📚 Alkanes Exam Revision for IB & OCR Chemistry | IB OCR 化学：烷烃 考点精讲
Alkanes are the simplest organic compounds, consisting entirely of carbon and hydrogen atoms with single covalent bonds. In IB and OCR A‑Level Chemistry, they form a foundational topic that links organic nomenclature, physical properties, reaction mechanisms, and environmental chemistry. This guide covers every essential concept you need to master – from IUPAC naming and structural isomerism to free‑radical substitution and combustion analysis – with paired English‑Chinese explanations to support bilingual learners and exam candidates.
烷烃是最简单的有机化合物，仅由碳氢两种元素通过单键连接而成。在 IB 和 OCR A‑Level 化学中，烷烃是一个将有机命名、物理性质、反应机理和环境化学串联起来的基础课题。本指南涵盖你需要掌握的每一个核心概念——从 IUPAC 命名、结构异构到自由基取代反应和燃烧分析——并通过中英双语对照讲解，帮助双语学习者与考生高效备考。
1. Structure and Bonding in Alkanes | 烷烃的结构与键合
Alkanes have the general formula CnH2n+2. Each carbon atom is sp³ hybridised and forms four sigma (σ) bonds, leading to a tetrahedral geometry with bond angles of approximately 109.5°. The C–C and C–H bonds are non‑polar, so alkane molecules are held together only by weak London dispersion forces.
烷烃的通式为 CnH2n+2。每个碳原子均为 sp³ 杂化，形成四个 σ 键，呈四面体构型，键角约 109.5°。由于 C–C 键和 C–H 键均为非极性键，烷烃分子间仅靠微弱的伦敦色散力维系。
Because these intermolecular forces increase with molecular surface area, larger alkanes have higher boiling points. The carbon chain can be straight or branched, and branching lowers the boiling point because the molecules cannot pack as closely together, reducing the contact area for dispersion forces.
由于分子间作用力随分子表面积增大而增强，较大烷烃的沸点更高。碳链可为直链或支链；支链化使分子难以紧密堆积，色散力接触面积减小，沸点随之降低。
2. IUPAC Nomenclature | IUPAC 命名规则
Naming alkanes follows a systematic set of rules: identify the longest continuous carbon chain to determine the parent name (meth‑, eth‑, prop‑, but‑, pent‑, hex‑, etc.), number the chain from the end nearest a substituent, and name each branch as an alkyl group (methyl, ethyl, etc.) with its position number. Multiple identical substituents use prefixes di‑, tri‑, tetra‑.
烷烃命名遵循系统规则：找到最长的连续碳链确定母体名称（甲、乙、丙、丁、戊、己等），从最靠近取代基的一端开始编号，将支链命名为烷基（甲基、乙基等）并标明位次。相同取代基出现多次时使用二、三、四等前缀。
For example, the compound (CH3)2CHCH2CH3 is named 2‑methylbutane, not 3‑methylbutane, because the numbering must give the lowest set of locants. In OCR and IB exams, you will often be asked to draw structures from names and vice‑versa.
例如，化合物 (CH3)2CHCH2CH3 命名为 2‑甲基丁烷，而非 3‑甲基丁烷，因为编号需使位次之和最小。在 OCR 和 IB 考试中，经常要求根据名称画出结构，或根据结构写出名称。
3. Structural Isomerism | 结构异构
Alkanes with four or more carbon atoms exhibit chain isomerism. Isomers have the same molecular formula but different arrangements of the carbon skeleton. For C4H10, two isomers exist: butane and 2‑methylpropane. For C5H12, three isomers: pentane, 2‑methylbutane, and 2,2‑dimethylpropane.
含有四个及以上碳原子的烷烃会出现碳链异构。异构体分子式相同但碳骨架排列不同。C4H10 有两种异构体：丁烷和 2‑甲基丙烷。C5H12 有三种异构体：戊烷、2‑甲基丁烷和 2,2‑二甲基丙烷。
Isomers have different physical properties (e.g., boiling point, density) but the same chemical characteristics because the functional group – or lack thereof – is unchanged. Drawing all possible isomers for a given formula is a typical exam question that tests systematic thinking.
异构体具有不同的物理性质（如沸点、密度），但由于缺少官能团，其化学性质相同。根据给定分子式画出所有可能的异构体是典型的考题，考察系统思维能力。
4. Physical Properties and Trends | 物理性质与变化趋势
The boiling points of straight‑chain alkanes increase steadily with molar mass because the strength of London dispersion forces grows with the number of electrons. Methane (‑162 °C) is a gas, while octane (125 °C) is a liquid, and eicosane (C20H42) is a waxy solid at room temperature.
直链烷烃的沸点随摩尔质量增加而稳定升高，因为伦敦色散力强度随电子数增多而增强。甲烷（‑162 °C）为气体，辛烷（125 °C）为液体，二十烷（C20H42）在室温下为蜡状固体。
| Alkane / 烷烃 | Formula / 分子式 | Mr | b.p. / °C |
|---|---|---|---|
| Methane | CH4 | 16 | ‑162 |
| Ethane | C2H6 | 30 | ‑89 |
| Propane | C3H8 | 44 | ‑42 |
| Butane | C4H10 | 58 | ‑0.5 |
| Pentane | C5H12 | 72 | 36 |
Branched isomers always boil at lower temperatures than their straight‑chain counterparts. Alkanes are insoluble in water but dissolve in non‑polar solvents. These trends are frequently assessed in data‑based questions.
支链异构体的沸点始终低于对应的直链异构体。烷烃不溶于水，但可溶于非极性溶剂。这些趋势常以数据分析题的形式进行考察。
5. Sources and Fractional Distillation | 来源与分馏
Crude oil is a mixture of hydrocarbons, mainly alkanes, formed from ancient marine organisms. Fractional distillation separates the mixture into fractions with similar boiling ranges. The column is hottest at the bottom and cools towards the top; small molecules with low boiling points rise to the top as gases, while heavier fractions condense lower down.
原油是主要由烷烃组成的烃类混合物，源自远古海洋生物。分馏利用沸点差异将混合物分离为沸程相近的馏分。分馏塔底部温度最高，向上逐渐冷却；沸点低的小分子升至塔顶呈气态，较重的馏分则在较低处冷凝。
- Refinery gas → bottled gas / 炼厂气 → 瓶装燃气
- Gasoline / petrol → fuel for cars / 汽油 → 汽车燃料
- Naphtha → chemical feedstock / 石脑油 → 化工原料
- Kerosene → jet fuel / 煤油 → 航空燃料
- Diesel → fuel for buses and lorries / 柴油 → 巴士与卡车燃料
- Bitumen → road surfacing / 沥青 → 铺路
Understanding the link between chain length, intermolecular forces, and viscosity is crucial for both IB and OCR specifications.
理解链长、分子间作用力与黏度之间的关系，对于 IB 和 OCR 考纲都至关重要。
6. Combustion Reactions | 燃烧反应
Alkanes undergo complete combustion in excess oxygen to produce carbon dioxide and water, releasing a large amount of energy. The general equation is:
烷烃在过量氧气中发生完全燃烧，生成二氧化碳和水，释放大量能量。其通式为：
CnH2n+2 + (1.5n+0.5) O2 → n CO2 + (n+1) H2O
For example, the complete combustion of butane:
例如，丁烷的完全燃烧：
2 C4H10 + 13 O2 → 8 CO2 + 10 H2O
Incomplete combustion occurs when the oxygen supply is limited, producing carbon monoxide (toxic) or carbon (soot) alongside water. Carbon monoxide binds irreversibly to haemoglobin, reducing the blood’s oxygen‑carrying capacity.
当氧气供应不足时发生不完全燃烧，除水外还会生成有毒的一氧化碳或碳（炭黑）。一氧化碳与血红蛋白不可逆结合，降低血液携氧能力。
Oxides of nitrogen (NOx) and unburnt hydrocarbons also contribute to photochemical smog. Catalytic converters in vehicles help reduce these emissions, a topic linking alkane chemistry to environmental science.
氮氧化物和未燃烧的烃类还会导致光化学烟雾。汽车尾气催化转化器有助于减少这些排放，将烷烃化学与环境科学联系起来。
7. Halogenation: Free Radical Substitution | 卤代反应：自由基取代
Alkanes react with halogens (Cl2 or Br2) in the presence of ultraviolet (UV) light or heat to form halogenoalkanes. This is a photochemical free‑radical substitution reaction. The overall equation for methane chlorination is:
烷烃在紫外光或加热条件下与卤素（Cl2 或 Br2）反应生成卤代烷，属于光化学自由基取代反应。甲烷氯代的总反应式为：
CH4 + Cl2 → CH3Cl + HCl
Further substitution can occur, leading to a mixture of products: CH2Cl2, CHCl3, and CCl4. Bromination is slower and more selective due to the higher bond enthalpy of H–Br compared to H–Cl, but the mechanism is identical.
反应可继续进行，生成 CH2Cl2、CHCl3 和 CCl4 的混合物。溴代反应因 H–Br 键焓更高，速率较慢且选择性更强，但机理完全相同。
8. Mechanism: Initiation, Propagation, Termination | 机理：引发、增长、终止
The free‑radical mechanism consists of three stages.
自由基机理包含三个阶段。
Initiation / 引发: UV light provides energy to break the Cl–Cl bond homolytically, generating two chlorine radicals. Each radical has an unpaired electron, shown with a dot.
引发：紫外光提供能量使 Cl–Cl 键均裂，产生两个氯自由基。每个自由基带有一个未成对电子，用圆点表示。
Cl2 → 2 Cl•
Propagation / 增长: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical (•CH3). The methyl radical then reacts with a Cl2 molecule, producing chloromethane and regenerating a Cl• radical.
增长：氯自由基夺取甲烷中的一个氢原子，生成 HCl 和甲基自由基（•CH3）。甲基自由基再与 Cl2 分子反应，生成氯甲烷并再生 Cl• 自由基。
Cl• + CH4 → HCl + •CH3
•CH3 + Cl2 → CH3Cl + Cl•
Termination / 终止: able molecule, removing radicals from the system. Possible recombination steps include: Cl• + Cl• → Cl2; •CH3 + Cl• → CH3Cl; •CH3 + •CH3 → C2H6.
终止：两个自由基结合为稳定分子，将自由基从体系中移除。可能的结合方式包括：Cl• + Cl• → Cl2；•CH3 + Cl• → CH3Cl；•CH3 + •CH3 → C2H6。
In exams, you must be able to write equations for each step using displayed formulae or condensed structural formulae, and to recognise that a small amount of ethane indicates the presence of methyl radicals.
考试中要求你能够用结构式或简写式写出各步骤方程式，并认识到少量乙烷的生成是甲基自由基存在的证据。
9. Energetics and Bond Enthalpies | 能量学与键焓
The chlorination of methane is exothermic overall because the energy released in forming new bonds (C–Cl and H–Cl) exceeds the energy required to break existing bonds (Cl–Cl and C–H). Using average bond enthalpies, the enthalpy change ΔH can be estimated:
甲烷的氯代反应总过程是放热的，因为生成新键（C–Cl 和 H–Cl）所释放的能量大于断裂原有键（Cl–Cl 和 C–H）所需的能量。利用平均键焓可估算焓变 ΔH：
ΔH ≈ Σ (bond enthalpies of bonds broken) − Σ (bond enthalpies of bonds formed)
For CH4 + Cl2 → CH3Cl + HCl, typical values give ΔH ≈ ‑ d once initiated. Propagation steps are the key determinant of the overall energy profile.
对于 CH4 + Cl2 → CH3Cl + HCl，典型值给出 ΔH ≈ ‑104 kJ mol⁻¹。这种放热特性一旦反应被引发即可推动反应持续进行。增长步骤是决定总能量变化的关键。
10. Environmental Impact and Cracking | 环境影响与裂化
Burning alkane fuels contributes to the greenhouse effect (CO2), acid rain (SO2 from impurities, NOx from high‑temperature combustion), and photochemical smog (unburnt hydrocarbons and NOx). These impacts drive the need for cleaner fuels and emission control.
燃烧烷烃燃料会加剧温室效应（CO2）、酸雨（杂质产生的 SO2 和高温燃烧产生的 NOx）以及光化学烟雾（未燃烃与 NOx）。这些问题推动了对清洁燃料和排放控制的需求。
Cracking converts long‑chain alkanes from heavier fractions into shorter, more useful hydrocarbons. Thermal cracking uses high temperature and pressure to produce alkenes, while catalytic cracking uses zeolite catalysts at moderate temperatures to yield branched alkanes and cycloalkanes for petrol blending. Cracking equations often appear in stoichiometry questions.
裂化将重质馏分中的长链烷烃转变为更短、更有用的烃类。热裂化利用高温高压生成烯烃，催化裂化则在适中温度下使用沸石催化剂，生成支链烷烃和环烷烃用于汽油调和。裂化方程式经常出现在化学计量题中。
For example, the cracking of decane can be represented as:
例如，癸烷的裂化可表示为：
C10H22 → C5H12 + C3H6 + C2H4
Reforming converts straight‑chain alkanes into branched alkanes and cyclic hydrocarbons to improve the octane rating of petrol. Both cracking and reforming are essential industrial processes covered in the syllabus.
重整将直链烷烃转化为支链烷烃和环烃以提高汽油辛烷值。裂化和重整都是考纲涵盖的基本工业过程。
11. Common Exam Pitfalls | 常见考试易错点
Students often lose marks by forgetting to number the carbon chain to give the lowest set of locants, or by miswriting radical mechanisms without showing the unpaired electron. In combustion questions, balancing equations with fractional coefficients for O2 can cause errors – always check that the total number of atoms is conserved.
考生常因未按最低位次编号，或写自由基机理时未标出未成对电子而失分。在燃烧题中，使用 O2 的分系数配平容易出错——务必核对原子总数是否守恒。
Another common mistake is confusing initiation with propagation steps; initiation always produces radicals from a non‑radical species, while propagation consumes one radical and generates another. Also, remember that UV light is essential to start the reaction – simply mixing methane and chlorine in the dark gives no reaction.
另一个常见错误是混淆引发与增长步骤；引发总是由非自由基物种产生自由基，而增长则消耗一个自由基并生成另一个。此外，切记紫外光是启动反应的必要条件——仅将甲烷与氯气在黑暗中混合不会发生反应。
12. Summary and Revision Tips | 总结与复习建议
Alkanes may appear simple, but the topic weaves together bonding, energetics, mechanisms, and industrial applications. As you revise, practise drawing all isomers for C5H12 and C6H14, write out the full free‑radical mechanism for ethane chlorination, and calculate ΔH using given bond enthalpies. Past‑paper questions on fractional distillation and environmental consequences are also very common.
烷烃看似简单，却将键合、能量学、机理与工业应用紧密串联起来。复习时请练习画出 C5H12 和 C6H14 的所有异构体，完整写出乙烷氯代的自由基机理，并利用给定键焓计算 ΔH。分馏与环境影响的历年真题也极为常见。
By mastering these core ideas, you will build a solid foundation for more advanced organic chemistry topics such as alkenes, alcohols, and polymers. Keep a glossary of key terms in both English and Chinese to reinforce bilingual recall.
掌握这些核心概念后，你将为烯烃、醇和聚合物等更高级的有机化学主题奠定坚实基础。建议你制作一张中英对照的关键术语表，以强化双语记忆。
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