Alkanes: Key Exam Points for IB & CIE Chemistry | 烷烃：IB与CIE化学考点精讲

📚 Alkanes: Key Exam Points for IB & CIE Chemistry | 烷烃：IB与CIE化学考点精讲

Alkanes are the simplest family of hydrocarbons, containing only carbon and hydrogen atoms joined by single bonds. They form the basis of organic chemistry and appear frequently in both IB and CIE examinations. This article reviews the key concepts—structure, nomenclature, isomerism, physical properties, reactions, and environmental aspects—that you need to master for top marks.

烷烃是最简单的烃类化合物，仅由碳和氢原子通过单键连接而成。它们构成有机化学的基础，在IB和CIE考试中频繁出现。本文梳理了结构、命名、异构现象、物理性质、反应和环境方面的重要概念，帮助你掌握取得高分的核心考点。

1. Introduction to Alkanes | 烷烃简介

Alkanes are saturated hydrocarbons, meaning all carbon–carbon bonds are single, and each carbon atom forms four sigma bonds. They are generally unreactive compared to other organic compounds, but they undergo combustion and substitution reactions under specific conditions. In both IB and CIE syllabuses, alkanes serve as the starting point for understanding homologous series.

烷烃是饱和烃，即所有碳-碳键均为单键，每个碳原子形成四个σ键。与其他有机化合物相比，它们一般不活泼，但在特定条件下可发生燃烧和取代反应。在IB和CIE课程中，烷烃是理解同系物的起点。

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

Alkanes follow the general formula CₙH₂ₙ₊₂, where n is the number of carbon atoms. This formula applies to straight-chain and branched alkanes alike. The members of the alkane homologous series differ by a –CH₂– unit, show a gradual trend in physical properties, and share similar chemical behaviour. Examination questions often ask you to deduce the molecular formula of an alkane from its carbon count or to identify a homologous pair.

烷烃遵循通式 CₙH₂ₙ₊₂，其中 n 代表碳原子数。该通式适用于直链和支链烷烃。烷烃同系物彼此相差一个 –CH₂– 单元，物理性质呈现渐变趋势，化学性质相似。考试题目常要求根据碳原子数推导烷烃分子式，或判断同系物关系。

3. Structural Isomerism | 结构异构

With four or more carbon atoms, alkanes exhibit structural isomerism (chain isomerism). For example, butane (C₄H₁₀) has two isomers: n-butane and 2-methylpropane (isobutane). As the carbon number increases, the number of possible isomers rises rapidly. Examiners like to test your ability to draw and name the isomers of alkanes such as pentane or hexane, and to recognise that branched isomers usually have lower boiling points due to reduced surface contact.

从四个碳原子开始，烷烃出现结构异构（链异构）。例如丁烷 C₄H₁₀ 有两种异构体：正丁烷和2-甲基丙烷（异丁烷）。随着碳数目增多，可能的异构体数目迅速增加。考官喜欢考查绘制并命名戊烷或己烷异构体的能力，以及识别支链异构体因分子间接触面积减小而沸点较低的现象。

4. IUPAC Nomenclature Rules | IUPAC命名规则

Naming alkanes follows a systematic IUPAC procedure: identify the longest continuous carbon chain (parent chain), number the chain to give the substituents the lowest possible locants, and name the branches as alkyl groups (methyl, ethyl, propyl, etc.). If multiple identical branches exist, use prefixes di-, tri-, tetra-. IB and CIE mark schemes expect you to hyphenate numbers and words, and to write commas between numbers. For instance, 2,2-dimethylbutane, not 2,2-dimethyl butane. Being able to name a given structure and to draw the correct structure from a name is essential.

烷烃的命名遵循系统IUPAC规则：选择最长的连续碳链作为主链；给主链编号，使取代基获得尽可能小的位次；支链命名为烷基（甲基、乙基、丙基等）。如有多个相同支链，使用二、三、四等词头。IB和CIE评分标准要求数字与单词之间用连字符，数字之间用逗号，例如2,2-二甲基丁烷，而非2,2-二甲基丁烷。能够对给出的结构命名，又能根据名称画出正确结构，是必考技能。

5. Physical Properties: Boiling Points & Solubility | 物理性质：沸点和溶解度

Alkanes are non-polar molecules; the only intermolecular forces are weak instantaneous dipole–induced dipole (London dispersion) forces. Boiling points increase with increasing molecular mass and chain length because larger molecules have more electrons and a larger surface area for interactions. Among isomers, branching lowers the boiling point because the molecule becomes more spherical, reducing the surface contact available for intermolecular forces. Alkanes are insoluble in water but dissolve in non-polar solvents such as hexane itself. These trends are frequently assessed through data analysis questions.

烷烃是非极性分子，唯一的分子间作用力是微弱的瞬时偶极-诱导偶极（London色散力）。随着分子质量和链长增加，沸点升高，因为较大的分子拥有更多电子和更大的相互作用表面积。在同分异构体中，支链降低沸点，因为分子形状更接近球形，减少了分子间接触面积。烷烃不溶于水，但溶于非极性溶剂如己烷。这些变化趋势常通过数据分析题型考查。

6. Combustion of Alkanes | 烷烃的燃烧

Alkanes release a large amount of energy when burned in excess oxygen, producing carbon dioxide and water. The complete combustion of methane is represented by:

烷烃在过量氧气中燃烧放出大量能量，生成二氧化碳和水。甲烷完全燃烧的方程式为：

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion occurs when the oxygen supply is limited, yielding carbon monoxide (CO) or carbon (soot) along with water. These products are toxic and hazardous. Exam questions often require writing balanced equations for both complete and incomplete combustion, and linking CO production to dangers such as poisoning or carbon deposits.

氧气供应不足时发生不完全燃烧，产生一氧化碳（CO）或碳（炭黑）以及水。这些产物有毒且危险。考题常要求书写完全和不完全燃烧的配平方程式，并将CO的生成与中毒或积碳等危险联系起来。

7. Free-Radical Substitution: Overview | 自由基取代反应概述

Alkanes react with halogens (Cl₂, Br₂) in the presence of ultraviolet (UV) light or heat to form haloalkanes. This is a free-radical substitution mechanism, characteristic of alkanes. The reaction does not occur in the dark at room temperature. The overall equation for methane chlorination is:

烷烃在有紫外光（UV）或加热条件下与卤素（Cl₂、Br₂）反应生成卤代烷。这是烷烃特有的自由基取代机理。该反应在室温避光条件下不发生。甲烷氯化的总反应方程式为：

CH₄ + Cl₂ → CH₃Cl + HCl (UV light)

Further substitution can produce a mixture of chloromethane, dichloromethane, trichloromethane, and tetrachloromethane. Both syllabuses expect you to explain this in terms of the radical chain mechanism.

进一步取代可产生一氯甲烷、二氯甲烷、三氯甲烷和四氯化碳的混合物。两者课程都要求你用自由基链式机理进行解释。

8. Mechanism of Methane Chlorination | 甲烷氯化机理

The free-radical substitution mechanism proceeds in three stages: initiation, propagation, and termination. Initiation: Cl–Cl bond breaks homolytically under UV light, forming two chlorine radicals.

自由基取代机理分三个阶段：引发、增长和终止。引发：紫外光下Cl–Cl键均裂，生成两个氯自由基。

Cl₂ → 2Cl•

Propagation: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical. The methyl radical then reacts with a chlorine molecule, producing chloromethane and another chlorine radical that continues the chain.

增长：氯自由基从甲烷中夺取一个氢原子，生成HCl和甲基自由基。然后甲基自由基与氯分子反应，生成氯甲烷和另一个氯自由基，使链反应持续。

CH₄ + Cl• → •CH₃ + HCl

•CH₃ + Cl₂ → CH₃Cl + Cl•

Termination: Two radicals combine to form a stable molecule, ending the chain. Possible termination steps include Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, and •CH₃ + •CH₃ → C₂H₆. Marks are awarded for correctly showing the movement of single electrons using half-arrowed or fishhook arrows (IB often requires curly arrows with single heads for radical movements).

终止：两个自由基结合生成稳定分子，链终止。可能的终止步骤包括 Cl• + Cl• → Cl₂，•CH₃ + Cl• → CH₃Cl 和 •CH₃ + •CH₃ → C₂H₆。考试中正确画出单电子的半箭头（鱼钩箭头）能得分（IB常要求用带单箭头的弯箭头表示自由基移动）。

9. Environmental Impact of Alkanes | 烷烃的环境影响

The primary environmental concern with alkanes is the production of carbon dioxide, a greenhouse gas, during combustion. Incomplete combustion releases carbon monoxide, which binds irreversibly to haemoglobin, reducing oxygen transport, and soot, which causes respiratory problems. Additionally, unburnt hydrocarbons from vehicle exhausts contribute to photochemical smog. CIE questions may ask you to evaluate catalytic converters as a solution, while IB may link these to the option on energy.

与烷烃相关的主要环境问题是燃烧产生的温室气体二氧化碳。不完全燃烧释放一氧化碳和炭黑：一氧化碳与血红蛋白不可逆结合，降低输氧能力；炭黑引发呼吸系统疾病。此外，汽车尾气中的未燃烧烃会促成光化学烟雾。CIE考题可能要求评估催化转化器作为解决方案，IB则可能将其与能源选修内容联系。

10. Cracking of Alkanes | 烷烃的裂化

Cracking converts long-chain, less useful alkane fractions into shorter-chain alkanes and alkenes, which are in higher demand. Thermal cracking uses high temperature (700–1200 K) and high pressure to produce mainly alkenes; catalytic cracking uses a zeolite catalyst at lower temperature and pressure to produce branched alkanes and aromatic compounds, which are better for petrol. CIE particularly emphasises the economic importance of cracking, while IB may include it in the organic chemistry or energy topics.

裂化将长链低价值烷烃馏分转化为需求量更高的短链烷烃和烯烃。热裂化在高温（700–1200 K）高压下产生以烯烃为主的产物；催化裂化使用沸石催化剂，在较低温度和压力下产生支链烷烃和芳香族化合物，更适用于汽油。CIE尤其强调裂化的经济意义，IB可能将其纳入有机化学或能源主题。

11. Summary of Important Exam Tips | 重要考点总结

Focus on mastering the general formula CₙH₂ₙ₊₂ and its application. Be able to draw and name structural isomers for C₄–C₆ alkanes. Explain boiling point trends using London forces and molecular shape. Write balanced equations for complete and incomplete combustion, emphasising CO toxicity. Describe the free-radical substitution mechanism in three clear steps, drawing correct fishhook arrows. Connect the chemistry of alkanes to real‑world issues such as fuel quality, greenhouse emissions, and catalytic cracking. Practice past-paper questions on these topics, as they are regularly examined in both IB Papers and CIE Structured Questions.

重点掌握通式 CₙH₂ₙ₊₂ 及其应用。能够画出和命名C₄–C₆烷烃的结构异构体。用London力和分子形状解释沸点趋势。写出完全和不完全燃烧的配平方程式，强调CO的毒性。按三步清晰描述自由基取代机理，画出正确的鱼钩箭头。将烷烃化学与燃料品质、温室气体排放和催化裂化等实际问题相联系。多做相关真题练习，因为这些知识点在IB试卷和CIE结构题中高频出现。

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Alkanes: Key Exam Points for IB & CIE Chemistry | 烷烃：IB与CIE化学考点精讲