📚 Alkanes: A-Level AQA Chemistry Exam Essentials | 烷烃：A-Level AQA 化学考点精讲
Alkanes are the simplest family of hydrocarbons, containing only carbon and hydrogen atoms joined by single bonds. In AQA A-Level Chemistry, you must be confident with their nomenclature, isomerism, physical trends, radical substitution mechanism, combustion, industrial cracking and reforming, as well as the environmental consequences of their use. This article breaks down every key specification point in a dual-language format to support both your conceptual understanding and your exam technique.
烷烃是最简单的烃类家族，仅由碳和氢以单键相连。在 AQA A-Level 化学中，你需要熟练掌握烷烃的命名、异构、物理性质变化规律、自由基取代机理、燃烧反应、工业裂化和重整，以及其使用对环境的影响。本文以中英双语逐点梳理每个重要考点，帮助你既夯实概念理解，又提升答题技巧。
1. Introduction: What Are Alkanes? | 烷烃简介
Alkanes are saturated hydrocarbons, meaning every carbon atom forms four single covalent bonds, either to other carbon atoms or to hydrogen atoms. They are the main components of crude oil and natural gas and serve as essential fuels and chemical feedstocks.
烷烃是饱和烃，也就是说每个碳原子都形成四个单共价键，连向其他碳原子或氢原子。它们是原油和天然气的主要成分，既是不可或缺的燃料，也是重要的化工原料。
Because all bonds are σ bonds, alkanes have relatively low reactivity except under vigorous conditions such as combustion or UV‑initiated free‑radical substitution. The C–C and C–H bonds are strong and non‑polar, so alkanes do not react with acids, bases, or oxidising agents under normal conditions.
由于所有键都是 σ 键，烷烃除了在剧烈条件（如燃烧或紫外光引发的自由基取代）下，相对而言反应性较低。C–C 键和 C–H 键强度大且非极性，因此烷烃在通常条件下不与酸、碱或氧化剂反应。
In the AQA specification, alkanes are introduced in Year 12 organic chemistry and their reactions are revisited in the context of fuels and industrial processes. You will need to link their structure to both physical properties and chemical behaviour.
在 AQA 大纲中，烷烃在 Year 12 有机化学部分引入，随后在燃料和工业过程的背景下再次深入。你需要将它们的结构与物理性质及化学行为联系起来。
2. General Formula and Homologous Series | 通式与同系物
Alkanes form a homologous series with the general formula CnH2n+2, where n is the number of carbon atoms. They differ from one another by a –CH2– unit and show gradual trends in physical properties.
烷烃构成一个通式为 CnH2n+2 的同系物序列，其中 n 为碳原子数。相邻烷烃相差一个 –CH2– 单元，物理性质呈现规律性变化。
The simplest alkane is methane (CH4), followed by ethane (C2H6), propane (C3H8), butane (C4H10), and so on. Every member has a name ending in -ane, and all conform to the same general formula.
最简单的烷烃是甲烷 (CH4)，随后是乙烷 (C2H6)、丙烷 (C3H8)、丁烷 (C4H10) 等。每个成员的名称都以 -ane 结尾，且都符合同一通式。
For a given number of carbon atoms, the molecular formula uniquely identifies an alkane. However, from butane onward, structural isomerism becomes possible because the carbon skeleton can be arranged in different ways.
给定碳原子数目时，分子式唯一确定一种烷烃。但从丁烷开始，由于碳骨架可以有不同的连接方式，便出现了结构异构现象。
3. Nomenclature of Alkanes | 烷烃命名法
IUPAC names for alkanes are built by identifying the longest continuous carbon chain. The number of carbon atoms gives the stem: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10).
烷烃的 IUPAC 命名通过找出最长的连续碳链来构建。碳原子数决定词干： meth- (1)、eth- (2)、prop- (3)、but- (4)、pent- (5)、hex- (6)、hept- (7)、oct- (8)、non- (9)、dec- (10)。
If a branched chain is present, the name includes the positions and names of side chains (alkyl groups) as prefixes. The main chain is numbered from the end that gives the substituents the lowest possible numbers, and the substituents are listed in alphabetical order, ignoring any multipliers like di-, tri-, etc.
如果有支链，命名时会将侧链（烷基）的位置和名称作为前缀。主链从最靠近取代基的一端开始编号，使取代基的位置编号尽可能小；然后按字母顺序列出取代基，忽略 di-、tri- 等倍数词。
For example, a six‑carbon chain with a methyl group on carbon 2 and an ethyl group on carbon 4 is named 4-ethyl-2-methylhexane. Practice with AQA‑style naming questions is essential, as systematic nomenclature is frequently tested.
例如，一个六碳主链，在 2 号碳上有一个甲基、4 号碳上有一个乙基，其名称为 4-ethyl-2-methylhexane。多做 AQA 风格的命名练习至关重要，因为系统命名法经常出现在考题中。
4. Structural Isomerism in Alkanes | 烷烃的结构异构
Structural isomers are molecules with the same molecular formula but different structural formulae. In alkanes, isomerism arises from the different ways the carbon skeleton can be branched. For C4H10, there are two isomers: butane and 2-methylpropane (isobutane). For C5H12, three isomers exist: pentane, 2-methylbutane and 2,2-dimethylpropane.
结构异构体是指分子式相同而结构式不同的分子。在烷烃中，异构现象源自碳骨架不同的支链方式。C4H10 有两种异构体：丁烷和 2-甲基丙烷（异丁烷）。C5H12 则有三种异构体：戊烷、2-甲基丁烷和 2,2-二甲基丙烷。
Recognising and drawing all the chain isomers for a given formula is a key skill. You should also be able to name each isomer correctly and identify which isomer has the highest or lowest boiling point based on the degree of branching.
能够识别并画出给定分子式的所有碳链异构体是一项关键技能。你还应能为每个异构体正确命名，并根据支链程度判断哪种异构体的沸点最高或最低。
Remember that cycloalkanes (e.g. cyclopentane, C5H10) are not alkanes; they have the general formula CnH2n and belong to a different homologous series, so do not confuse them with alkane isomers.
注意，环烷烃（如环戊烷 C5H10）不属于烷烃，它们的通式是 CnH2n，属于不同的同系物，不要与烷烃的异构体混淆。
5. Physical Properties of Alkanes | 烷烃的物理性质
Alkanes are non‑polar molecules; the only intermolecular forces present are London (van der Waals) forces. As the length of the carbon chain increases, the number of electrons grows, leading to stronger instantaneous dipole–induced dipole interactions and increasingly higher boiling points.
烷烃是非极性分子，仅存在伦敦（范德华）力。随着碳链长度增加，电子数增多，瞬时偶极–诱导偶极作用增强，沸点逐渐升高。
Branching reduces the surface area over which molecules can pack closely together, weakening the van der Waals forces. Consequently, a branched alkane has a lower boiling point than its straight‑chain isomer.
支链使分子紧密堆积的表面积减小，范德华力减弱。因此支链烷烃的沸点低于其直链异构体。
The table below illustrates the variation in boiling point with chain length for straight‑chain alkanes.
下表展示了直链烷烃沸点随碳链长度的变化。
| Alkane | Formula | Boiling point / °C |
|---|---|---|
| Methane | CH4 | −162 |
| Ethane | C2H6 | −89 |
| Propane | C3H8 | −42 |
| Butane | C4H10 | −0.5 |
| Pentane | C5H12 | 36 |
Alkanes are virtually insoluble in water because they cannot form hydrogen bonds with water molecules, but they dissolve well in non‑polar solvents like other hydrocarbons.
烷烃几乎不溶于水，因为它们无法与水分子形成氢键，但可溶于其他碳氢化合物等非极性溶剂。
6. Combustion of Alkanes | 烷烃的燃烧反应
Alkanes are excellent fuels. In a plentiful supply of oxygen, they undergo complete combustion to form carbon dioxide and water, releasing large amounts of energy. For example, the complete combustion of methane is:
烷烃是优良的燃料。在氧气充足时，它们发生完全燃烧，生成二氧化碳和水，并释放大量能量。例如，甲烷的完全燃烧为：
CH4 + 2O2 → CO2 + 2H2O
In restricted oxygen supply, incomplete combustion occurs, producing carbon monoxide (CO) and/or solid carbon (soot). Carbon monoxide is a toxic gas because it binds permanently to haemoglobin, reducing the blood’s oxygen‑carrying capacity.
在氧气不足时，发生不完全燃烧，生成一氧化碳 (CO) 和/或固态碳（炭黑）。一氧化碳是有毒气体，因为它与血红蛋白不可逆地结合，降低血液的携氧能力。
Incomplete combustion also releases unburnt hydrocarbons and particulate matter, contributing to air pollution. AQA questions often ask you to write balanced equations for both complete and incomplete combustion, and to explain the health and environmental effects of the products.
不完全燃烧还会排放未燃烧的烃类及颗粒物，加剧空气污染。AQA 试题常要求你书写完全燃烧和不完全燃烧的配平方程式，并阐述产物的健康及环境影响。
You should also be aware that enthalpy changes of combustion for alkanes become more negative as the chain length increases, because more C–C and C–H bonds are broken and more CO2 and H2O bonds are formed.
你还应知道，随着碳链增长，烷烃的燃烧焓变更大（更负），因为将断裂更多的 C–C 和 C–H 键，并形成更多的 CO2 和 H2O 键。
7. Free Radical Substitution Mechanism | 自由基取代机理
Alkanes react with halogens (Cl2 or Br2) in the presence of UV light through a free‑radical substitution mechanism. The reaction replaces a hydrogen atom with a halogen atom, forming a halogenoalkane and a hydrogen halide. For methane and chlorine:
烷烃在紫外光照射下与卤素 (Cl2 或 Br2) 经自由基取代机理发生反应。反应将一个氢原子替换为卤原子，生成卤代烷和卤化氢。以甲烷和氯气为例：
CH4 + Cl2 → CH3Cl + HCl
The mechanism proceeds by three stages: initiation, propagation and termination.
该机理分三步进行：引发、传递和终止。
Initiation: Cl–Cl bond undergoes homolytic fission under UV light, generating two chlorine free radicals.
引发：Cl–Cl 键在紫外光下发生均裂，产生两个氯自由基。
Cl2 → 2Cl•
Propagation: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical; the methyl radical then attacks a Cl2 molecule, producing chloromethane and regenerating a chlorine radical.
传递：一个氯自由基夺取甲烷上的一个氢原子，生成 HCl 和甲基自由基；甲基自由基攻击 Cl2 分子，生成氯甲烷并再生一个氯自由基。
Cl• + CH4 → HCl + •CH3
•CH3 + Cl2 → CH3Cl + Cl•
Termination: Any two free radicals combine to form a stable molecule, such as Cl• + Cl• → Cl2, CH3• + CH3• → C2H6, or Cl• + CH3• → CH3Cl.
终止：任意两个自由基结合形成稳定分子，如 Cl• + Cl• → Cl2、CH3• + CH3• → C2H6 或 Cl• + CH3• → CH3Cl。
AQA will expect you to use curly, fish‑hook arrows to show the movement of single electrons in homolytic fission and radical propagation steps. In questions, you may be asked to write propagation equations for the formation of further substitution products such as dichloromethane, or to explain why a mixture of products is obtained.
AQA 要求你使用弯钩箭头（鱼钩箭头）来表示均裂和自由基传递步骤中单电子的移动。考题可能要求你写出生成进一步取代产物（如二氯甲烷）的传递方程式，或解释为什么得到的总是混合物。
8. Cracking of Alkanes | 烷烃的裂化
Cracking is the process of breaking longer‑chain alkanes (less useful as fuels) into shorter, more useful molecules. There are two main types: thermal cracking and catalytic cracking.
裂化是将较长碳链的烷烃（作为燃料用途较少）断裂为较短且更有用的分子的过程。主要有两种类型：热裂化和催化裂化。
| Type | Conditions | Products |
|---|---|---|
| Thermal cracking | High temperature (700–1200 K) and high pressure (up to 7000 kPa) | Mainly alkenes (e.g. ethene) plus some shorter alkanes; used to produce alkenes for polymers |
| Catalytic cracking | Lower temperature (about 720 K), slight pressure, zeolite catalyst | Branched‑chain alkanes, cycloalkanes, aromatic hydrocarbons; high‑octane petrol fractions |
In thermal cracking, carbon–carbon bonds break homolytically, generating free radicals that quickly rearrange to form smaller alkanes and alkenes. This gives a high proportion of straight‑chain alkenes used in the polymer industry.
在热裂化中，碳–碳键发生均裂，产生自由基，自由基迅速重排生成较小的烷烃和烯烃。这样可以得到大量用于聚合物工业的直链烯烃。
Catalytic cracking uses a zeolite catalyst to lower the activation energy, allowing the reaction to proceed at lower temperatures and pressures. The catalyst also promotes the formation of branched and cyclic hydrocarbons, which are better for petrol because they have a higher octane rating and cause less knocking in engines.
催化裂化使用沸石催化剂降低活化能，使反应能在较低的温度和压力下进行。催化剂还能促进支链和环状烃的生成，这些产物更适合用于汽油，因为它们辛烷值更高，发动机爆震更少。
9. Reforming and Isomerisation | 重整与异构化
To further improve the quality of petrol, straight‑chain alkanes from the naphtha fraction are subjected to reforming and isomerisation.
为了进一步提高汽油质量，来自石脑油馏分的直链烷烃需要经过重整和异构化。
Isomerisation converts a straight‑chain alkane into its branched isomer using a platinum catalyst at moderate temperature. For example, pentane can be isomerised to 2‑methylbutane. The branched isomer burns more smoothly in an engine, giving a higher octane number.
异构化 是在中等温度和铂催化剂作用下，将直链烷烃转变为支链异构体。例如，戊烷可以异构成 2‑甲基丁烷。支链异构体在发动机中燃烧更平稳，具有更高的辛烷值。
Reforming transforms straight‑chain alkanes into cycloalkanes and arenes (aromatic hydrocarbons) in the presence of a platinum or platinum‑rhenium catalyst at about 770 K. For instance, hexane can be reformed to cyclohexane, which then undergoes dehydrogenation to form benzene. These aromatic compounds also greatly increase the octane rating of petrol.
重整 是在铂或铂‑铼催化剂、约 770 K 的温度下，将直链烷烃转变为环烷烃和芳烃。例如，己烷可重整为环己烷，后者再脱氢生成苯。这些芳香族化合物同样能大幅提升汽油的辛烷值。
Reforming also produces hydrogen gas, which is an important by‑product used in the Haber process for ammonia manufacture. You should be able to link reforming to the need for high‑octane fuels and to the production of aromatic feedstocks for the chemical industry.
重整还会产生氢气，这是工业上哈伯法合成氨的重要副产物。你需要能够将重整与高辛烷值燃料需求以及为化工行业提供芳烃原料联系起来。
10. Environmental Impact of Alkanes as Fuels | 烷烃作为燃料的环境影响
Although alkanes are convenient fuels, their combustion releases several pollutants that cause significant environmental problems.
尽管烷烃是方便的燃料，其燃烧会释放多种污染物，引发严重的环境问题。
Carbon dioxide (CO2): Formed in complete combustion, it is a greenhouse gas that traps infrared radiation, contributing to global warming and climate change.
二氧化碳 (CO2)： 完全燃烧的产物，是一种温室气体，能吸收红外辐射，导致全球变暖和气候变化。
Carbon monoxide (CO): Produced during incomplete combustion; it is toxic and reduces the oxygen‑carrying capacity of the blood.
一氧化碳 (CO)： 不完全燃烧的产物；有毒，降低血液携氧能力。
Soot (unburnt carbon particles): Released during incomplete combustion; these particulates cause respiratory illnesses and can trigger asthma attacks.
炭黑（未燃烧的碳颗粒）： 不完全燃烧时释放；这些颗粒物可引发呼吸系统疾病和哮喘发作。
Nitrogen oxides (NOx): Formed in vehicle engines at high temperatures when nitrogen and oxygen from the air react. Nitrogen oxides contribute to acid rain (forming nitric acid) and photochemical smog, and they irritate the lungs.
氮氧化物 (NOx)： 在车辆发动机的高温下，空气中的氮气和氧气反应生成。氮氧化物会导致酸雨（形成硝酸）和光化学烟雾，并刺激肺部。
Sulfur dioxide (SO2): Many crude oil fractions contain sulfur impurities. When the fuel burns, SO2 is released, which dissolves in atmospheric water to form sulfurous acid and sulfuric acid, leading to acid rain that damages ecosystems and buildings. Sulfur can be removed from fuels before combustion by hydrodesulfurisation.
二氧化硫 (SO2)： 许多原油馏分含有硫杂质。燃料燃烧时会释放 SO2，溶解在大气中的水里形成亚硫酸和硫酸，导致酸雨，破坏生态系统和建筑物。硫可以通过加氢脱硫
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