📚 A-Level WJEC Chemistry: Alkanes | A-Level WJEC 化学：烷烃
Alkanes are the simplest family of hydrocarbons, containing only carbon and hydrogen atoms joined by single covalent bonds. In the WJEC A-Level Chemistry specification, alkanes form a foundational topic that links organic nomenclature, physical properties, reaction mechanisms, and industrial processes. Understanding alkanes thoroughly not only helps with exam questions on structure and reactivity but also prepares students for more complex organic chemistry topics such as alkenes, haloalkanes, and polymerisation.
烷烃是最简单的烃类家族，仅由碳和氢原子通过单共价键连接。在 WJEC A-Level 化学大纲中，烷烃是一个将有机命名、物理性质、反应机理和工业过程联系起来的基础课题。彻底理解烷烃不仅有助于解答关于结构和反应性的考题，也为学习烯烃、卤代烷和聚合等更复杂的有机化学主题做好准备。
1. Homologous Series and General Formula | 同系物与通式
Alkanes belong to the homologous series with the general formula CₙH₂ₙ₊₂, where n is the number of carbon atoms. Each member differs from the next by a –CH₂– unit. This series displays a gradual trend in physical properties, such as increasing boiling point with increasing chain length, due to stronger London dispersion forces.
烷烃属于通式为 CₙH₂ₙ₊₂ 的同系物，其中 n 为碳原子数。每相邻两个成员相差一个 –CH₂– 单元。由于伦敦色散力增强，随着碳链增长，该系列在物理性质上呈现逐渐变化的趋势，例如沸点升高。
The simplest alkane is methane (CH₄), followed by ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). All alkanes are saturated hydrocarbons, meaning they contain the maximum possible number of hydrogen atoms per carbon atom; there are no double or triple bonds.
最简单的烷烃是甲烷 (CH₄)，然后依次是乙烷 (C₂H₆)、丙烷 (C₃H₈) 和丁烷 (C₄H₁₀)。所有烷烃都是饱和烃，即每个碳原子上都连有最大可能数量的氢原子；分子中没有双键或三键。
2. Nomenclature and Structural Isomerism | 命名与结构异构
IUPAC rules for naming alkanes require identification of the longest continuous carbon chain as the parent name (e.g., meth-, eth-, prop-, but-, pent-, hex-). Substituents, such as alkyl groups, are named as prefixes with their position numbers, ensuring the lowest possible set of locants. For example, 2-methylpentane indicates a methyl group on carbon 2 of a five-carbon chain.
IUPAC 命名规则要求识别最长的连续碳链作为母体名称（如甲、乙、丙、丁、戊、己）。取代基如烷基作为前缀，并标明其位次编号，使定位数之和尽可能小。例如，2-甲基戊烷表示在五碳链的 2 号碳上有一个甲基。
Structural isomerism arises when molecules share the same molecular formula but differ in the arrangement of atoms. Alkanes with four or more carbon atoms exhibit chain isomerism. For instance, C₄H₁₀ can exist as straight-chain butane or as branched 2-methylpropane (isobutane). These isomers have different physical properties, such as boiling points and densities, which is a common exam focus.
结构异构产生于分子共享相同分子式但原子排列不同的情况。含有四个或更多碳原子的烷烃会出现碳链异构。例如，C₄H₁₀ 可以以直链丁烷或支链 2-甲基丙烷（异丁烷）的形式存在。这些异构体具有不同的物理性质，如沸点和密度，这是常见的考点。
3. Physical Properties of Alkanes | 烷烃的物理性质
At room temperature, the first four alkanes (methane to butane) are gases, pentane (C₅H₁₂) to heptadecane (C₁₇H₃₆) are liquids, and higher alkanes are waxy solids. Boiling points increase with molecular mass because larger molecules have more electrons, leading to stronger instantaneous dipole–induced dipole interactions (London forces).
室温下，前四种烷烃（甲烷至丁烷）为气体，戊烷 (C₅H₁₂) 至十七烷 (C₁₇H₃₆) 为液体，更高级的烷烃为蜡状固体。沸点随分子量增加而升高，因为较大的分子拥有更多电子，从而产生更强的瞬时偶极-诱导偶极相互作用（伦敦力）。
Branched isomers have lower boiling points than their straight-chain counterparts because branching reduces the surface area available for intermolecular contact, weakening the London forces. Alkanes are non-polar and therefore insoluble in water but dissolve in non-polar organic solvents such as cyclohexane.
支链异构体的沸点低于其直链异构体，因为支链减少了分子间可接触的表面积，削弱了伦敦力。烷烃是非极性的，因此不溶于水，但可溶于非极性有机溶剂如环己烷。
4. Sources and Fractional Distillation | 来源与分馏
Crude oil is the primary natural source of alkanes. It is a complex mixture of hydrocarbons that must be separated into useful fractions by fractional distillation. The process relies on the fact that different alkanes have different boiling points. The crude oil is heated to around 350°C in a furnace, vaporised, and passed into a fractionating column where the temperature decreases with height.
原油是烷烃的主要天然来源。它是一种复杂的烃类混合物，必须通过分馏将其分离成有用的馏分。该过程依赖于不同烷烃具有不同沸点这一事实。原油在炉中加热至约 350°C，汽化后送入分馏塔，塔内温度随高度升高而降低。
As the hot vapour rises, fractions condense at different levels: bitumen and fuel oil at the bottom, diesel, kerosene, naphtha, and petrol (gasoline) in the middle, and refinery gases at the top. Each fraction contains a range of alkanes with similar boiling points and has distinct uses, from LPG to lubricating oils.
随着热蒸气上升，各馏分在不同高度冷凝：沥青和燃料油在底部，柴油、煤油、石脑油和汽油在中部，炼厂气在顶部。每个馏分含有沸点相近的一系列烷烃，并具有不同的用途，从液化石油气到润滑油。
5. Chemical Reactivity and Bonding | 化学反应性与键合
Alkanes are relatively unreactive because they consist of strong C–C and C–H σ-bonds, which are non-polar and have high bond enthalpies. The electronegativity difference between carbon (2.5) and hydrogen (2.1) is very small, so the bonds are essentially non-polar. This makes alkanes resistant to attack by nucleophiles, electrophiles, and common oxidising or reducing agents. However, they do undergo combustion and, under specific conditions, free radical substitution.
烷烃相对不活泼，因为它们由强 C–C 和 C–H σ 键构成，这些键非极性且键焓高。碳 (2.5) 和氢 (2.1) 之间的电负性差极小，因此键基本上是非极性的。这使得烷烃不易受亲核试剂、亲电试剂及常见氧化剂或还原剂的进攻。然而，它们确实能发生燃烧反应，并在特定条件下发生自由基取代反应。
Exam tip: remember that the main reactions of alkanes required for WJEC are combustion (complete and incomplete) and radical substitution with halogens. Be prepared to explain why alkanes are generally inert using bond and polarity arguments.
应考提示：记住 WJEC 要求掌握的烷烃主要反应是燃烧（完全和不完全）以及与卤素的自由基取代。准备好用键和极性的论点解释烷烃为何通常呈惰性。
6. Combustion Reactions | 燃烧反应
ic, which is why alkanes are excellent fuels. For example, the complete combustion of propane is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. The amount of energy released per mole of CO₂ produced depends on the carbon chain length and structure.
完全燃烧发生在氧气充足的情况下，烷烃燃烧生成二氧化碳和水。该反应高度放热，这就是烷烃是优良燃料的原因。例如，丙烷的完全燃烧：C₃H₈ + 5O₂ → 3CO₂ + 4H₂O。每生成一摩尔 CO₂ 所释放的能量取决于碳链长度和结构。
Incomplete combustion takes place when the oxygen supply is limited, leading to the formation of carbon monoxide (CO) or even carbon particulates (soot). This is hazardous because carbon monoxide is a poisonous gas that binds irreversibly to haemoglobin. Soot particles can cause respiratory problems and contribute to global dimming.
当氧气供应有限时会发生不完全燃烧，导致生成一氧化碳 (CO) 甚至碳颗粒（烟灰）。这是危险的，因为一氧化碳是一种有毒气体，能与血红蛋白不可逆结合。烟灰颗粒可引起呼吸系统疾病并导致全球变暗。
+ 3H₂O, which is doubled to 2C₂H₆ + 5O₂ → 4CO + 6H₂O. Balancing these equations is a common calculation task.
书写不完全燃烧的配平方程式需要系统的方法；例如乙烷仅生成 CO 和 H₂O：C₂H₆ + 2½O₂ → 2CO + 3H₂O，将其加倍得到 2C₂H₆ + 5O₂ → 4CO + 6H₂O。配平这类方程式是常见的计算任务。
7. Free Radical Substitution Mechanism | 自由基取代机理
Alkanes react with halogens (Cl₂, Br₂) in the presence of UV light to form haloalkanes via a radical chain mechanism. This is the principal mechanism you must know for WJEC. The reaction with chlorine proceeds through three stages: initiation, propagation, and termination.
烷烃在紫外光存在下与卤素（Cl₂、Br₂）通过自由基链式机理反应生成卤代烷。这是你必须为 WJEC 掌握的主要机理。与氯的反应通过三个阶段进行：引发、传递和终止。
Initiation: UV light provides energy to homolytically cleave the Cl–Cl bond, producing two chlorine radicals: Cl₂ → 2Cl•. Each chlorine radical has an unpaired electron and is highly reactive.
引发：紫外光提供能量使 Cl–Cl 键均裂，生成两个氯自由基：Cl₂ → 2Cl•。每个氯自由基带有一个未成对电子，反应性极高。
Propagation: acts a hydrogen atom from an alkane such as methane, generating HCl and a methyl radical (•CH₃): Cl• + CH₄ → HCl + • n reacts with a chlorine molecule to form chloromethane and regenerate a chlorine radical: •CH₃ + Cl₂ → CH₃Cl + Cl•. This chlorine radical can continue the chain.
传递：氯自由基从烷烃如甲烷中夺取一个氢原子，生成 HCl 和甲基自由基 (•CH₃)：Cl• + CH₄ → HCl + •CH₃。然后甲基自由基与氯分子反应形成氯甲烷并再生一个氯自由基：•CH₃ + Cl₂ → CH₃Cl + Cl•。这个氯自由基可继续传递链反应。
Termination: table molecule, removing radicals from the mixture. Possible termination steps include: Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, •CH₃ + •CH₃ → C₂H₆. Termination stops the chain reaction.
终止：任意两个自由基可以结合形成稳定分子，从混合物中移除自由基。可能的终止步骤包括：Cl• + Cl• → Cl₂，•CH₃ + Cl• → CH₃Cl，•CH₃ + •CH₃ → C₂H₆。终止使链反应停止。
A mixture of products arises because further substitution can occur. For example, mono-, di-, tri-, and tetra-substituted methanes (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄) are all formed. The reaction is not a good method for preparing a single haloalkane in high yield.
由于可能发生进一步取代，会生成混合物产物。例如，会生成一、二、三和四取代甲烷 (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄)。该反应不是高收率制备单一卤代烷的好方法。
8. Cracking and Its Importance | 裂化及其重要性
Cracking is the process of breaking longer-chain alkanes from heavy fractions into shorter, more useful hydrocarbons. There are two types: thermal cracking and catalytic cracking. Both involve breaking C–C bonds and C–H bonds at high temperatures, but catalytic cracking uses a zeolite catalyst and lower temperatures (around 500°C), while thermal cracking requires higher temperatures (up to 900°C) and high pressure.
裂化是将重质馏分中的长链烷烃断裂为更短、更有用的烃类的过程。有两种类型：热裂化和催化裂化。两者都涉及在高温下断裂 C–C 键和 C–H 键，但催化裂化使用沸石催化剂且在较低温度（约 500°C），而热裂化需要更高温度（高达 900°C）和高压。
Catalytic cracking is more commonly used because it produces a higher proportion of branched alkanes and aromatic hydrocarbons, which improve the octane rating of petrol. Thermal cracking tends to give a higher proportion of alkenes, which are valuable feedstocks for polymers and other chemicals.
催化裂化更常用，因为它生成更高比例的支链烷烃和芳香烃，可提高汽油的辛烷值。热裂化往往生成更高比例的烯烃，这是聚合物和其他化学品的重要原料。
kane plus an alkene). WJEC often asks students to identify the products or write cracking equations based on given molecular formulae.
裂化方程式通常写为：C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀（一个烷烃加一个烯烃）。WJEC 常常要求学生根据给定的分子式确定产物或书写裂化方程式。
9. Environmental Impact of Alkanes | 烷烃的环境影响
The combustion of alkane fuels releases carbon dioxide, a greenhouse gas that contributes to climate change. Although CO₂ is a natural part of the carbon cycle, the massive scale of fossil fuel use has disrupted this balance. Incomplete combustion produces carbon monoxide and particulates, which have direct health effects and contribute to atmospheric pollution.
烷烃燃料的燃烧释放二氧化碳，一种导致气候变化的温室气体。尽管 CO₂ 是碳循环的自然组成部分，但化石燃料的大规模使用打破了这一平衡。不完全燃烧产生的一氧化碳和颗粒物对健康有直接影响，并造成大气污染。
Sulfur impurities in crude oil lead to the formation of sulfur dioxide during combustion, causing acid rain. Although modern refining and flue-gas desulfurisation reduce these emissions, the environmental impact remains a significant topic in WJEC syllabi, often linked to the discussion of alternative fuels and the role of catalytic converters.
原油中的硫杂质在燃烧过程中生成二氧化硫，导致酸雨。尽管现代炼油和烟气脱硫减少了这些排放，环境影响仍是 WJEC 大纲中的重要主题，常与替代燃料的讨论和催化转化器的作用相关联。
10. Key Calculations and Equation Balancing | 关键计算与方程式配平
eps. For combustion, write the alkane, add O₂, then balance C to CO₂ and H to H₂O, and finally balance O₂ using fractions or whole numbers. For example, to balance the complete combustion of heptane (C₇H₁₆): C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O.
你必须能够配平烷烃的完全和不完全燃烧反应、裂化反应以及自由基取代步骤的方程式。对于燃烧，先写出烷烃，加上 O₂，然后将 C 配成 CO₂、H 配成 H₂O，最后用分数或整数配平 O₂。例如，配平庚烷 (C₇H₁₆) 的完全燃烧：C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O。
n involve enthalpy changes of combustion. You may be given data and asked to calculate ΔcH⦵ mcΔT) or to use bond enthalpies to estimate the enthalpy change. Pay attention to sign conventions and units (kJ mol⁻¹).
计算常涉及燃烧焓变。你可能会被给数据并要求根据实验测量 (q = mcΔT) 计算 ΔcH⦵，或使用键焓来估算焓变。注意符号惯例和单位 (kJ mol⁻¹)。
Additionally, you should be able to calculate atom economy for reactions such as the radical substitution of methane with chlorine, showing why the process is inefficient for making a specific haloalkane.
此外，你应该能够计算诸如甲烷与氯的自由基取代等反应的原子经济性，以说明为什么该过程在制备特定卤代烷方面效率低下。
11. Common Exam Pitfalls and Revision Tips | 常见考试陷阱与复习技巧
Pitfall 1: Forgetting that London forces increase with molecular mass but also depend on molecular shape and surface contact. In branched alkanes, lower boiling points are due to reduced surface area for intermolecular forces, not lower molecular mass.
陷阱 1：忘记伦敦力随分子量增加而增强，但也取决于分子形状和表面接触。在支链烷烃中，沸点较低是由于分子间作用力的表面积减小，而非分子量较低。
Pitfall 2: Omitting the UV light condition when writing free radical substitution equations or mechanisms. Without UV or high temperature, the reaction does not proceed. Always write ‘UV’ over the arrow.
陷阱 2：在书写自由基取代方程式或机理时遗漏紫外光条件。没有紫外光或高温，反应不会进行。务必在箭头上方写上‘UV’。
Pitfall 3: Confusing termination steps with propagation steps. Remember that termination produces stable molecules and consumes radicals, while propagation steps both consume and produce radicals, thus sustaining the chain.
陷阱 3：混淆终止步骤与传递步骤。记住，终止产生稳定分子并消耗自由基，而传递步骤既消耗又产生自由基，从而维持链反应。
Create summary tables for the homologous series, boiling point trends, and reaction conditions. Practice drawing structures with correct bond angles (109.5° for tetrahedral carbon) and labelling radical mechanisms with appropriate curly arrows (single fishhook arrows for single electron movement). Use past papers to familiarise yourself with the style of WJEC questions.
为同系物、沸点趋势和反应条件制作总结表。练习绘制具有正确键角（四面体碳为 109.5°）的结构式，并用适当的卷曲箭头（单电子移动用单钩箭头）标注自由基机理。利用历年真题熟悉 WJEC 的出题风格。
12. Summary of Alkanes for WJEC A-Level | WJEC A-Level 烷烃总结
Alkanes are saturated hydrocarbons with general formula CₙH₂ₙ₊₂, known for their low reactivity due to strong σ-bonds. Key reactions include combustion (complete and incomplete) and free radical substitution with chlorine or bromine under UV light. Physical properties such as boiling points depend on London forces and molecular shape. Industrially, alkanes are obtained from crude oil fractional distillation and are converted into more valuable products by cracking. Master the radical mechanism, environmental considerations, and balancing equations to excel in this topic.
烷烃是通式为 CₙH₂ₙ₊₂ 的饱和烃，因其强 σ 键而具有低反应性。关键反应包括燃烧（完全和不完全）以及在紫外光下与氯或溴的自由基取代。沸点等物理性质取决于伦敦力和分子形状。工业上，烷烃通过原油分馏获取，并通过裂化转化为更有价值的产品。掌握自由基机理、环境因素和方程式配平，即可在本课题中取得优异成绩。
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