Alkanes: Key Exam Points for IB & Edexcel Chemistry | IB Edexcel 化学：烷烃考点精讲

📚 Alkanes: Key Exam Points for IB & Edexcel Chemistry | IB Edexcel 化学：烷烃考点精讲

Alkanes are the simplest family of hydrocarbons, consisting only of carbon and hydrogen atoms linked by single bonds. They serve as the foundation for understanding organic chemistry and appear frequently in both IB and Edexcel examinations. This revision guide covers essential concepts, reaction mechanisms, properties, and exam techniques to help you master alkanes.

烷烃是仅由碳和氢原子通过单键连接构成的最简单的烃类家族。它们是有机化学的基础，在 IB 与 Edexcel 考试中频繁涉及。本复习指南涵盖核心概念、反应机理、性质及应试技巧，助你彻底掌握烷烃。

1. Introduction to Alkanes | 烷烃简介

Alkanes are saturated hydrocarbons because they contain only C–C and C–H single bonds, with the maximum possible number of hydrogen atoms per carbon. Their general molecular formula for non-cyclic alkanes is CₙH₂ₙ₊₂. In IB and Edexcel, you need to recognise that carbon atoms in alkanes are sp³ hybridised, leading to a tetrahedral geometry with bond angles of approximately 109.5°. They form a homologous series, where each successive member differs by a –CH₂– unit.

烷烃为饱和烃，因其只含 C–C 与 C–H 单键，每碳原子价电子均达饱和。链状烷烃的通式为 CₙH₂ₙ₊₂。在 IB 与 Edexcel 考试中，必须认清烷烃中的碳为 sp³ 杂化，形成四面体构型，键角约 109.5°。它们组成同系物，相邻成员相差一个 –CH₂– 单元。

Because of their strong, nonpolar sigma bonds, alkanes are relatively unreactive under standard conditions. They are often used as fuels and chemical feedstocks, making their reactions central to both syllabi.

由于含有牢固的非极性 σ 键，烷烃在标准条件下相对不活泼。它们常被用作燃料和化工原料，因此其反应在两个课程大纲中均占据核心地位。

2. General Formula and Homologous Series | 通式与同系物

The general formula CₙH₂ₙ₊₂ applies to straight-chain and branched alkanes. Cycloalkanes, which contain a single ring, follow the formula CₙH₂ₙ. Each member of the alkane homologous series has similar chemical properties, but physical properties such as boiling point change gradually with chain length.

通式 CₙH₂ₙ₊₂ 适用于直链及支链烷烃。含单环的环烷烃通式为 CₙH₂ₙ。烷烃同系物中各成员化学性质相似，但沸点等物理性质随链长增加而渐变。

This concept is frequently tested: you must be able to predict the molecular formula of any alkane given its carbon number or identify whether a given formula fits the alkane series. For example, C₆H₁₂ is not an alkane (it fits an alkene or cycloalkane).

这一概念常考：需要能根据碳原子数写出任意烷烃的分子式，或判断某个分子式是否属于烷烃系列。例如，C₆H₁₂ 不属于烷烃（它符合烯烃或环烷烃）。

3. Structure and Bonding | 结构与键

Each carbon in an alkane forms four sigma (σ) bonds, which result from the end-on overlap of sp³ hybrid orbitals with hydrogen 1s or another carbon sp³ orbital. Sigma bonds allow free rotation about the C–C axis, giving rise to different conformations. In exams, you may be asked to explain why alkanes can exist in many conformers but not as geometric isomers; the free rotation prevents cis–trans isomerism.

烷烃中每个碳形成四个 σ 键，由 sp³ 杂化轨道与氢的 1s 或另一碳的 sp³ 轨道端端重叠而成。σ 键允许绕 C–C 轴自由旋转，产生不同构象。考试中可能要求解释为何烷烃存在众多构象却无顺反异构：自由旋转阻止了顺反异构的形成。

The bond angles of ~109.5° and bond lengths (C–C ~154 pm, C–H ~110 pm) are consistent across alkanes. Remember that ‘saturated’ does not mean unreactive in all contexts; under radical conditions, even these strong bonds can break.

键角约 109.5°，键长（C–C ~154 pm，C–H ~110 pm）在各类烷烃中保持一致。请记住，“饱和”并不意味在所有条件下都不反应；在自由基条件下，这些牢固的键也会断裂。

4. Nomenclature of Alkanes | 烷烃命名

IUPAC naming for alkanes requires identifying the longest continuous carbon chain, numbering it to give substituents the lowest possible locants, and assembling the name with prefixes (methyl-, ethyl-, etc.) in alphabetical order. For example, 2-methylbutane is correct, not 3-methylbutane, because numbering must minimise the sum of substituent positions.

烷烃 IUPAC 命名需找出最长连续碳链，编号使取代基位次尽可能小，并按字母顺序组合前缀（甲基-、乙基-等）。例如，正确名称是 2-甲基丁烷，而非 3-甲基丁烷，因为编号要使取代基位次总和最小。

Common exam pitfalls include forgetting to use di-, tri- for multiple identical substituents and omitting hyphens or commas. The IB often requires drawing structural formulas from names or vice versa. Edexcel also expects you to recognise common trivial names like isobutane.

常见考试陷阱包括忘记用二-、三-表示多个相同取代基，或遗漏连字符与逗号。IB 常要求根据名称画出结构式或反之。Edexcel 也期望识别异丁烷等常见俗名。

IUPAC Name	Molecular Formula	Structural Skeleton
Butane	C₄H₁₀	CH₃CH₂CH₂CH₃
2-Methylpropane	C₄H₁₀	(CH₃)₃CH

5. Isomerism in Alkanes | 烷烃同分异构

Alkanes exhibit structural isomerism, primarily chain isomerism. The first alkane with isomers is butane (C₄H₁₀), which has two isomers: butane and 2-methylpropane. As the carbon number increases, the number of possible isomers rises sharply. Pentane (C₅H₁₂) has three isomers, while hexane (C₆H₁₄) has five.

烷烃表现出构造异构，主要是碳链异构。首个存在异构体的烷烃是丁烷（C₄H₁₀），有丁烷和 2-甲基丙烷两种异构体。随着碳数增加，可能异构体数目急剧上升。戊烷（C₅H₁₂）有三种异构体，己烷（C₆H₁₄）有五种。

In exam questions, you will often be asked to draw and name all isomers of a given molecular formula. Remember to check for duplicates and to ensure each carbon maintains four bonds. Cycloalkanes add another layer: for C₄H₈, both alkene and cyclobutane structures appear, but alkane isomers require saturated frameworks.

考试中常要求画出并命名给定分子式的所有异构体。注意检查是否有重复结构，并确保每个碳均形成四个键。环烷烃增加了复杂性：对 C₄H₈，既有烯烃又有环丁烷结构，但烷烃异构体必须为饱和骨架。

6. Physical Properties: Trends and Explanation | 物理性质：趋势与解释

Boiling points of straight-chain alkanes increase with molecular mass because larger molecules have more surface area, leading to stronger London dispersion forces between molecules. Branching lowers boiling point: more branched isomers have a more compact shape, reducing intermolecular contact and thus weaker dispersion forces.

直链烷烃的沸点随分子质量增大而升高，因分子越大表面积越大，分子间伦敦色散力越强。支链会降低沸点：支链越多的异构体形状越紧凑，分子间接触减少，色散力减弱。

Alkanes are nonpolar and insoluble in water, but they dissolve in nonpolar solvents. Their melting points also generally increase with carbon number, though odd–even effects from solid-state packing can cause slight irregularities. Both IB and Edexcel ask for explanations linking structure to physical properties.

烷烃为非极性分子，不溶于水，但可溶于非极性溶剂。其熔点通常随碳数增加而升高，但因固态堆积中的奇偶效应可能导致细微波动。IB 与 Edexcel 均要求联系结构解释物理性质。

7. Chemical Reactions: Combustion | 化学反应：燃烧

Complete combustion of alkanes in excess oxygen produces carbon dioxide and water, releasing large amounts of energy. For example, the combustion of octane (a component of petrol):

烷烃在过量氧气中完全燃烧生成二氧化碳和水，释放大量能量。例如，辛烷（汽油组分）的燃烧：

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

Incomplete combustion, due to limited oxygen, yields carbon monoxide and/or soot (carbon particulates). This reaction is highly relevant to environmental and health topics in the syllabus. IB may require calculation of oxygen volumes using molar volume at STP, while Edexcel links combustion to enthalpy changes and fuel efficiency.

氧气不足时发生不完全燃烧，生成一氧化碳和／或炭黑（碳颗粒）。该反应与课程中环境和健康议题紧密相关。IB 可能要求在 STP 下用摩尔体积计算氧气量，而 Edexcel 则将燃烧与焓变及燃料效率联系起来。

You must be able to write balanced equations for both complete and incomplete combustion, recognising that the products of incomplete combustion can be CO, C, or a mixture. The dangers of CO poisoning, due to its binding to haemoglobin, are often cited.

必须能够书写完全与不完全燃烧的配平方程式，并认识到不完全燃烧产物可为 CO、C 或二者混合物。CO 与血红蛋白结合导致中毒的危险性常被提及。

8. Chemical Reactions: Free Radical Substitution | 化学反应：自由基取代

Alkanes react with halogens (Cl₂, Br₂) in the presence of ultraviolet light or heat to form halogenoalkanes. This is a photochemical free-radical substitution reaction. The overall equation for the monochlorination of methane is:

烷烃在紫外光或加热下与卤素（Cl₂、Br₂）反应生成卤代烷。这是一个光化学自由基取代反应。甲烷一氯化的总反应为：

CH₄ + Cl₂ → CH₃Cl + HCl

The reaction yields a mixture of products because multiple substitution steps can occur. In iodine, the reaction is endothermic and does not proceed well. Fluorine reacts explosively, while bromine is less reactive, requiring stronger heating. Both IB and Edexcel focus on chlorine and bromine substitution.

由于可能发生多次取代，反应通常得到混合物。碘的反应因吸热而难以进行；氟反应剧烈爆炸，溴则活性较低，需较强加热。IB 与 Edexcel 均重点关注氯与溴的取代反应。

9. Mechanism of Free Radical Substitution | 自由基取代机理详解

The mechanism proceeds in three stages: initiation, propagation, and termination.

反应机理分三步：引发、增长、终止。

Initiation: The halogen molecule undergoes homolytic fission under UV light, forming two halogen radicals.

引发：卤素分子在紫外光下发生均裂，生成两个卤素自由基。

Cl–Cl → 2 Cl·

Propagation: A halogen radical abstracts a hydrogen atom from an alkane, forming a hydrogen halide and an alkyl radical. The alkyl radical then attacks a new halogen molecule, generating the halogenoalkane and regenerating a halogen radical to continue the chain.

增长：卤自由基夺取烷烃中的氢原子，生成卤化氢与烷基自由基。烷基自由基随后进攻另一个卤素分子，生成卤代烷并再生卤自由基，维持链反应。

CH₄ + Cl· → ·CH₃ + HCl

·CH₃ + Cl₂ → CH₃Cl + Cl·

Termination: Any two radicals combine, ending the chain. Possible termination steps include:

终止：任意两个自由基结合，链反应终止。可能的终止步骤有：

Cl· + Cl· → Cl₂

·CH₃ + Cl· → CH₃Cl

·CH₃ + ·CH₃ → C₂H₆

In IB, you may be required to use ‘half-arrows’ (fish-hook arrows) to show single electron movement. Edexcel expects you to write and explain each step, including the conditions and the formation of minor products like ethane.

IB 可能要求用半箭头（鱼钩箭头）表示单电子转移。Edexcel 则期望书写并解释各步，包括反应条件及乙烷等副产物的生成。

10. Cracking and Reforming | 裂解与重整

Long-chain alkanes from crude oil can be broken into shorter, more useful molecules via cracking. Thermal cracking uses high temperature (~700–1200 K) and pressure to produce a mixture of alkanes and alkenes. Catalytic cracking employs zeolite catalysts at lower temperatures to give more branched alkanes and aromatic compounds, which improve fuel octane rating.

原油中的长链烷烃可通过裂解转化为更短、更有用的分子。热裂化在高温（约 700–1200 K）和高压下产生烷烃和烯烃混合物。催化裂化使用沸石催化剂在较低温下产生更多支链烷烃与芳烃，可提高燃料辛烷值。

Reforming rearranges straight-chain alkanes into branched isomers or cycloalkanes without changing the carbon number. This enhances fuel properties. In exams, you may be asked to write equations for cracking a given alkane, producing an alkane and an alkene, and to appreciate the industrial importance of these processes.

重整在碳数不变下将直链烷烃重排为支链异构体或环烷烃，改善燃料性能。考试可能要求写出某烷烃裂解的方程式（生成一个烷烃和一个烯烃），并理解这些工艺的工业重要性。

11. Environmental Impact and Fuels | 环境影响与燃料

Alkanes are the primary constituents of natural gas and petroleum fuels. Their combustion produces CO₂, a greenhouse gas contributing to climate change. Incomplete combustion releases CO (toxic) and particulates (soot), which cause respiratory problems. Sulphur impurities in fossil fuels produce SO₂, leading to acid rain.

烷烃是天然气与石油燃料的主要成分。其燃烧产生温室气体 CO₂，加剧气候变化。不完全燃烧释放有毒的 CO 和颗粒物（碳烟），引发呼吸系统疾病。化石燃料中的硫杂质生成 SO₂，导致酸雨。

Both IB and Edexcel require you to suggest measures to reduce pollution, such as catalytic converters that oxidise CO and unburnt hydrocarbons to CO₂ and H₂O, and the use of low-sulphur fuels. Understanding the carbon neutrality debate of biofuels, contrasting with fossil alkanes, can earn high marks in evaluation questions.

IB 与 Edexcel 均要求提出减轻污染的措施，如催化转化器将 CO 与未燃烧烃氧化为 CO₂ 和 H₂O，以及使用低硫燃料。理解生物燃料与化石烷烃在碳中和性上的争论，可在评价类题目中获高分。

12. Common Exam Pitfalls and Tips | 常见考试陷阱与提示

1) Never confuse ‘saturated’ with ‘unreactive’: alkanes do react under radical conditions. 2) When writing the substitution mechanism, always show the radical dot clearly on the atom carrying the unpaired electron. 3) In naming, check for the longest chain – many students misidentify it in branched structures. 4) When balancing combustion equations, ensure all atoms balance; a common error is forgetting that O₂ is diatomic. 5) For the propagation step of bromination, note that the abstraction of H by Br· is endothermic and slower, which makes bromination more selective.

1) 勿将“饱和”等同于“不反应”：烷烃在自由基条件下能反应。2) 书写取代机理时，务必在带有单电子的原子上清晰标出自由基点。3) 命名时检查最长碳链——许多学生在含支链的结构中识别错误。4) 配平燃烧方程式时确保所有原子守恒；常见错误是忘记 O₂ 为双原子分子。5) 溴化反应增长步中，Br· 夺取 H 是吸热且较慢的，这使溴化选择性更高。

Additionally, practice drawing both displayed and skeletal formulas, as these are regularly tested. Use model answers to perfect your mechanistic notation, and link physical properties to intermolecular forces, not just ‘molecular weight’.

此外，练习绘制完整结构式和键线式，这两种形式均常考。参考标准答案完善机理表示法，并将物理性质与分子间作用力关联，而非仅归因于“分子量”。

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Alkanes: Key Exam Points for IB & Edexcel Chemistry | IB Edexcel 化学：烷烃 考点精讲