₂=CH₂ → (–CH₂–CH₂–)ₙ
The repeat unit is the smallest section of the polymer that repeats, and it must be written inside brackets with a subscript n outside. Notice that the repeat unit contains a C-C single bond, never a double bond.
重复单元是聚合物中不断重复的最小片段，必须写在括号内，并在括号外标注下标 n。注意重复单元内只有 C-C 单键，绝不能再出现双键。
| Monomer 单体 | Polymer 聚合物 | Repeat unit 重复单元 |
| ethene 乙烯 | poly(ethene) 聚乙烯 | (–CH₂–CH₂–)ₙ |
| propene 丙烯 | poly(propene) 聚丙烯 | (–CH₂–CH(CH₃)–)ₙ |
| chloroethene 氯乙烯 | poly(chloroethene) PVC 聚氯乙烯 | (–CH₂–CHCl–)ₙ |
| tetrafluoroethene 四氟乙烯 | PTFE 聚四氟乙烯 | (–CF₂–CF₂–)ₙ |
Do not confuse addition polymerisation with condensation polymerisation. In condensation polymerisation two different monomers react and a small molecule such as water is eliminated; in addition polymerisation the monomer is the only reactant and there is no by-product.
不要把加聚反应与缩聚反应混淆：缩聚反应由两种不同单体反应并脱去水等小分子；加聚反应中单体是唯一反应物，没有副产物。
11. Uses and Environmental Issues of Polymers | 聚合物的用途与环境问题
Poly(ethene) is used for carrier bags, bottles and plastic film because it is flexible, waterproof and unreactive. Poly(propene) is stronger and is used for crates, ropes and car parts. PVC is used for pipes and window frames, and PTFE is used for non-stick coatings because it is very unreactive and heat resistant.
聚乙烯柔韧、防水且不活泼，常用于购物袋、瓶子和塑料薄膜。聚丙烯强度更高，用于周转箱、绳索和汽车部件。聚氯乙烯用于管道和窗框；聚四氟乙烯因极不活泼且耐热，用于不粘涂层。
The problem is that addition polymers are generally non-biodegradable, so they persist in landfill and in the ocean. Burning them can release toxic gases, and burning PVC can release hydrogen chloride. Recycling, reusing and developing biodegradable alternatives are the main ways to reduce the impact.
问题在于加聚产物通常难以生物降解，会在垃圾填埋场和海洋中长期存在。焚烧可能释放有毒气体，聚氯乙烯焚烧还会释放氯化氢。回收利用、重复使用以及开发可生物降解的替代品，是减少影响的主要途径。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Most marks are lost through careless detail rather than a lack of knowledge. Always check the command word, the conditions and the state of the reactants before you write your answer.
大多数失分源于细节疏忽，而不是知识欠缺。动笔前务必看清指令词、反应条件和反应物状态。
- Write "addition" for alkenes and "substitution" for alkanes - never swap them. 烯烃答"加成"，烷烃答"取代"，不可混淆。
- Show the double bond as two lines in displayed formulas. 展示式中双键必须画成两条线。
- Include the number in the name: but-1-ene, not just butene. 命名必须带位次数字。
- Give conditions: nickel catalyst for hydrogenation, H₃PO₄ and 300 °C for hydration. 写出条件：加氢用镍催化，加水用磷酸、300 °C。
- In polymers, draw the repeat unit with a single bond and brackets with n. 聚合物重复单元内用单键，并加括号和下标 n。
- Say "decolourised" for the bromine water test, not "clear". 溴水检验写"褪色"，不写"变清澈"。
- Balance every equation and include state symbols if the question asks for them. 方程式要配平，若题目要求则写状态符号。
Useful equations to memorise in one place:
需要背熟的方程式汇总：
C₂H₄ + Br₂ → C₂H₄Br₂ | C₂H₄ + H₂ → C₂H₆ | C₂H₄ + H₂O → C₂H₅OH | n CH₂=CH₂ → (–CH₂–CH₂–)ₙ
Finally, practise drawing monomers from a given polymer repeat unit and vice versa. This reverse question appears regularly and is easy to score in if you remember that the double bond must be restored in the monomer.
最后，要练习由聚合物重复单元反推单体、以及由单体写出重复单元。这类逆向题目经常出现，只要记住"单体中必须把双键还原"，就能轻松得分。
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