AS AQA Chemistry Scheme of Work 4.2: Alkanes | AS AQA 化学教学计划 4.2：烷烃

📚 AS AQA Chemistry Scheme of Work 4.2: Alkanes | AS AQA 化学教学计划 4.2：烷烃

The AQA AS Chemistry specification (7404) is often delivered through a structured scheme of work. Section 4.2 of this scheme typically covers the chemistry of alkanes, a fundamental family of saturated hydrocarbons. This article provides a comprehensive revision guide for that topic, aligned with the AQA AS-level requirements, focusing on structures, properties, reactions, and exam-relevant mechanisms.

AQA AS 化学教学大纲（7404）通常通过结构化的教学计划来实施。该教学计划的第 4.2 部分通常涵盖烷烃的化学——一类基本的饱和烃。本文为这一主题提供全面的复习指南，与 AQA AS 级别要求保持一致，重点讲解结构、性质、反应以及考试相关的反应机理。

1. What Are Alkanes? | 什么是烷烃？

Alkanes are saturated hydrocarbons, meaning they contain only carbon and hydrogen atoms with single covalent bonds. Their general formula is CₙH₂ₙ₊₂, where n is the number of carbon atoms. The simplest member is methane (CH₄), followed by ethane (C₂H₆), propane (C₃H₈), and so on. Because all carbon–carbon bonds are single, alkanes are often described as ‘saturated’.

烷烃是饱和烃，即分子中只含有碳和氢原子，且全部以单键共价键结合。其通式为 CₙH₂ₙ₊₂，其中 n 为碳原子数。最简单的成员是甲烷（CH₄），其次是乙烷（C₂H₆）、丙烷（C₃H₈）等。由于所有碳–碳键均为单键，烷烃常被描述为“饱和的”。

Carbon atoms in alkanes adopt a tetrahedral geometry, with bond angles of approximately 109.5°. This shape arises from sp³ hybridisation of each carbon atom’s orbitals.

烷烃中的碳原子采取四面体几何构型，键角约为 109.5°。这一形状源于每个碳原子的 sp³ 杂化轨道。

2. Nomenclature and Isomerism | 命名与同分异构现象

The IUPAC naming system for alkanes uses the prefix to indicate the number of carbon atoms (meth-, eth-, prop-, but-, pent-, etc.) and the suffix ‘-ane’ to show that the compound is an alkane. For branched alkanes, the longest continuous carbon chain is chosen as the parent chain, and alkyl substituents (e.g., methyl, ethyl) are listed alphabetically with their position numbers.

烷烃的 IUPAC 命名法使用词头表示碳原子数（甲、乙、丙、丁、戊等），并以后缀“-ane”表明该化合物是烷烃。对于支链烷烃，选择最长的连续碳链作为主链，烷基取代基（如甲基、乙基）按字母顺序列出，并标注其位置编号。

Compound	Structure	Name
CH₄	Methane	Methane
CH₃CH₂CH₂CH₃	Butane	Butane
(CH₃)₂CHCH₃	2-methylpropane	Methylpropane (isobutane)

Isomerism in alkanes arises from different arrangements of the carbon skeleton. For example, butane (C₄H₁₀) has two structural isomers: n-butane and 2-methylpropane. As the number of carbon atoms increases, the number of possible isomers grows rapidly.

烷烃的同分异构现象源于碳骨架的不同排列。例如，丁烷（C₄H₁₀）有两种结构异构体：正丁烷和 2-甲基丙烷。随着碳原子数增加，可能的异构体数目迅速增多。

3. Physical Properties | 物理性质

The physical properties of alkanes are largely determined by their non-polar nature. The only intermolecular forces between alkane molecules are London dispersion forces (induced dipole–dipole interactions), which become stronger as molecular size increases. This explains several trends.

烷烃的物理性质在很大程度上取决于其非极性特性。烷烃分子之间的唯一分子间作用力是伦敦色散力（诱导偶极–偶极相互作用），它随分子尺寸增大而增强。这解释了许多变化趋势。

Boiling points: Increase with chain length due to larger surface area and stronger dispersion forces. Branched isomers have lower boiling points than their linear counterparts because branching reduces surface contact.
溶解度： 沸点：随着链长增加而升高，因为表面积增大、色散力增强。支链异构体的沸点低于直链异构体，因为支链减少了分子间的接触面积。
Solubility: Alkanes are non-polar and insoluble in water, but they dissolve in non-polar solvents such as hexane.
溶解度： 烷烃是非极性的，不溶于水，但可溶于非极性溶剂（如己烷）。
Density: All alkanes are less dense than water, so they float on water.
密度： 所有烷烃的密度都小于水，因此它们会浮在水面上。

4. Reactions with Oxygen: Combustion | 与氧气的反应：燃烧

Alkanes undergo complete combustion in excess oxygen to produce carbon dioxide and water. This reaction is highly exothermic, making alkanes valuable as fuels. For example:

烷烃在过量氧气中发生完全燃烧，生成二氧化碳和水。该反应高度放热，使烷烃成为重要的燃料。例如：

CH₄ + 2O₂ → CO₂ + 2H₂O

In limited oxygen, incomplete combustion can occur, producing carbon monoxide (a toxic gas) and/or carbon (soot), along with water. Incomplete combustion releases less energy and can be dangerous in poorly ventilated spaces.

在氧气不足时，可能发生不完全燃烧，生成一氧化碳（有毒气体）和/或碳（烟灰），以及水。不完全燃烧释放的能量较少，在通风不良的环境中可能造成危险。

5. Free-Radical Substitution with Halogens | 与卤素的自由基取代反应

Alkanes are generally unreactive due to the strength of their C–H and C–C bonds and the low polarity of these bonds. However, they do react with halogens in the presence of ultraviolet (UV) light or heat. This is a free-radical substitution reaction, commonly illustrated with methane and chlorine.

由于 C–H 和 C–C 键的强度较高且极性较低，烷烃通常不活泼。然而，在紫外线（UV）光或加热条件下，它们能与卤素反应。这是一个自由基取代反应，通常以甲烷和氯气为例说明。

The mechanism proceeds in three steps:

该反应机理分三步进行：

Initiation: Cl₂ → 2Cl• (UV light breaks the Cl–Cl bond homolytically)
引发： Cl₂ → 2Cl•（紫外光使 Cl–Cl 键均裂）
Propagation: Cl• + CH₄ → HCl + •CH₃; then •CH₃ + Cl₂ → CH₃Cl + Cl•
链增长： Cl• + CH₄ → HCl + •CH₃；随后 •CH₃ + Cl₂ → CH₃Cl + Cl•
Termination: Two radicals combine, e.g., Cl• + •CH₃ → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆
链终止： 两个自由基结合，例如 Cl• + •CH₃ → CH₃Cl，或 •CH₃ + •CH₃ → C₂H₆

The reaction produces a mixture of chlorinated products (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄) because further substitution can occur. This is a limitation of the reaction for synthetic purposes.

由于进一步取代可能发生，该反应会生成多种氯代产物的混合物（CH₃Cl、CH₂Cl₂、CHCl₃、CCl₄）。这是该反应在合成用途上的一个局限。

6. Environmental Importance: CFCs and the Ozone Layer | 环境意义：氟氯烃与臭氧层

Chlorofluorocarbons (CFCs) were once widely used as refrigerants and propellants. They are stable in the troposphere but decompose in the stratosphere under UV radiation, releasing chlorine radicals (Cl•). These radicals catalyse the breakdown of ozone (O₃) to oxygen (O₂).

氟氯烃（CFCs）曾广泛用作制冷剂和推进剂。它们在对流层中稳定，但在平流层中受到紫外线辐射会分解，释放氯自由基（Cl•）。这些自由基催化臭氧（O₃）分解为氧气（O₂）。

The overall chain reaction is:

总链反应为：

Cl• + O₃ → ClO• + O₂

ClO• + O• → Cl• + O₂

One chlorine radical can destroy thousands of ozone molecules. This knowledge is linked to the free-radical substitution mechanism, as it shows the reactivity of radicals in chain reactions.

一个氯自由基可以破坏成千上万个臭氧分子。这一知识与自由基取代反应机理相关联，因为它展示了自由基在链式反应中的活性。

7. Comparing Alkanes with Other Hydrocarbons | 烷烃与其他烃类的比较

Alkanes are less reactive than alkenes (which contain C=C bonds) because the C=C bond provides a region of high electron density, making alkenes more susceptible to electrophilic attack. Alkanes, in contrast, undergo only free-radical substitution under harsh conditions. This comparison is essential for understanding the broader organic chemistry module.

烷烃的活性低于烯烃（含有 C=C 键），因为 C=C 键提供了高电子密度区域，使烯烃更容易受到亲电试剂进攻。相比之下，烷烃仅在苛刻条件下发生自由基取代。这种比较对于理解更广泛的有机化学模块至关重要。

For example, alkanes do not decolourise bromine water in the dark, while alkenes do immediately. This distinguishes saturated from unsaturated hydrocarbons.

例如，烷烃在暗处不能使溴水褪色，而烯烃能立即使其褪色。这可用于区分饱和烃和不饱和烃。

8. Key Exam Points and Common Misconceptions | 考试要点与常见误区

In AQA AS exams, students are often asked to:

在 AQA AS 考试中，学生常被要求：

Write balanced equations for complete and incomplete combustion of alkanes.
写出烷烃完全燃烧和不完全燃烧的配平方程式。
Identify and name isomers of alkanes up to hexane.
识别并命名至己烷的烷烃异构体。
Describe the free-radical substitution mechanism using curly arrows and dot-and-cross diagrams.
使用弯箭头和点叉图描述自由基取代反应机理。
Explain the environmental consequences of CFCs on the ozone layer.
解释氟氯烃对臭氧层的环境影响。

Common mistakes include misdrawing the propagation steps, forgetting that UV light is needed for initiation, and writing incomplete combustion products incorrectly (e.g., only CO, not C or CO₂ mix).

常见错误包括：画错链增长步骤、忘记引发需要紫外光，以及错误写出不完全燃烧产物（例如只写 CO，而不写 C 或 CO₂ 的混合物）。

9. Practicing Calculation of Combustion Enthalpies | 燃烧焓计算练习

Although alkanes are not directly assessed for enthalpy changes in the AS scheme, they often appear in thermochemistry questions. The complete combustion enthalpy of an alkane can be calculated using bond enthalpies, though experimental values differ due to the formation of strong bonds in CO₂ and H₂O.

虽然 AS 教学计划中并不直接考察烷烃的焓变，但它们常出现在热化学题目中。可利用键焓计算烷烃的完全燃烧焓，不过由于 CO₂ 和 H₂O 中强键的形成，实验值会有所不同。

For example, the combustion of propane:

例如，丙烷的燃烧：

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Students should be able to use Hess’s law cycles to relate combustion enthalpies to formation enthalpies.

学生应能利用赫斯定律循环将燃烧焓与生成焓联系起来。

10. Conclusion | 总结

Alkanes are a cornerstone of organic chemistry at AS level. Their saturated nature, non-polar behaviour, and free-radical substitution mechanism are all essential knowledge for the AQA examination. By mastering the structure, naming, physical properties, and reactions of alkanes, students build a solid foundation for more advanced organic topics, including alkenes and halogenoalkanes, in later sections.

烷烃是 AS 阶段有机化学的基石。它们的饱和性质、非极性行为以及自由基取代反应机理都是 AQA 考试中的必备知识。通过掌握烷烃的结构、命名、物理性质和反应，学生可以为后续更高级的有机主题（如烯烃和卤代烷烃）打下坚实基础。

Remember to practise drawing mechanisms, writing balanced equations, and applying the concepts to unfamiliar contexts, as these are the skills that yield top marks in AQA chemistry papers.

请记住，要练习画反应机理、写配平方程式，并将这些概念应用到陌生情境中，因为这是在 AQA 化学试卷中获得高分的关键技能。

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AS AQA Chemistry Scheme of Work 4.2: Alkanes | AS AQA 化学教学计划 4.2：烷烃