📚 IGCSE WJEC Chemistry: Alkanes Key Points | IGCSE WJEC 化学：烷烃 考点精讲
Alkanes are fundamental hydrocarbons in organic chemistry, and the WJEC IGCSE specification expects you to understand their structure, naming, isomerism, physical trends, key reactions, and environmental impact. This guide breaks down every essential concept with clear examples and paired explanations.
烷烃是有机化学中基础的烃类化合物，WJEC IGCSE 大纲要求掌握其结构、命名、同分异构、物理性质递变规律、核心反应以及环境影响。本文通过清晰的例题和双语对照解释，逐一梳理所有必考要点。
1. What Are Alkanes? | 烷烃是什么？
Alkanes are saturated hydrocarbons containing only carbon and hydrogen atoms, with all carbon–carbon bonds being single covalent bonds. They form a homologous series with the general formula CnH2n+2. Each member differs from the next by a –CH2– unit, giving similar chemical properties but gradually changing physical properties. Because they have the maximum possible number of hydrogen atoms per carbon atom, they are described as ‘saturated’.
烷烃是饱和烃，只含碳和氢原子，所有碳碳键均为单共价键。它们构成通式为 CnH2n+2 的同系物。每相邻两个成员相差一个 –CH2– 单元，因此化学性质相似，物理性质呈递变性。由于每个碳原子上结合了尽可能多的氢原子，烷烃被称为“饱和”烃。
2. Naming Straight-Chain Alkanes | 直链烷烃的命名
The names of the first six straight-chain alkanes follow the prefixes: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), all with the ending ‘-ane’. For example, CH4 is methane, C2H6 is ethane, C3H8 propane, C4H10 butane, C5H12 pentane, and C6H14 hexane. When naming branched isomers, you must identify the longest continuous carbon chain and use prefixes like methyl or ethyl to indicate branches.
前六个直链烷烃按前缀命名：甲（meth-）、乙（eth-）、丙（prop-）、丁（but-）、戊（pent-）、己（hex-），词尾均为“-ane”。例如 CH4 为甲烷，C2H6 为乙烷，C3H8 为丙烷，C4H10 为丁烷，C5H12 为戊烷，C6H14 为己烷。命名支链异构体时，需找出最长的连续碳链，并用甲基（methyl）、乙基（ethyl）等前缀标明支链。
3. Structural Formulas and Bonding | 结构式与化学键
Alkanes can be represented by molecular formulas (e.g. C4H10), displayed formulas showing all atoms and bonds, or shortened structural formulas such as CH3CH2CH2CH3. Each carbon atom forms four sigma (σ) covalent bonds, giving a tetrahedral arrangement around each carbon with bond angles of approximately 109.5°. The single bonds allow free rotation, which leads to conformational flexibility.
烷烃可用分子式（如 C4H10）、展示所有原子和键的结构式，或简写的结构式如 CH3CH2CH2CH3 来表示。每个碳原子形成四个 σ 共价键，使碳原子周围呈四面体排布，键角约为 109.5°。单键可自由旋转，赋予了分子构象灵活性。
4. Isomerism in Alkanes | 烷烃的同分异构现象
Structural isomers have the same molecular formula but different arrangements of atoms. Butane (C4H10) has two isomers: straight-chain butane and branched 2-methylpropane. Pentane (C5H12) has three isomers: pentane, 2-methylbutane, and 2,2-dimethylpropane. When drawing isomers, ensure that you do not accidentally duplicate structures by merely rotating the carbon skeleton.
结构异构体是指分子式相同但原子排列方式不同的化合物。丁烷 (C4H10) 有两种异构体：直链丁烷和支链的 2-甲基丙烷。戊烷 (C5H12) 有三种异构体：戊烷、2-甲基丁烷和 2,2-二甲基丙烷。绘制异构体时，要确保不会因简单旋转碳骨架而重复画出相同结构。
5. Physical Properties and Trends | 物理性质及变化趋势
As the number of carbon atoms in a straight-chain alkane increases, its boiling point, melting point, viscosity, and density all increase, while flammability decreases. This is due to stronger London (van der Waals’) forces between larger molecules. Shorter alkanes are gases at room temperature, mid-length alkanes are liquids, and very long chains are waxy solids. Volatility decreases as molecular size increases.
随着直链烷烃碳原子数增加，其沸点、熔点、黏度和密度均上升，而可燃性下降。这是因为更大的分子间存在更强的伦敦（范德华）力。短链烷烃室温下为气体，中等链长者为液体，长链烷烃则为蜡状固体。挥发性随分子增大而降低。
6. Combustion of Alkanes | 烷烃的燃烧反应
Alkanes burn readily in oxygen, making them excellent fuels. Complete combustion produces carbon dioxide and water and releases plenty of energy. For methane:
CH4 + 2O2 → CO2 + 2H2O
In limited oxygen, incomplete combustion occurs, generating toxic carbon monoxide (CO) and/or carbon particulates (soot). Balanced equations for incomplete combustion must account for the formation of CO or C.
烷烃在氧气中容易燃烧，因此是优良的燃料。完全燃烧生成二氧化碳和水，并释放大量能量。以甲烷为例：
CH4 + 2O2 → CO2 + 2H2O
当氧气不足时则发生不完全燃烧，生成有毒的一氧化碳（CO）和/或炭微粒（碳烟）。书写不完全燃烧的配平方程式时，需正确表示 CO 或 C 的生成。
7. Substitution Reactions with Halogens | 与卤素的取代反应
In the presence of ultraviolet (UV) light, alkanes react with halogens such as chlorine or bromine to form haloalkanes and hydrogen halide. For example, methane reacts with chlorine in a substitution reaction:
CH4 + Cl2 → CH3Cl + HCl
This reaction can continue, replacing further hydrogen atoms to produce a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane. The UV light provides the energy to break the Cl–Cl bond, generating reactive chlorine radicals that initiate the process.
在紫外光（UV）照射下，烷烃与氯或溴等卤素发生取代反应，生成卤代烷和卤化氢。例如，甲烷与氯气发生取代反应：
CH4 + Cl2 → CH3Cl + HCl
该反应可继续进行，进一步取代氢原子，得到一氯甲烷、二氯甲烷、三氯甲烷和四氯甲烷的混合物。紫外光提供能量断裂 Cl–Cl 键，产生具有反应活性的氯自由基从而引发反应。
8. Environmental Impact of Alkane Combustion | 烷烃燃烧的环境影响
The combustion of alkane-based fuels releases carbon dioxide, a greenhouse gas that contributes to global warming. Incomplete combustion produces carbon monoxide, a colourless, odourless gas that binds irreversibly to haemoglobin, reducing the blood’s oxygen-carrying capacity. Unburnt hydrocarbons and soot particulates can cause respiratory problems and smog. Additionally, many fuels contain sulfur impurities; when burned, they produce sulfur dioxide (SO2), which leads to acid rain.
烷烃燃料燃烧释放二氧化碳，这是一种导致全球变暖的温室气体。不完全燃烧产生的一氧化碳是一种无色无味的气体，能与血红蛋白不可逆结合，降低血液的携氧能力。未燃烧的碳氢化合物和碳粒会导致呼吸系统疾病与光化学烟雾。此外，许多燃料含硫杂质，燃烧时生成二氧化硫（SO2），从而引发酸雨。
9. Sources and Uses of Alkanes | 烷烃的来源和用途
Alkanes are primarily obtained from crude oil through fractional distillation, where the mixture is separated into fractions based on boiling point ranges. Natural gas consists mainly of methane. Shorter-chain alkanes are used as LPG and petrol, while longer-chain alkanes serve as diesel, kerosene, lubricating oils and bitumen. Alkanes are also used as solvents and as starting materials for producing plastics and other chemicals via cracking.
烷烃主要通过分馏原油获得，利用沸点范围差异将混合物分离成不同的馏分。天然气的主要成分是甲烷。短链烷烃用作液化石油气和汽油，长链烷烃则用作柴油、煤油、润滑油和沥青。烷烃还可用作溶剂，并可通过裂解作为生产塑料及其他化学品的原料。
10. Common Mistakes and Exam Tips | 常见错误与应试技巧
When writing displayed or structural formulas, ensure every carbon atom has exactly four bonds. In naming branched alkanes, always number the chain from the end nearest the branch, and include the position number even if it is ‘2-‘ for a single branch. When balancing combustion equations, check both carbon and oxygen atoms carefully – a common error is forgetting to include the oxygen in CO or CO2 correctly. For substitution reactions, remember that UV light is a necessary condition, and that the reaction yields a mixture of products, not just the monosubstituted haloalkane.
书写结构式或展示式时，确保每个碳原子恰好形成四个键。命名支链烷烃时，始终从离支链最近的一端编号，即使支链在 2-位上也要标明数字。配平燃烧方程式时，仔细核对碳原子和氧原子的数量——常见错误是未能正确地计入 CO 或 CO2 中的氧原子。对于取代反应，牢记紫外光照是必要条件，且反应生成混合物，而不仅仅是单取代产物。
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