A-Level Chemistry: Alkanes Revision Guide | A-Level 化学：烷烃考点精讲

📚 A-Level Chemistry: Alkanes Revision Guide | A-Level 化学：烷烃考点精讲

Alkanes are the simplest organic molecules, yet they form the foundation for understanding structure, bonding, and reactivity in organic chemistry. This guide covers essential A-Level content including nomenclature, isomerism, physical properties, combustion, free‑radical substitution, and environmental considerations.

烷烃是最简单的有机分子，但它们构成了理解有机化学中结构、键合和反应性的基础。本指南涵盖了A-Level的核心内容，包括命名法、同分异构、物理性质、燃烧、自由基取代反应以及环境因素。

1. Structure and Bonding | 结构与键合

Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. Each carbon atom is sp³ hybridised and forms four sigma (σ) bonds in a tetrahedral geometry with bond angles of approximately 109.5°. The bonds are non‑polar or only very weakly polar, so the main intermolecular forces are London dispersion forces.

烷烃是通式为 CₙH₂ₙ₊₂ 的饱和碳氢化合物。每个碳原子均为 sp³ 杂化，形成四个 σ 键，呈四面体构型，键角约为 109.5°。这些键是非极性或极性极弱的，因此主要的分子间作用力是色散力。

Sigma bonds: formed by head‑on overlap of orbitals, allowing free rotation.
σ 键：由轨道头对头重叠形成，可自由旋转。
Tetrahedral shape: gives alkanes their characteristic zig‑zag chain arrangement.
四面体形状：使烷烃具有特征的锯齿形链排列。

2. Nomenclature | 命名法

IUPAC names for alkanes are built from a prefix indicating the number of carbons together with the suffix -ane. Substituents (alkyl groups) are named as prefixes with locants to show their position on the longest continuous chain.

烷烃的 IUPAC 名称由表示碳原子数的前缀加上后缀“-烷”组成。取代基（烷基）作为前缀，并用位次标明它们在最长连续链上的位置。

Number of carbons	Prefix	Name	碳原子数	前缀	名称
1	Meth-	Methane	1	甲-	甲烷
2	Eth-	Ethane	2	乙-	乙烷
3	Prop-	Propane	3	丙-	丙烷
4	But-	Butane	4	丁-	丁烷
5	Pent-	Pentane	5	戊-	戊烷
6	Hex-	Hexane	6	己-	己烷

When naming branched alkanes: identify the longest chain, number the chain to give substituents the lowest possible numbers, and list substituents in alphabetical order with di-, tri- prefixes if needed.

命名支链烷烃时：找出最长链，给链编号使取代基位次尽可能小，并按字母顺序列出取代基，必要时使用二-、三-等前缀。

3. Isomerism | 同分异构

Alkanes can exhibit chain isomerism and, for cyclic alkanes, geometric stereoisomerism. Chain isomers have the same molecular formula but different arrangements of the carbon skeleton.

烷烃可以表现出碳链异构，对于环烷烃还可表现出顺反异构。碳链异构体具有相同的分子式，但碳骨架的排列不同。

Chain isomerism: For C₄H₁₀, butane and 2‑methylpropane are chain isomers.
碳链异构：对于 C₄H₁₀，丁烷和 2‑甲基丙烷互为碳链异构体。
Cycloalkanes: have the general formula CₙH₂ₙ and are structural isomers of alkenes, not straight‑chain alkanes.
环烷烃：通式为 CₙH₂ₙ，是烯烃的结构异构体，而非直链烷烃的异构体。

4. Physical Properties | 物理性质

Boiling points of alkanes increase with molecular mass due to stronger London forces as the electron cloud becomes larger and more polarisable. Branched alkanes have lower boiling points than their straight‑chain isomers because the more spherical shape reduces surface contact, weakening intermolecular forces.

烷烃的沸点随分子质量的增加而升高，这是因为电子云变大且更易极化，色散力更强。支链烷烃的沸点低于其直链异构体，因为更接近球形的形状减少了表面接触，削弱了分子间作用力。

Solubility: Alkanes are non‑polar and insoluble in water but dissolve in non‑polar organic solvents.
溶解性：烷烃是非极性的，不溶于水，但可溶于非极性有机溶剂。
Density: Liquid alkanes are less dense than water (≈0.7–0.8 g cm⁻³).
密度：液态烷烃的密度小于水（约 0.7–0.8 g cm⁻³）。

5. Combustion | 燃烧反应

Alkanes burn in excess oxygen to produce carbon dioxide and water, releasing large amounts of energy. This makes them valuable fuels. The general equation for complete combustion is:

烷烃在过量氧气中燃烧生成二氧化碳和水，并释放大量能量，因此是重要的燃料。完全燃烧的通式为：

CₙH₂ₙ₊₂ + (1.5n+0.5) O₂ → n CO₂ + (n+1) H₂O

Incomplete combustion occurs when oxygen is limited, producing carbon monoxide (CO) or carbon (soot). Carbon monoxide is toxic because it binds irreversibly to haemoglobin, reducing oxygen transport.

当氧气不足时会发生不完全燃烧，生成一氧化碳 (CO) 或碳（烟灰）。一氧化碳有毒，因为它与血红蛋白不可逆地结合，降低了氧气输送能力。

Complete: CH₄ + 2 O₂ → CO₂ + 2 H₂O
完全燃烧：CH₄ + 2 O₂ → CO₂ + 2 H₂O
Incomplete: 2 CH₄ + 3 O₂ → 2 CO + 4 H₂O or CH₄ + O₂ → C + 2 H₂O
不完全燃烧：2 CH₄ + 3 O₂ → 2 CO + 4 H₂O 或 CH₄ + O₂ → C + 2 H₂O

6. Environmental Impact of Combustion | 燃烧的环境影响

Complete combustion of alkanes produces CO₂, a greenhouse gas that contributes to global warming. Impurities in fossil fuels, such as sulfur compounds, lead to SO₂ emissions causing acid rain. High‑temperature combustion can also produce nitrogen oxides (NOₓ) which contribute to acid rain and photochemical smog.

烷烃的完全燃烧产生 CO₂，这是一种导致全球变暖的温室气体。化石燃料中的杂质（如硫化合物）会导致 SO₂ 排放，形成酸雨。高温燃烧还会产生氮氧化物 (NOₓ)，加剧酸雨和光化学烟雾。

SO₂: forms sulfurous and sulfuric acid in the atmosphere.
SO₂：在大气中形成亚硫酸和硫酸。
Catalytic converters: reduce CO, NOₓ and unburnt hydrocarbons in car exhausts.
催化转化器：减少汽车尾气中的 CO、NOₓ 和未燃烧碳氢化合物。

7. Reactivity and Activation Energy | 反应性与活化能

Alkanes are generally unreactive because the C–C and C–H σ bonds are strong and non‑polar. They do not react with acids, bases, oxidising or reducing agents under normal conditions. However, they do undergo combustion and free‑radical substitution when given sufficient energy.

烷烃通常不活泼，因为 C–C 和 C–H σ 键强且非极性。在正常条件下它们不与酸、碱、氧化剂或还原剂反应。但给予足够能量时，它们可以发生燃烧和自由基取代反应。

Bond enthalpy: C–H ≈ 413 kJ mol⁻¹, C–C ≈ 347 kJ mol⁻¹
键能：C–H ≈ 413 kJ mol⁻¹，C–C ≈ 347 kJ mol⁻¹
UV light or heat: provides the energy to initiate radical reactions.
紫外光或加热：提供启动自由基反应的能量。

8. Free‑Radical Substitution Mechanism | 自由基取代机理

Alkanes react with halogens (e.g. Cl₂, Br₂) in the presence of UV light via a free‑radical chain mechanism. The reaction produces a mixture of halogenoalkanes.

烷烃在紫外光存在下与卤素（如 Cl₂、Br₂）通过自由基链式机理反应，生成卤代烷的混合物。

The mechanism proceeds in three stages:

该机理分三个阶段进行：

Initiation: Homolytic fission of the halogen molecule by UV light produces two halogen radicals.

链引发：紫外光使卤素分子发生均裂，产生两个卤素自由基。

Cl₂ → 2 Cl•

Propagation: Radicals react with alkane molecules to form hydrogen halide and an alkyl radical, which then reacts with another halogen molecule, regenerating the halogen radical.

链增长：自由基与烷烃分子反应，生成卤化氢和一个烷基自由基，后者再与另一卤素分子反应，重新生成卤素自由基。

CH₄ + Cl• → •CH₃ + HCl

•CH₃ + Cl₂ → CH₃Cl + Cl•

Termination: Two radicals combine to form a stable molecule, removing radicals from the system.

链终止：两个自由基结合形成稳定分子，从体系中移除自由基。

Cl• + Cl• → Cl₂

•CH₃ + Cl• → CH₃Cl

•CH₃ + •CH₃ → C₂H₆

9. Further Substitution and Product Mixtures | 进一步取代与产物混合物

Because the substitution is random, further halogenation occurs, leading to a mixture of mono-, di-, and poly-substituted products. The product distribution depends on the halogen:alkane ratio.

由于取代是随机的，会进一步发生卤代，导致一取代、二取代和多取代产物的混合物。产物分布取决于卤素与烷烃的比例。

Excess alkane: favours monosubstitution.
烷烃过量：有利于一取代。
Excess halogen: favours polysubstitution.
卤素过量：有利于多取代。
Example: Methane + Cl₂ gives CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄.
示例：甲烷 + Cl₂ 生成 CH₃Cl、CH₂Cl₂、CHCl₃ 和 CCl₄。

10. Uses of Alkanes | 烷烃的用途

Alkanes are primarily used as fuels due to their high energy release upon combustion. They also serve as raw materials in the petrochemical industry for producing plastics, solvents, and as starting materials for synthesis of more complex organic molecules through cracking and reforming.

烷烃主要用作燃料，因为它们燃烧时释放大量能量。它们还作为石化工业的原料，用于生产塑料、溶剂，并通过裂化和重整作为合成更复杂有机分子的起始原料。

Natural gas: mainly methane, used for heating and electricity generation.
天然气：主要成分是甲烷，用于取暖和发电。
LPG: propane/butane mixtures for portable stoves and BBQs.
液化石油气：丙烷/丁烷混合物，用于便携式炉具和烧烤。
Petrol (gasoline): a mixture of C₅–C₁₂ alkanes.
汽油：C₅–C₁₂ 烷烃的混合物。

11. Cracking and Reforming | 裂化与重整

Cracking breaks long‑chain alkanes into shorter, more useful alkanes and alkenes. Thermal cracking uses high temperature (700–1200 K) and high pressure, producing mainly alkenes. Catalytic cracking uses a zeolite catalyst at lower temperature and pressure, producing branched alkanes and cycloalkanes for motor fuels.

裂化将长链烷烃断裂成更短、更有用的烷烃和烯烃。热裂化使用高温 (700–1200 K) 和高压，主要生成烯烃。催化裂化使用沸石催化剂，在较低的温度和压力下进行，生成支链烷烃和环烷烃，用于机动车燃料。

Reforming: converts straight‑chain alkanes into branched or cyclic alkanes to improve octane rating.
重整：将直链烷烃转化为支链或环状烷烃，以提高辛烷值。
Equation example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄
反应式示例：C₁₀H₂₂ → C₈H₁₈ + C₂H₄

12. Exam Tips and Common Errors | 考试技巧与常见错误

Ensure you can draw displayed and skeletal formulae accurately. When writing free‑radical substitution mechanisms, use curly half‑arrows (fish‑hooks) to show single electron movement, and clearly label initiation, propagation and termination steps. Avoid using ‘H’ as a radical; remember it is a halogen radical that initiates the reaction.

确保能准确画出结构式和骨架式。书写自由基取代机理时，使用弯箭头（鱼钩箭头）表示单电子转移，并清楚地标记链引发、链增长和链终止步骤。避免把“H”写成自由基；记住引发反应的是卤素自由基。

Common mistake: writing a propagation step as CH₄ + Cl₂ → CH₃Cl + Cl• — this is not an elementary step; it must be broken into two steps.
常见错误：将链增长步骤写成 CH₄ + Cl₂ → CH₃Cl + Cl• —— 这不是基元步骤，必须分解为两步。
Bond angle: in alkanes is 109.5°, not 120° or 180°.
键角：烷烃中为 109.5°，而非 120° 或 180°。

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A-Level Chemistry: Alkanes Revision Guide | A-Level 化学：烷烃 考点精讲