📚 Alkanes: Key Concepts for IB and WJEC Chemistry | 烷烃：IB与WJEC化学核心考点精讲
Alkanes are the simplest family of hydrocarbons, forming the foundation of organic chemistry. Whether you are following the IB Diploma Programme or the WJEC specification, a solid understanding of alkanes – their structure, nomenclature, isomerism, physical properties, and reactivity – is essential for success. This article covers the core concepts examined by both syllabi, highlighting key similarities and any subtle differences, all in a clear bilingual format to support your revision.
烷烃是最简单的烃类化合物，构成有机化学的基础。无论你学习的是IB文凭课程还是WJEC考试大纲，扎实掌握烷烃的结构、命名、异构现象、物理性质及反应活性都是成功的关键。本文以清晰的双语形式，梳理两个课程体系共同考查的核心知识点，并指出细微差异，助力你的复习备考。
1. What Are Alkanes? | 什么是烷烃？
Alkanes are saturated hydrocarbons, meaning they contain only carbon and hydrogen atoms linked exclusively by single covalent bonds. Their general molecular formula is CₙH₂ₙ₊₂ for straight-chain and branched non-cyclic alkanes. Because all carbon atoms are sp³ hybridised with tetrahedral geometry (bond angles ~109.5°), the molecules are non-polar and exhibit only weak intermolecular forces.
烷烃是饱和烃，意味着只含碳和氢原子，且所有键均为单键。直链和支链非环烷烃的通式为 CₙH₂ₙ₊₂。由于所有碳原子均为sp³杂化，呈四面体几何构型（键角约109.5°），分子无极性，仅存在微弱的分子间作用力。
2. Nomenclature (IUPAC Rules) | 命名法（IUPAC规则）
Both IB and WJEC require accurate naming of alkanes up to at least ten carbons. The systematic approach involves identifying the longest continuous carbon chain as the parent, numbering it to give substituents the lowest possible locants, and naming alkyl branches (methyl, ethyl, propyl, etc.) as prefixes. When multiple identical branches exist, use prefixes di-, tri-, tetra-; for different branches, list them alphabetically.
IB和WJEC均要求掌握至少十个碳以内烷烃的正确命名。系统命名法要点：确定最长连续碳链为母体，从离取代基较近的一端开始编号，使位次最小；支链烷基（甲基、乙基、丙基等）作为前缀，多个相同取代基用二、三、四等表示，不同取代基按字母顺序排列。
3. Structural Isomerism in Alkanes | 烷烃的结构异构
Alkanes exhibit chain isomerism from C₄ upwards. Butane (C₄H₁₀) has two isomers: butane and 2-methylpropane. The number of isomers increases rapidly with chain length; for example, pentane has three, hexane has five. IB HL and WJEC often ask you to draw and name all possible isomers for a given molecular formula, and to recognise that isomers are distinct compounds with different physical properties.
从丁烷（C₄）开始烷烃出现碳链异构。丁烷（C₄H₁₀）有两种异构体：正丁烷和2-甲基丙烷。随着碳链增长，异构体数目急剧增加，戊烷有三种，己烷有五种。IB HL和WJEC常要求根据分子式画出并命名所有可能的异构体，并认识到异构体是物理性质不同的不同化合物。
4. Physical Properties: Trends and Explanations | 物理性质：递变规律及解释
Boiling points of straight-chain alkanes increase with molar mass because of stronger London dispersion forces between larger electron clouds. Branched alkanes have lower boiling points than their straight-chain isomers due to reduced surface contact and thus weaker intermolecular forces. Alkanes are insoluble in water but dissolve in non-polar solvents, a consequence of their inability to form hydrogen bonds.
直链烷烃的沸点随摩尔质量增大而升高，因为更大的电子云产生更强的伦敦色散力。支链烷烃的沸点低于其直链异构体，原因是分子间接触面积减小，分子间力更弱。烷烃不溶于水，但溶于非极性溶剂，这是由于其无法形成氢键。
| Alkane | Formula | Boiling Point (°C) |
|---|---|---|
| Methane | CH₄ | -162 |
| Ethane | C₂H₆ | -89 |
| Propane | C₃H₈ | -42 |
| Butane | C₄H₁₀ | -0.5 |
| Pentane | C₅H₁₂ | 36 |
5. Combustion Reactions | 燃烧反应
Complete combustion of alkanes in excess oxygen yields carbon dioxide and water, releasing large amounts of energy, which is why they are widely used as fuels. The general equation is: CₙH₂ₙ₊₂ + (1.5n+0.5)O₂ → nCO₂ + (n+1)H₂O. Incomplete combustion, occurring when oxygen is limited, produces carbon monoxide (toxic) and/or carbon (soot). Both IB and WJEC expect you to write balanced equations and discuss environmental impacts.
烷烃在过量氧气中完全燃烧生成二氧化碳和水，并释放大量能量，因此被广泛用作燃料。通式为：CₙH₂ₙ₊₂ + (1.5n+0.5)O₂ → nCO₂ + (n+1)H₂O。当氧气不足时发生不完全燃烧，产生有毒的一氧化碳和/或碳（炭黑）。IB和WJEC均要求书写配平方程式并讨论环境影响。
CH₄ + 2O₂ → CO₂ + 2H₂O (complete)
2CH₄ + 3O₂ → 2CO + 4H₂O (incomplete)
6. Free-Radical Substitution Mechanism | 自由基取代机理
Alkanes undergo halogenation via a free-radical chain mechanism involving three stages: initiation, propagation, and termination. For chlorination of methane under UV light: initiation produces chlorine radicals (Cl•) via homolytic fission; propagation steps involve H-atom abstraction by Cl• to form HCl and a methyl radical (•CH₃), which then reacts with Cl₂ to give chloromethane and regenerate Cl•; termination occurs when two radicals combine. IB HL requires drawing curly arrows for initiation and propagation steps, while WJEC expects correct equation sequences and conditions.
烷烃通过自由基链式机理发生卤代反应，包括链引发、链增长和链终止三个阶段。以甲烷在紫外光下的氯代为例：引发阶段通过均裂产生氯自由基（Cl•）；增长阶段中，Cl•夺取氢原子生成HCl和甲基自由基（•CH₃），•CH₃再与Cl₂反应生成氯甲烷并再生Cl•；终止阶段两个自由基结合。IB HL要求用弯箭头表示引发和增长步骤，WJEC则侧重方程顺序和反应条件。
7. Structural Formulas and Representation | 结构式与表示方法
You must be adept at interconverting molecular formulas, full structural formulas, condensed structural formulas, and skeletal formulas. For instance, pentane can be written as CH₃CH₂CH₂CH₂CH₃ (condensed) or with a zigzag skeletal diagram where each vertex and endpoint represents a carbon atom. Both syllabi test the ability to interpret these forms, but IB may also explore three-dimensional representations like wedge-dash notation.
你必须熟练转换分子式、完整结构式、简写结构式和骨架式。例如，戊烷可写作CH₃CH₂CH₂CH₂CH₃（简写），或用锯齿形骨架图表示，每个顶点和端点代表一个碳原子。两个大纲都考查这些形式的解读能力，但IB还可能涉及楔形-虚线式的三维表示。
8. Environmental and Health Aspects | 环境与健康议题
Burning alkanes contributes to CO₂ emissions, a greenhouse gas. Incomplete combustion releases CO (toxic) and particulates. Unburnt hydrocarbons from vehicle exhausts contribute to photochemical smog. WJEC often examines catalytic converters and the use of alternative fuels, while IB links these topics to broader Energy and Green Chemistry options. Both expect you to evaluate the environmental impact of alkane usage.
烷烃燃烧导致温室气体CO₂排放。不完全燃烧释放有毒的CO及颗粒物。汽车尾气中未燃烧的烃类会促进光化学烟雾的形成。WJEC常考查催化转化器和替代燃料的使用，IB则将这些议题与更广泛的能源与绿色化学选修内容关联。两者都要求你对烷烃使用的环境影响进行评估。
9. Cracking and Industrial Relevance | 裂化与工业意义
Long-chain alkanes from crude oil are cracked into shorter, more useful hydrocarbons (alkanes and alkenes) via thermal or catalytic cracking. Catalytic cracking uses zeolite catalysts at lower temperatures, producing branched alkanes and cycloalkanes for high-octane petrol. IB students may encounter this in Option C or the core, while WJEC explicitly covers the conditions, products, and economic importance of cracking.
来自原油的长链烷烃通过热裂化或催化裂化转化为更短、更有用的烃类（烷烃和烯烃）。催化裂化采用沸石催化剂，温度较低，产物为用于高辛烷值汽油的支链烷烃和环烷烃。IB学生可能在选修C或核心部分接触此内容，WJEC则明确考查裂化的条件、产物及经济意义。
10. Key Differences Between IB and WJEC Approaches | IB与WJEC考查方式的差异
While the core content is nearly identical, IB Chemistry (especially HL) places greater emphasis on mechanistic understanding (electron-pushing arrows) and quantitative aspects like enthalpy of combustion calculations. The WJEC specification tends to embed alkanes within wider contexts such as fuels, sustainability, and industrial processes. Be aware that IB may require explanations in terms of bond enthalpies and energy profiles, whereas WJEC often links alkane chemistry to practical scenarios in Wales and the UK.
虽然核心内容几乎一致，IB化学（特别是HL）更强调机理理解（弯箭头表示电子转移）和燃烧焓计算等定量方面。WJEC大纲倾向于将烷烃嵌入更广泛的背景，如燃料、可持续性和工业流程。注意，IB可能要求用键能和能量曲线解释反应，而WJEC常将烷烃化学与威尔士及英国的实际情境相联系。
11. Summary and Revision Checklist | 复习清单与总结
Ensure you can confidently: name and draw alkanes up to C₁₀; identify and draw chain isomers; explain physical trends using intermolecular forces; write and balance equations for complete and incomplete combustion; outline the free-radical substitution mechanism with relevant equations; discuss environmental impacts; and compare the relevant examination requirements of IB and WJEC. Practice with past paper questions and apply your knowledge to unfamiliar alkanes.
确保你能自信地完成：命名和画出C₁₀以内的烷烃；识别和绘制碳链异构体；用分子间作用力解释物理性质递变；书写并配平完全与不完全燃烧方程式；用相关方程式概述自由基取代机理；讨论环境影响；比较IB与WJEC的考试要求。通过历年真题进行练习，并将知识应用于陌生的烷烃。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导