📚 Alkanes – Key Points in IGCSE CIE Chemistry | IGCSE CIE 化学：烷烃 考点精讲
Alkanes are a foundational topic in IGCSE CIE Chemistry, appearing in questions about organic chemistry, fuels, and environmental impact. They are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. This guide brings together all the essential facts, reactions, and trends you need to remember, with worked examples and clear explanations to help you master the topic.
烷烃是 IGCSE CIE 化学的基础课题，常出现在有机化学、燃料和环境影响的相关问题中。它们属于饱和烃，即碳原子之间只含有单键。本指南汇总了你需要牢记的所有重要事实、反应和变化规律，并配有例题和清晰的解释，帮助你彻底掌握这一考点。
1. Introduction to Alkanes | 烷烃简介
Alkanes are organic compounds made up of only carbon and hydrogen atoms. They are described as saturated because all carbon-carbon bonds are single covalent bonds. This saturation makes them relatively unreactive compared to alkenes. The simplest alkane is methane, CH₄. Alkanes occur naturally in crude oil and natural gas, and they are used extensively as fuels and as starting materials for many chemical products.
烷烃是仅由碳和氢原子组成的有机化合物。它们被称为饱和烃，因为所有碳-碳键都是单共价键。这种饱和结构使它们与烯烃相比反应性较低。最简单的烷烃是甲烷 CH₄。烷烃天然存在于原油和天然气中，被广泛用作燃料以及许多化学产品的起始原料。
2. General Formula and Homologous Series | 通式与同系物
All alkanes fit the same general molecular formula: CₙH₂ₙ₊₂, where n is the number of carbon atoms (n = 1, 2, 3, …). Because each member differs from the next by a –CH₂– unit, they form a homologous series. Members of a homologous series share similar chemical properties and show gradual trends in physical properties such as boiling point. For example, methane (CH₄, n=1) and ethane (C₂H₆, n=2) both follow this rule.
所有烷烃都符合同一通式：CₙH₂ₙ₊₂，其中 n 是碳原子数（n = 1, 2, 3, …）。由于每个成员与下一个成员相差一个 –CH₂– 单元，它们构成一个同系物系列。同系物的成员具有相似的化学性质，并且在物理性质（如沸点）上呈现出逐渐变化的趋势。例如，甲烷（CH₄, n=1）和乙烷（C₂H₆, n=2）都遵循这一规律。
3. Naming Alkanes (IUPAC) | 烷烃命名（IUPAC）
The names of straight-chain alkanes are based on the number of carbon atoms, ending with ‘-ane’. The first ten are: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane. For branched alkanes, identify the longest continuous carbon chain as the parent name. Number the chain to give the substituent (alkyl group) the lowest possible number. Common alkyl groups include methyl (–CH₃) and ethyl (–C₂H₅). The position and name of the substituent are placed before the parent name. For example, 2-methylpropane has a three-carbon parent chain with a methyl group attached to carbon 2.
直链烷烃的命名基于碳原子数，词尾为‘-ane’。前十个依次是：甲烷、乙烷、丙烷、丁烷、戊烷、己烷、庚烷、辛烷、壬烷、癸烷。对于有支链的烷烃，找出最长的连续碳链作为母体名称。给碳链编号，使取代基（烷基）的位次尽可能小。常见的烷基包括甲基 (–CH₃) 和乙基 (–C₂H₅)。取代基的位次和名称写在母体名称之前。例如，2-甲基丙烷具有三个碳的母链，并在碳 2 上连接着一个甲基。
4. Structural Isomerism | 同分异构现象
Structural isomers are molecules that have the same molecular formula but different structural formulae. Alkanes with four or more carbon atoms can form isomers. For butane (C₄H₁₀), there are two isomers: straight-chain butane and branched 2-methylpropane. For pentane (C₅H₁₂), three isomers exist: pentane, 2-methylbutane and 2,2-dimethylpropane. Isomers have different physical properties, such as boiling points, because the shape of the molecule affects the strength of intermolecular forces. In the exam you may be asked to draw and name all possible isomers for a given molecular formula.
同分异构体是指具有相同分子式但不同结构式的分子。碳原子数为四或更多的烷烃可以形成异构体。丁烷（C₄H₁₀）有两种异构体：直链丁烷和支链 2-甲基丙烷。戊烷（C₅H₁₂）存在三种异构体：戊烷、2-甲基丁烷和 2,2-二甲基丙烷。异构体具有不同的物理性质（如沸点），因为分子的形状会影响分子间力的强度。在考试中，你可能会被要求画出并命名给定分子式的所有可能异构体。
5. Physical Properties | 物理性质
The physical properties of alkanes change in a predictable way as the length of the carbon chain increases. Melting points and boiling points rise, viscosity (thickness) increases, and the alkanes become less flammable. These trends are explained by stronger intermolecular forces (London dispersion forces) between larger molecules, which require more energy to overcome. The table below summarises the first few straight-chain alkanes.
随着碳链增长，烷烃的物理性质呈现出可预测的变化：熔点和沸点升高，黏度（稠度）增大，可燃性降低。这些趋势可以用较大分子间更强的分子间力（伦敦分散力）来解释，克服这些力需要更多的能量。下表总结了前几种直链烷烃的性质。
| Alkane | Molecular Formula | Boiling Point (°C) | State at Room Temp. |
|---|---|---|---|
| Methane | CH₄ | -162 | Gas |
| Ethane | C₂H₆ | -89 | Gas |
| Propane | C₃H₈ | -42 | Gas |
| Butane | C₄H₁₀ | -0.5 | Gas |
| Pentane | C₅H₁₂ | 36 | Liquid |
上表中文翻译：随着碳链增长，沸点升高；前四个烷烃在室温下为气体，戊烷为液体。这些数据在解释分馏原理时常常用到。
6. Combustion Reactions | 燃烧反应
Alkanes burn in excess oxygen to produce carbon dioxide and water, releasing a large amount of heat energy. This is complete combustion. The general equation for the complete combustion of an alkane is: CₙH₂ₙ₊₂ + (1.5n+0.5) O₂ → n CO₂ + (n+1) H₂O. For example, the combustion of propane:
烷烃在过量氧气中燃烧生成二氧化碳和水，同时释放大量热能，这称为完全燃烧。烷烃完全燃烧的通式为：CₙH₂ₙ₊₂ + (1.5n+0.5) O₂ → n CO₂ + (n+1) H₂O。例如丙烷的燃烧：
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
If there is not enough oxygen, incomplete combustion occurs. This produces carbon monoxide (a toxic gas) or carbon (soot) together with water. For instance, incomplete combustion of methane may produce carbon monoxide:
如果氧气不足，则发生不完全燃烧。这会产生一氧化碳（有毒气体）或碳（炭黑）以及水。例如甲烷的不完全燃烧可能生成一氧化碳：
2CH₄ + 3O₂ → 2CO + 4H₂O
Incomplete combustion is dangerous because CO is colourless, odourless, and prevents the blood from carrying oxygen. You should be able to write balanced equations for the complete combustion of given alkanes and recognise the products of incomplete combustion.
不完全燃烧是危险的，因为一氧化碳无色无味，而且会阻止血液携氧。你应当能够写出给定烷烃完全燃烧的配平方程式，并识别不完全燃烧的产物。
7. Substitution Reaction with Halogens | 与卤素的取代反应
Alkanes react with halogens (such as chlorine or bromine) in the presence of ultraviolet (UV) light. This is a substitution reaction, where a hydrogen atom in the alkane is replaced by a halogen atom. The reaction is a photochemical reaction because it requires light energy to break the halogen bond and start the chain mechanism. For methane and chlorine, the overall reaction is:
烷烃在紫外光（UV）光照下与卤素（如氯或溴）发生反应。这是一个取代反应，烷烃中的一个氢原子被卤素原子替换。该反应是光化学反应，因为它需要光能来断裂卤素键并启动链式机理。甲烷与氯气的总反应为：
CH₄ + Cl₂ → CH₃Cl + HCl
Further substitution can occur, replacing more hydrogen atoms to form dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃) and tetrachloromethane (CCl₄). The reaction happens slowly in the dark at room temperature, but quickly when exposed to UV light. In the IGCSE exam, you need to describe the conditions and write simple substitution equations. Remember that this reaction turns bromine water from brown to colourless, but only when UV light is present.
进一步取代可以发生，置换出更多的氢原子，生成二氯甲烷 (CH₂Cl₂)、三氯甲烷 (CHCl₃) 和四氯甲烷 (CCl₄)。在室温暗处反应缓慢，但在紫外光照射下反应迅速。在 IGCSE 考试中，你需要描述反应条件并书写简单的取代方程式。请记住，此反应可将溴水由棕褐色变为无色，但只限于有紫外光存在的情况下。
8. Alkanes as Fuels | 作为燃料的烷烃
The primary use of alkanes is as fuels. Short-chain alkanes like methane (natural gas), propane, and butane (LPG) are used for domestic cooking and heating. Longer-chain alkanes are found in petrol, kerosene, diesel, and fuel oil. The combustion of alkanes is highly exothermic, making them excellent energy sources. Fractional distillation of crude oil separates the alkane mixture into useful fractions according to boiling points. In the exam, you should relate the chain length to the fraction’s use: small molecules ignite easily and are used for bottled gas; larger molecules are used for diesel engines.
烷烃的主要用途是作为燃料。短链烷烃如甲烷（天然气）、丙烷和丁烷（液化石油气 LPG）用于家庭烹饪和取暖。长链烷烃存在于汽油、煤油、柴油和燃料油中。烷烃的燃烧是高度放热的，这使它们成为极好的能源。通过原油的分馏，根据沸点将烷烃混合物分离为有用的馏分。在考试中，你应将碳链长度与馏分的用途联系起来：小分子容易点燃，用于瓶装气体；较大的分子则用于柴油发动机。
9. Environmental Issues | 环境问题
Burning alkane fuels has significant environmental consequences. Complete combustion produces carbon dioxide, a greenhouse gas that contributes to global warming. Incomplete combustion releases carbon monoxide (toxic) and particulate carbon (soot), which cause respiratory problems and smog. Additionally, impurities in fuels, such as sulfur compounds, produce sulfur dioxide when burned. This gas leads to acid rain, which damages buildings, forests, and aquatic life. In many exam questions, you will be expected to link the combustion of alkanes to these environmental problems and suggest solutions, such as using catalytic converters in cars or removing sulfur from fuels before combustion.
燃烧烷烃燃料会带来严重的环境问题。完全燃烧产生二氧化碳，这是一种加剧全球变暖的温室气体。不完全燃烧释放出一氧化碳（有毒）和碳微粒（炭黑），会导致呼吸系统疾病和烟雾。此外，燃料中的杂质（如硫化合物）燃烧时生成二氧化硫。这种气体会导致酸雨，破坏建筑物、森林和水生生物。在许多考试题目中，你需要将烷烃的燃烧与这些环境问题联系起来，并提出解决方案，例如在汽车中使用催化转化器，或在燃烧前去除燃料中的硫。
10. Summary of Key Concepts | 考点总结
To quickly revise, keep these core points in mind:
快速复习时，请牢记以下核心要点：
- Alkanes are saturated hydrocarbons with single bonds and the general formula CₙH₂ₙ₊₂.
- They form a homologous series with gradual physical trends, such as increasing boiling points with chain length.
- IUPAC naming requires identifying the longest chain and the lowest-numbered substituents.
- Structural isomers have the same molecular formula but different structures; example: butane and 2-methylpropane.
- Alkanes undergo complete combustion (CO₂ + H₂O) and incomplete combustion (CO/C + H₂O).
- With halogens and UV light, they take part in substitution reactions forming halogenoalkanes.
- They are vital fuels, but burning them causes CO₂ emissions, CO poisoning, soot, and acid rain.
- 烷烃是只含单键的饱和烃，通式为 CₙH₂ₙ₊₂。
- 它们组成同系物，物理性质呈现渐变，如沸点随碳链增长而升高。
- IUPAC 命名要求找出最长碳链并采用最小编号的取代基。
- 同分异构体分子式相同而结构不同，例如丁烷与 2-甲基丙烷。
- 烷烃可发生完全燃烧（生成 CO₂ + H₂O）和不完全燃烧（生成 CO/C + H₂O）。
- 在紫外光下与卤素发生取代反应，生成卤代烷。
- 它们是重要的燃料，但燃烧会引起 CO₂ 排放、CO 中毒、炭黑和酸雨问题。
Reviewing these concepts together with the reactions and trends above will give you a very strong foundation for the exam.
将上述概念与反应和变化规律一起复习，会为你应对考试打下坚实的基础。
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