📚 Alkanes Key Points for CCEA A-Level Chemistry | A-Level CCEA 化学：烷烃 考点精讲
Alkanes are the simplest family of hydrocarbons, containing only carbon and hydrogen atoms linked by single covalent bonds. In CCEA A-Level Chemistry, a thorough understanding of alkanes is essential, covering their structure, nomenclature, physical properties, and especially their chemical reactions – most notably combustion and free-radical substitution. This article consolidates the key syllabus points to help you revise effectively, pairing every English explanation with a Chinese translation.
烷烃是最简单的烃类家族，只含有碳和氢原子，并以单共价键连接。在 CCEA A-Level 化学中，全面理解烷烃至关重要，涵盖其结构、命名、物理性质，尤其是它们的化学反应——最显著的是燃烧和自由基取代反应。本文整合了核心考点，帮助你高效复习，每条英文讲解均配有中文翻译。
1. Introduction to Alkanes | 烷烃简介
Alkanes are saturated hydrocarbons with the general molecular formula CₙH₂ₙ₊₂ for open-chain, non-cyclic structures. Each carbon atom in an alkane forms four sigma (σ) bonds, resulting in a tetrahedral geometry with bond angles of approximately 109.5°. Because the molecules are non-polar and only experience weak van der Waals’ forces, alkanes are relatively unreactive compared to other organic compounds, but they do undergo combustion and substitution reactions under suitable conditions.
烷烃是饱和烃，对于开链非环结构，其通式为 CₙH₂ₙ₊₂。烷烃中的每个碳原子形成四个 σ 键，呈四面体几何结构，键角约 109.5°。由于分子为非极性且只存在微弱的范德华力，烷烃相对其他有机化合物而言较为不活泼，但在适当条件下仍能发生燃烧和取代反应。
Cycloalkanes, which are also saturated, have the general formula CₙH₂ₙ. They feature a ring structure and possess slightly different chemical properties due to angle strain in small rings. Both acyclic and cyclic alkanes are important feedstocks in the chemical industry.
环烷烃也是饱和烃，通式为 CₙH₂ₙ。它们具有环状结构，由于小环中的角张力，化学性质略有不同。开链烷烃和环烷烃都是化学工业中的重要原料。
2. Nomenclature of Alkanes | 烷烃的命名
IUPAC nomenclature rules for alkanes require identifying the longest continuous carbon chain to determine the parent name (meth-, eth-, prop-, but-, pent-, hex-, etc.). Number the chain from the end nearest a substituent to give the lowest possible numbers to alkyl groups (e.g., methyl, ethyl). List substituents alphabetically with appropriate di-, tri- prefixes. For example, 2,3-dimethylpentane indicates a pentane backbone with methyl groups on carbons 2 and 3.
IUPAC 命名法规则要求找到最长的连续碳链作为母体名称（甲、乙、丙、丁、戊、己等）。从最靠近取代基的一端开始给主链编号，使烷基（如甲基、乙基）获最小位号。按字母顺序列出取代基，并使用二、三等前缀。例如，2,3-二甲基戊烷表示戊烷主链，在碳 2 和 3 上有甲基。
When multiple different alkyl groups are present, they are named in alphabetical order (e.g., 4-ethyl-2-methylheptane). Halogenoalkanes are named similarly, with halogen substituents indicated by fluoro-, chloro-, bromo-, iodo-.
当存在多个不同烷基时，按字母顺序命名（如 4-乙基-2-甲基庚烷）。卤代烷的命名类似，卤素取代基用氟、氯、溴、碘表示。
3. Structural Isomerism | 结构异构
Alkanes with four or more carbon atoms exhibit structural (chain) isomerism. Isomers have the same molecular formula but different arrangements of atoms in the carbon skeleton. For example, C₅H₁₂ has three isomers: pentane, 2-methylbutane, and 2,2-dimethylpropane. As the number of carbon atoms increases, the number of possible structural isomers rises sharply.
含四个或更多碳原子的烷烃存在结构（碳链）异构。异构体具有相同的分子式，但碳骨架中原子排列不同。例如，C₅H₁₂ 有三种异构体：戊烷、2-甲基丁烷和 2,2-二甲基丙烷。随着碳原子数增加，可能的结构异构体数目急剧上升。
Branching lowers the boiling point because the molecule becomes more compact, reducing the surface area available for van der Waals’ forces. Thus, 2,2-dimethylpropane (bp 9.5 °C) has a lower boiling point than pentane (bp 36 °C).
支链化会降低沸点，因为分子变得更紧凑，减少了可用于产生范德华力的表面积。因此，2,2-二甲基丙烷（沸点 9.5 °C）的沸点低于戊烷（沸点 36 °C）。
4. Physical Properties | 物理性质
Alkanes are colourless, odourless (in pure form) and generally non-polar. Their boiling points increase with increasing relative molecular mass due to stronger van der Waals’ forces. Among isomers, the more branched the alkane, the lower the boiling point. Melting points also generally increase with molecular mass, although symmetry can sometimes cause anomalies (e.g., pentane vs 2,2-dimethylpropane).
烷烃为无色、无味（纯净状态下）且通常为非极性。随着相对分子质量增大，范德华力增强，沸点升高。在异构体中，支链越多沸点越低。熔点通常也随分子质量增大而升高，但对称性有时会导致反常（如戊烷与 2,2-二甲基丙烷）。
Alkanes are insoluble in water but dissolve in non-polar organic solvents such as hexane. They are less dense than water, so they form an upper layer when mixed with water. These properties influence how alkanes are separated during fractional distillation of crude oil.
烷烃不溶于水，但溶于非极性有机溶剂如己烷。它们密度小于水，因此与水混合时浮在上层。这些性质影响着原油分馏时烷烃的分离方式。
5. Chemical Properties: Combustion | 化学性质：燃烧
Complete combustion of alkanes in an excess of oxygen produces carbon dioxide and water, releasing a large amount of energy as heat. The general equation is: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O. For example, propane: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O. This exothermic reaction makes alkanes excellent fuels.
烷烃在过量氧气中完全燃烧生成二氧化碳和水，并释放大量热能。通式为：CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O。例如丙烷：C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O。这种放热反应使烷烃成为优良的燃料。
Incomplete combustion happens when the oxygen supply is limited, yielding carbon monoxide (CO) or elemental carbon (soot, C). Carbon monoxide is a toxic gas that binds irreversibly to haemoglobin, preventing oxygen transport. Soot can cause respiratory problems and blacken buildings.
当氧气供应不足时发生不完全燃烧，产生一氧化碳 (CO) 或单质碳（炭黑 C）。一氧化碳是有毒气体，能与血红蛋白不可逆结合，阻碍氧气运输。炭黑会导致呼吸问题并熏黑建筑物。
Environmental concerns related to alkane combustion include the greenhouse effect from CO₂, acid rain from any sulfur impurities producing SO₂, and NOₓ formation at high temperatures. Catalytic converters in vehicles reduce CO and NOₓ emissions.
与烷烃燃烧相关的环境问题包括 CO₂ 导致的温室效应、硫杂质生成的 SO₂ 引起的酸雨，以及高温下生成的氮氧化物 (NOₓ)。汽车中的催化转换器可减少 CO 和 NOₓ 排放。
6. Chemical Properties: Halogenation | 化学性质：卤代反应
Alkanes react with halogens (Cl₂, Br₂) in the presence of ultraviolet (UV) light or heat to form halogenoalkanes via a free-radical substitution mechanism. The reaction with chlorine is vigorous, while bromine requires more energy. Iodine does not react appreciably, and fluorine reacts explosively. The typical reaction is:
CH₄ + Cl₂ → CH₃Cl + HCl
烷烃与卤素（Cl₂、Br₂）在紫外光或加热条件下发生自由基取代反应，生成卤代烷。与氯的反应较为剧烈，溴需要更高能量。碘基本不反应，氟则发生爆炸。典型反应为：
CH₄ + Cl₂ → CH₃Cl + HCl
Further substitution can occur, producing a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane unless an excess of methane is used. The reaction is not stereospecific, and mixtures are common in synthetic pathways.
若甲烷过量不足，可能会发生进一步取代，生成氯甲烷、二氯甲烷、三氯甲烷和四氯甲烷的混合物。该反应不具备立体专一性，混合物在合成路径中十分常见。
7. Free Radical Substitution Mechanism | 自由基取代机理
The mechanism proceeds in three stages: initiation, propagation, and termination.
该机理分三个阶段：引发、增长和终止。
Initiation: The halogen molecule undergoes homolytic fission under UV light to generate two halogen free radicals. For chlorine:
Cl₂ → 2 Cl•
These radicals are highly reactive due to the unpaired electron.
引发：卤素分子在紫外光下发生均裂，生成两个卤素自由基。以氯为例：
Cl₂ → 2 Cl•
这些自由基因带有未成对电子而高度活泼。
Propagation: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical (CH₃•). The methyl radical then reacts with a chlorine molecule to produce chloromethane and a new chlorine radical. This step repeats in a chain reaction.
CH₄ + Cl• → CH₃• + HCl
CH₃• + Cl₂ → CH₃Cl + Cl•
增长：一个氯自由基从甲烷中夺取一个氢原子，生成 HCl 和一个甲基自由基 (CH₃•)。甲基自由基再与氯分子反应，生成氯甲烷和一个新的氯自由基。这一步骤以链式反应反复进行。
Termination: Two free radicals combine to form a stable molecule, ending the chain. Possible termination steps include:
Cl• + Cl• → Cl₂
CH₃• + CH₃• → C₂H₆
CH₃• + Cl• → CH₃Cl
终止：两个自由基结合生成稳定分子，使链式反应终止。可能的终止步骤如上所示。
8. Relative Rates of Halogenation | 卤代反应速率比较
The reactivity of halogens with alkanes follows the order: F₂ > Cl₂ > Br₂ > I₂. Fluorine is so reactive that the reaction is explosive and difficult to control. Chlorine reacts readily under UV light, bromine more slowly and often requires heating. Iodine is essentially unreactive because the H–I bond formed is relatively weak and the I• radical is insufficiently reactive to abstract a hydrogen atom.
卤素与烷烃的反应活性顺序为：F₂ > Cl₂ > Br₂ > I₂。氟的反应活性极高，反应往往爆炸且难以控制。氯在紫外光下容易反应，溴则较慢且常需加热。碘基本不反应，因为生成的 H–I 键相对较弱，且 I• 自由基的活性不足以夺取氢原子。
The strength of the C–H bond influences which hydrogen is substituted. Tertiary hydrogens are replaced most easily, then secondary, then primary, due to the relative stability of the resulting alkyl radical (tertiary > secondary > primary). This selectivity is especially pronounced with bromine, which is more selective than chlorine.
C–H 键的强度影响哪个氢被取代。由于生成的烷基自由基稳定性顺序为三级 > 二级 > 一级，三级氢最容易被取代，其次为二级，一级最难。这种选择性在溴代反应中尤其明显，溴比氯更具选择性。
9. Cracking of Alkanes | 烷烃的裂化
Cracking is the thermal or catalytic decomposition of long-chain alkanes into shorter, more useful hydrocarbons. Thermal cracking uses high temperature (700–1200 K) and high pressure, producing a high proportion of alkenes. Catalytic cracking uses a zeolite catalyst at lower temperature (about 720 K) and slight pressure, producing branched alkanes and aromatic hydrocarbons suitable for petrol.
裂化是将长链烷烃热分解或催化分解为较短、更有用的烃类的过程。热裂化使用高温（700–1200 K）和高压，生成高比例的烯烃。催化裂化使用沸石催化剂，在较低温度（约 720 K）和轻微压力下，生成适合汽油的支链烷烃和芳烃。
Both processes are essential to meet the demand for lighter fractions such as petrol and ethene (for polymers). Cracking also produces hydrogen gas, a valuable industrial feedstock. The C–C bonds are broken heterolytically or homolytically under these conditions.
这两种过程对于满足汽油、乙烯（用于聚合物）等轻质馏分的需求至关重要。裂化还会生成氢气，一种宝贵的工业原料。在此条件下 C–C 键发生异裂或均裂。
10. Environmental Issues | 环境问题
Burning alkanes contributes to greenhouse gas emissions (CO₂) and, if impurities are present, SO₂ which leads to acid rain. Incomplete combustion releases toxic CO and particulates (soot). Nitrogen oxides (NOₓ) are formed at high combustion temperatures by reaction of N₂ and O₂ from the air, contributing to photochemical smog and acid rain.
燃烧烷烃会增加温室气体 (CO₂) 排放，若存在杂质还会产生导致酸雨的 SO₂。不完全燃烧会释放有毒的 CO 和颗粒物（炭黑）。在高温燃烧条件下，空气中的 N₂ 和 O₂ 反应生成氮氧化物 (NOₓ)，导致光化学烟雾和酸雨。
Catalytic converters, fitted in vehicle exhaust systems, use platinum, palladium and rhodium catalysts to convert CO to CO₂, NOₓ to N₂, and unburnt hydrocarbons to CO₂ and H₂O. Flue gas desulfurisation in power stations removes SO₂ by reacting it with CaO or CaCO₃.
安装在车辆排气系统中的催化转换器，利用铂、钯和铑催化剂将 CO 转化为 CO₂，NOₓ 转化为 N₂，以及未燃烧烃类转化为 CO₂ 和 H₂O。发电站的烟气脱硫通过使 SO₂ 与 CaO 或 CaCO₃ 反应来去除 SO₂。
11. Cycloalkanes | 环烷烃
Cycloalkanes are saturated cyclic hydrocarbons with the general formula CₙH₂ₙ. They are named by adding the prefix ‘cyclo-’ to the corresponding alkane name, e.g., cyclopropane (C₃H₆), cyclobutane (C₄H₈). Small rings (cyclopropane and cyclobutane) have significant angle strain, making them more reactive than their open-chain counterparts.
环烷烃是通式为 CₙH₂ₙ 的饱和环状烃。命名时在相应的烷烃名称前加上前缀“环”，例如环丙烷 (C₃H₆)、环丁烷 (C₄H₈)。小环（环丙烷和环丁烷）具有显著的角张力，使其比开链类似物更活泼。
Cycloalkanes, like alkanes, undergo free-radical substitution with halogens and combustion. Small rings can also undergo ring-opening reactions under certain conditions, e.g., cyclopropane can react with H₂ (hydrogenation) in the presence of a metal catalyst to form propane, releasing ring strain.
环烷烃与烷烃一样，能与卤素发生自由基取代反应和燃烧。小环在某些条件下也能发生开环反应，例如环丙烷在金属催化剂存在下与 H₂ 发生加氢反应生成丙烷，释放环张力。
Conformations of cyclohexane (chair and boat) are important in advanced topics; the chair conformation is most stable due to minimised torsional and steric strain. This influences the reactivity of substituted cyclohexanes.
环己烷的构象（椅式和船式）在进阶内容中很重要；椅式构象最稳定，因为扭转张力和空间张力最小。这会影响取代环己烷的反应活性。
12. Summary of Key Reactions | 关键反应总结
For revision, commit these core reactions to memory:
复习时请牢记以下核心反应：
Complete combustion: CₙH₂ₙ₊₂ + excess O₂ → n CO₂ + (n+1) H₂O
完全燃烧： CₙH₂ₙ₊₂ + 过量 O₂ → n CO₂ + (n+1) H₂O
Free-radical substitution (monochlorination): RH + Cl₂ –(UV light)–> RCl + HCl
自由基取代（一氯代）： RH + Cl₂ –（紫外光）–> RCl + HCl
Thermal cracking: long alkane → shorter alkane + alkene(s) ( + H₂ sometimes)
热裂化： 长链烷烃 → 短链烷烃 + 烯烃（有时 + H₂）
Catalytic cracking: similar but using a zeolite catalyst, produces more branched hydrocarbons and aromatics.
催化裂化： 类似，但使用沸石催化剂，生成更多支链烃和芳烃。
Remember the free-radical mechanism steps, the selectivity of halogenation, and the environmental impact of alkane use. With these fundamentals, you will be well-prepared for CCEA exam questions on alkanes.
记住自由基取代机理的步骤、卤代反应的选择性，以及烷烃使用的环境影响。掌握了这些基础，你将能充分应对 CCEA 考试中关于烷烃的题目。
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