A-Level Chemistry: Structure and Properties of the Alkane Homologous Series | A-Level 化学：烷烃同系物的结构与性质

📚 A-Level Chemistry: Structure and Properties of the Alkane Homologous Series | A-Level 化学：烷烃同系物的结构与性质

Alkanes are the simplest family of organic compounds and form the foundation of the homologous series in organic chemistry. Understanding their structure and properties is essential for tackling CIE A-Level Chemistry questions on bonding, isomerism, reaction mechanisms, and industrial applications.

烷烃是最简单的有机化合物家族，也是有机化学中同系物的基础。理解它们的结构与性质，对于解答 CIE A-Level 化学中有关成键、异构、反应机理和工业应用的题目至关重要。

1. The Homologous Series and General Formula | 同系物与通式

An homologous series is a family of compounds with the same general formula, similar chemical properties, and successive members differing by a CH₂ unit. Alkanes fit this definition perfectly, with the general formula CₙH₂ₙ₊₂ for open-chain (acyclic) alkanes.

同系物是一族具有相同通式、化学性质相似、相邻成员相差一个 CH₂ 单元的化合物。烷烃完全符合这一定义，其开链（无环）烷烃的通式为 CₙH₂ₙ₊₂。

For example: CH₄ (methane, n = 1), C₂H₆ (ethane, n = 2), C₃H₈ (propane, n = 3), and C₄H₁₀ (butane, n = 4). Each member can be derived from the previous one by replacing a hydrogen atom with a methyl group, adding one carbon and two hydrogen atoms.

例如：CH₄（甲烷，n = 1）、C₂H₆（乙烷，n = 2）、C₃H₈（丙烷，n = 3）、C₄H₁₀（丁烷，n = 4）。每个成员都可看作将前一个成员中的某个氢原子替换为甲基，从而增加一个碳原子和两个氢原子。

General formula: CₙH₂ₙ₊₂ (n ≥ 1)

通式：CₙH₂ₙ₊₂（n ≥ 1）

A common exam trap is confusing alkanes with cycloalkanes. Cycloalkanes have the general formula CₙH₂ₙ and are not part of the alkane homologous series in the strict CIE sense, although they share some similar reactions.

一个常见的考试陷阱是将烷烃与环烷烃混淆。环烷烃的通式为 CₙH₂ₙ，从 CIE 严格定义的角度讲，它们不属于烷烃同系物，尽管某些反应有相似之处。

2. Structure and Bonding in Alkanes | 烷烃的结构与成键

Each carbon atom in an alkane is sp³ hybridised, forming four sigma (σ) bonds directed towards the corners of a regular tetrahedron. The bond angle is approximately 109.5°, which minimises electron-pair repulsion according to VSEPR theory.

烷烃中的每个碳原子都采取 sp³ 杂化，形成四个指向正四面体顶点的 σ（sigma）键。键角约为 109.5°，根据 VSEPR 理论，这一角度使电子对之间的排斥力最小化。

The C–C and C–H bonds are both sigma bonds, formed by the head-on overlap of atomic orbitals. The electron density in a σ bond lies directly between the two nuclei, making the bond relatively strong and freely rotating around the C–C axis.

C–C 键和 C–H 键都是 σ 键，由原子轨道的头碰头重叠形成。σ 键的电子密度位于两个原子核之间，因此键强度较高，且 C–C 单键可绕键轴自由旋转。

Because the electronegativity difference between carbon (2.55) and hydrogen (2.20) is small, the C–H bond is essentially non-polar. As a result, alkane molecules have no permanent dipole and are non-polar overall.

由于碳（2.55）和氢（2.20）的电负性差异很小，C–H 键基本上是非极性的。因此，烷烃分子没有永久偶极，整体呈现非极性。

Isomerism appears from C₄H₁₀ onwards. Butane has two isomers: n-butane (CH₃CH₂CH₂CH₃) and isobutane ((CH₃)₂CHCH₃). The number of possible constitutional isomers increases rapidly with carbon number.

从 C₄H₁₀ 开始出现异构现象。丁烷有两种异构体：正丁烷（CH₃CH₂CH₂CH₃）和异丁烷（(CH₃)₂CHCH₃）。可能的构造异构体数目随着碳原子数的增加而迅速增多。

3. Nomenclature of Alkanes | 烷烃的命名

According to IUPAC rules, the name of an alkane is derived from the longest continuous carbon chain, with the suffix “-ane”. Substituents (alkyl groups) are named by replacing “-ane” with “-yl”, such as methyl (-CH₃) and ethyl (-C₂H₅).

根据 IUPAC 命名规则，烷烃的名称来源于最长的连续碳链，后缀为”-ane”（烷）。取代基（烷基）的命名将”-ane”改为”-yl”（基），例如甲基（-CH₃）和乙基（-C₂H₅）。

Select the longest chain containing the maximum number of carbon atoms as the parent chain.

选择含碳原子数最多的最长碳链作为主链。
Number the chain from the end giving the lowest locant to the first substituent.

从离第一个取代基最近的一端开始编号，使取代基的位次最小。
List substituents alphabetically (not by size), using di-, tri-, etc. for identical groups, and separate numbers by commas and numbers from names by hyphens.

取代基按字母顺序排列（而非按大小），相同取代基使用”二、三”等前缀，位次之间用逗号分隔，位次与名称之间用连字符连接。

For example, CH₃CH(CH₃)CH₂CH₃ is named 2-methylbutane, not 3-methylbutane. A common mark scheme requirement is to write the locant and substituent in the correct order.

例如，CH₃CH(CH₃)CH₂CH₃ 命名为 2-甲基丁烷，而不是 3-甲基丁烷。评分标准常要求正确书写位次和取代基的顺序。

4. Physical Properties: Trends Down the Series | 物理性质：沿同系物的递变规律

The physical properties of alkanes change gradually with increasing molecular mass, reflecting the increasing strength of intermolecular forces (London dispersion forces). These forces arise from temporary induced dipoles due to electron movement.

烷烃的物理性质随相对分子质量的增加而逐渐变化，这反映了分子间作用力（伦敦色散力）的增强。色散力源于电子运动产生的瞬时诱导偶极。

Boiling point: As the number of carbon atoms increases, the molecular surface area grows, leading to stronger dispersion forces and a higher boiling point. For isomeric alkanes, the straight-chain isomer has a higher boiling point than its branched counterpart because branching reduces the surface area for contact.

沸点：随着碳原子数增加，分子表面积增大，色散力增强，沸点升高。对于同分异构的烷烃，直链异构体的沸点高于支链异构体，因为支链化减少了分子间的接触面积。

Melting point: Similar to boiling point, but a zig-zag “sawtooth” pattern is observed: alkanes with an even number of carbon atoms pack more efficiently in the solid state, giving higher melting points.

熔点：与沸点相似，但呈现”锯齿形”规律：含偶数个碳原子的烷烃在固态中堆积更紧密，因此熔点相对较高。

Alkane	Formula	Boiling point / °C
Methane	CH₄	-164
Ethane	C₂H₆	-89
Propane	C₃H₈	-42
Butane	C₄H₁₀	-0.5
Pentane	C₅H₁₂	36

Alkanes are insoluble in water because water is polar and highly hydrogen-bonded, whereas alkanes are non-polar and cannot form hydrogen bonds with water. They dissolve readily in non-polar organic solvents and are less dense than water.

烷烃不溶于水，因为水是极性强且具有氢键的溶剂，而烷烃是非极性分子，不能与水形成氢键。烷烃易溶于非极性有机溶剂，且密度小于水。

5. Bond Energies and Chemical Stability | 键能与化学稳定性

The C–C bond energy is about 348 kJ mol⁻¹ and the C–H bond energy is about 412 kJ mol⁻¹. These are relatively strong covalent bonds, which explains the general unreactivity of alkanes at room temperature towards acids, bases, and oxidising agents.

C–C 键能约为 348 kJ mol⁻¹，C–H 键能约为 412 kJ mol⁻¹。它们都是较强的共价键，这解释了烷烃在室温下对酸、碱和氧化剂通常不活泼的原因。

Alkanes are often described as “saturated” hydrocarbons because they contain only single C–C and C–H bonds. This saturation, combined with the non-polar nature of the bonds, means alkanes do not undergo electrophilic addition reactions like alkenes do.

烷烃通常被称为”饱和”烃，因为它们只含有 C–C 和 C–H 单键。这种饱和性加上键的非极性，意味着烷烃不像烯烃那样发生亲电加成反应。

However, alkanes are not completely inert. Under suitable conditions — high temperature, ultraviolet light, or in the presence of oxygen — they react readily, especially through combustion and free-radical substitution.

然而，烷烃并非完全惰性。在适当条件下——高温、紫外线或存在氧气时——它们会迅速反应，尤其是通过燃烧和自由基取代反应。

6. Combustion of Alkanes | 烷烃的燃烧

Complete combustion of alkanes in excess oxygen produces carbon dioxide and water, releasing a large amount of energy. This is the basis for their use as fuels.

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。这是它们用作燃料的基础。

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion occurs when the oxygen supply is limited, producing carbon monoxide (CO) and/or carbon (soot) instead of CO₂. Carbon monoxide is toxic because it binds to haemoglobin more strongly than oxygen.

当氧气供应不足时会发生不完全燃烧，生成一氧化碳（CO）和/或碳（烟灰），而不是二氧化碳。一氧化碳有毒，因为它与血红蛋白结合的能力比氧气更强。

2CH₄ + 3O₂ → 2CO + 4H₂O

The general equation for complete combustion of an alkane is CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O. Exam questions often ask you to balance such equations working from the general formula.

烷烃完全燃烧的通式为 CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O。考试中常要求你根据通式配平此类方程式。

7. Free Radical Substitution | 自由基取代反应

Under ultraviolet light, alkanes react with halogens — especially chlorine and bromine — in a free-radical substitution reaction. This is a key mechanism tested in CIE A-Level Chemistry.

在紫外光照射下，烷烃与卤素——尤其是氯气和溴——发生自由基取代反应。这是 CIE A-Level 化学的重要考点机理。

The reaction between methane and chlorine proceeds in three stages:

甲烷与氯气的反应分三个阶段进行：

Initiation: Cl₂ → 2Cl• (homolytic fission requires UV light).

链引发：Cl₂ → 2Cl•（均裂需要紫外光）。
Propagation: Cl• + CH₄ → •CH₃ + HCl; •CH₃ + Cl₂ → CH₃Cl + Cl•. The radicals are regenerated, sustaining the chain reaction.

链增长：Cl• + CH₄ → •CH₃ + HCl；•CH₃ + Cl₂ → CH₃Cl + Cl•。自由基再生，维持链式反应。
Termination: Two radicals combine, e.g. Cl• + •CH₃ → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆. No new radical is formed.

链终止：两个自由基结合，例如 Cl• + •CH₃ → CH₃Cl，或 •CH₃ + •CH₃ → C₂H₆。不再产生新的自由基。

The product is not a single chloromethane; further substitution leads to a mixture: CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄. In the exam, you must be able to identify the initiation step (homolytic fission) and explain why a mixture forms.

产物不是单一的氯甲烷；进一步取代会产生混合物：CH₃Cl、CH₂Cl₂、CHCl₃ 和 CCl₄。在考试中，你必须能够识别链引发步骤（均裂）并解释为什么生成混合物。

Relative reactivity of halogens decreases in the order F₂ > Cl₂ > Br₂ > I₂. Fluorine is dangerously explosive; iodine does not react under normal conditions. When alkanes have more than one type of hydrogen, substitution occurs preferentially at tertiary (3°) C–H bonds, then secondary (2°), then primary (1°), due to radical stability.

卤素反应活性顺序为 F₂ > Cl₂ > Br₂ > I₂。氟反应过于剧烈，有爆炸危险；碘在通常条件下不反应。当烷烃含有多种类型的氢时，取代优先发生在叔碳（3°）的 C–H 键上，其次为仲碳（2°）、伯碳（1°），这是因为自由基稳定性的差异。

8. Cracking and Industrial Importance | 裂解与工业应用

Alkanes are obtained primarily from crude oil and natural gas. Large alkane molecules are often converted into more valuable smaller molecules through cracking, which involves breaking C–C bonds at high temperatures and/or with a catalyst.

烷烃主要来源于原油和天然气。较大的烷烃分子通常通过裂解转化为更有价值的小分子，裂解是在高温和/或催化剂作用下断裂 C–C 键的过程。

Thermal cracking uses high temperature (about 700–1200 K) and high pressure to produce mostly alkenes, especially ethene. Catalytic cracking uses a zeolite catalyst at lower temperatures (about 700 K) to produce branched-chain alkanes and aromatic hydrocarbons for high-octane petrol.

热裂解使用高温（约 700–1200 K）和高压，主要生产烯烃尤其是乙烯。催化裂解使用沸石催化剂，在较低温度（约 700 K）下生产支链烷烃和芳香烃，用于高辛烷值汽油。

C₈H₁₈ → C₄H₁₀ + C₄H₈ (thermal cracking example)

C₈H₁₈ → C₄H₁₀ + C₄H₈（热裂解示例）

Cracking also produces hydrogen gas, which is used in the Haber process and hydrogenation of unsaturated oils. Understanding the purpose of cracking — matching supply of fractions to demand — is a common AO1/AO2 question.

裂解也产生氢气，用于哈伯法合成氨和不饱和油脂的氢化。理解裂解的目的——使石油馏分的供需匹配——是常见的 AO1/AO2 题目。

9. Preparation of Alkanes | 烷烃的制备

In addition to natural sources, alkanes can be prepared in the laboratory and in industry by hydrogenation of alkenes. Ethene reacts with hydrogen over a nickel catalyst at about 150 °C to form ethane.

除了天然来源，实验室和工业上还可以通过烯烃的加氢反应制备烷烃。乙烯在镍催化剂作用下，于约 150 °C 与氢气反应生成乙烷。

CH₂=CH₂ + H₂ → CH₃CH₃ (Ni catalyst, heat)

CH₂=CH₂ + H₂ → CH₃CH₃（镍催化剂，加热）

Reduction of haloalkanes with zinc and hydrochloric acid also yields alkanes: R–X + 2[H] → R–H + HX. This method is sometimes shown in synthetic routes, though it is less important at A-Level than catalytic hydrogenation.

卤代烃用锌和盐酸还原也可以得到烷烃：R–X + 2[H] → R–H + HX。这个方法有时出现在合成路线中，但 A-Level 阶段不如催化加氢重要。

In a synthetic pathway question, you should recognise that converting an alkene to an alkane requires H₂/Ni, converting a haloalkane to an alkane may require reduction, and converting an alcohol to an alkane is not straightforward at A-Level.

在合成路线题中，你应该能识别：烯烃转化为烷烃需要 H₂/Ni，卤代烃转化为烷烃需要还原，而醇直接转化为烷烃在 A-Level 中并不常见。

10. Environmental and Safety Considerations | 环境与安全考量

Methane, the simplest alkane, is a potent greenhouse gas. Its release from agriculture, landfill sites, and fossil-fuel extraction contributes to global warming. Carbon dioxide from alkane combustion is also a major greenhouse gas.

最简单的烷烃甲烷是一种强效温室气体。它从农业、垃圾填埋场和化石燃料开采中的释放会加剧全球变暖。烷烃燃烧产生的二氧化碳也是主要的温室气体。

Incomplete combustion produces toxic carbon monoxide, and nitrogen oxides are formed in vehicle engines at high temperatures. Alkanes are also volatile organic compounds (VOCs) that can contribute to photochemical smog in urban areas.

不完全燃烧会产生有毒的一氧化碳，汽车发动机中的高温还会生成氮氧化物。烷烃属于挥发性有机化合物（VOCs），在城市地区可能引发光化学烟雾。

The octane number of a fuel relates to its resistance to knocking in a petrol engine. Branched-chain alkanes and aromatic compounds have higher octane numbers than straight-chain alkanes, which is why catalytic cracking is valuable for petrol production.

燃料的辛烷值与其在汽油发动机中抗爆震的能力有关。支链烷烃和芳香烃的辛烷值高于直链烷烃，这就是催化裂解在汽油生产中很有价值的原因。

11. Common Exam Traps and Revision Tips | 常见考试陷阱与复习要点

Many students lose marks by making avoidable errors in alkane questions. Here are the most frequent pitfalls:

许多学生在烷烃题目中因可避免的错误而失分。以下是最常见的陷阱：

Using the wrong general formula: remember CₙH₂ₙ₊₂ for acyclic alkanes, not CₙH₂ₙ.

使用错误的通式：记住开链烷烃是 CₙH₂ₙ₊₂，不是 CₙH₂ₙ。
Forgetting that alkanes are non-polar and insoluble in water; do not attribute solubility to hydrogen bonding.

忘记烷烃是非极性且不溶于水；不要将其溶解性归因于氢键。
Writing the propagation steps with arrows that do not show electron movement or radicals correctly — radicals always have an unpaired electron (Cl•, •CH₃).

书写链增长步骤时箭头不能正确表示电子移动或自由基——自由基始终带有一个未成对电子（Cl•、•CH₃）。
Claiming that alkanes undergo addition reactions because they are hydrocarbons: saturation means no π bond, so addition is impossible.

声称烷烃因是烃类而能发生加成反应：饱和意味着没有 π 键，因此不可能发生加成反应。
Confusing the order of boiling points during isomerism: branching lowers the boiling point, not raises it.

比较异构体沸点时混淆顺序：支链化降低沸点，而不是升高。

When revising, draw the displayed formulae and consider the 3D shape at each carbon atom. Practise naming compounds from structural formulae and drawing isomers from molecular formulae.

复习时，画出结构式并思考每个碳原子周围的三维形状。练习根据结构式命名化合物，以及根据分子式画出所有异构体。

12. Summary: Structure Determines Properties | 总结：结构决定性质

The non-polar, saturated, tetrahedral structure of alkanes determines their physical inertness and their chemical behaviour. Weak intermolecular forces explain low boiling points for small alkanes, while strong C–H and C–C bonds explain their low reactivity.

烷烃非极性、饱和、四面体的结构决定了其物理惰性和化学行为。弱分子间作用力解释了小分子烷烃沸点低，而强 C–H 键和 C–C 键解释了它们反应活性低。

Under suitable activation — heat, light, or catalysts — alkanes undergo combustion, free-radical substitution, and cracking. These reactions underpin their role as fuels and as feedstocks for the petrochemical industry.

在适当的活化条件下——加热、光照或催化剂——烷烃可以发生燃烧、自由基取代和裂解反应。这些反应支撑了它们作为燃料和石油化工原料的重要作用。

For CIE A-Level exams, master the general formula, IUPAC naming, the three-step free-radical mechanism, and the trends in physical properties. These core ideas will allow you to solve both structured and multiple-choice questions with confidence.

对于 CIE A-Level 考试，掌握通式、IUPAC 命名、三步自由基机理和物理性质递变规律。这些核心概念将帮助你自信地解答结构题和选择题。

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A-Level Chemistry: Structure and Properties of the Alkane Homologous Series | A-Level 化学：烷烃同系物的结构与性质