A-Level OCR Chemistry: Alkanes – Key Exam Points | A-Level OCR 化学：烷烃考点精讲

📚 A-Level OCR Chemistry: Alkanes – Key Exam Points | A-Level OCR 化学：烷烃考点精讲

Alkanes are the simplest family of hydrocarbons, and they form the bedrock of organic chemistry in the OCR A-Level specification. Understanding their structure, naming, properties, and especially the free-radical substitution mechanism is essential for both AS and A2 exam success. This article distils every key concept, common pitfall, and exam-style link you need to master the alkanes topic.

烷烃是最简单的烃类家族，也是 OCR A-Level 有机化学的基石。掌握它们的结构、命名、性质，尤其是自由基取代机理，对于 AS 和 A2 阶段的考试都至关重要。本文提炼了所有关键概念、常见失分点和考试必考联系，帮助你彻底攻克烷烃这一专题。

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

Alkanes are saturated hydrocarbons, meaning they contain only carbon and hydrogen atoms joined exclusively by single covalent bonds. Each carbon atom is sp³ hybridised and forms four σ (sigma) bonds. These σ bonds are formed by the end-on overlap of atomic orbitals, creating a bond with free rotation and maximum electron density along the internuclear axis.

烷烃是饱和烃，意味着它们只由碳和氢原子以单共价键连接。每个碳原子都是 sp³ 杂化，形成四个 σ（西格玛）键。这些 σ 键由原子轨道沿键轴方向重叠而成，具有自由旋转的特性，电子密度最大处位于两核连线上。

The shape around each carbon atom is tetrahedral, with a bond angle of approximately 109.5°. The C–C and C–H bonds are non-polar (electronegativity difference is very small), so alkanes are non-polar molecules overall. This low polarity is one reason for their poor solubility in water and their low reactivity with polar reagents.

每个碳原子周围的形状是四面体型，键角约为 109.5°。C–C 和 C–H 键都是非极性的（电负性差极小），因此烷烃整体是非极性分子。这种低极性是它们难溶于水、对极性试剂的反应性低的原因之一。

2. General Formula and Systematic Nomenclature | 通式与系统命名

Straight-chain and branched alkanes follow the general formula C_nH_2n+2. Cycloalkanes, which are also saturated but contain a ring of carbon atoms, have the general formula C_nH_2n. For OCR exams, you must be able to name alkanes using IUPAC rules: identify the longest continuous carbon chain, number the chain to give substituents the lowest possible numbers, and name alkyl side groups (methyl, ethyl, propyl, etc.) as prefixes in alphabetical order, separated by hyphens and commas.

直链和支链烷烃的通式为 C_nH_2n+2。环烷烃虽也是饱和烃，但含有碳环，通式为 C_nH_2n。在 OCR 考试中，你必须能运用 IUPAC 规则命名烷烃：找出最长连续碳链，从最靠近取代基的一端给主链编号，使取代基位次最小，并将烷基支链（甲基、乙基、丙基等）按字母顺序作为前缀，中间用连字符和逗号分隔。

Example: CH₃–CH(CH₃)–CH₂–CH₃ → 2-methylbutane (not 3-methylbutane, because numbering must give the lowest number).
示例：CH₃–CH(CH₃)–CH₂–CH₃ → 2-甲基丁烷（不能叫 3-甲基丁烷，因为编号必须使位次最小）。

3. Isomerism: Chain and Positional Variations | 异构现象：碳链异构与位置异构

Alkanes with four or more carbon atoms can exhibit structural isomerism. Chain isomers have different arrangements of the carbon skeleton (e.g., butane and 2-methylpropane). Positional isomers are not possible for simple alkanes because all carbon positions in an unbranched chain are equivalent; however, when a functional group is introduced in later topics, positional isomerism becomes important. For alkanes themselves, focus on drawing all possible chain isomers for a given molecular formula.

含有四个及以上碳原子的烷烃可以出现结构异构。碳链异构体具有不同的碳骨架排列方式（例如丁烷和 2-甲基丙烷）。对于简单烷烃本身，“位置异构”并不存在，因为直链上所有碳位是等同的；但当后续引入官能团后位置异构才变得重要。对于烷烃本身，重点是根据分子式画出所有可能的碳链异构体。

OCR frequently asks students to deduce the number of structural isomers for a formula like C₅H₁₂ or C₆H₁₄. C₅H₁₂ has three chain isomers: pentane, 2-methylbutane, and 2,2-dimethylpropane. Always check for symmetry to avoid drawing the same isomer twice.

OCR 经常要求学生推断如 C₅H₁₂ 或 C₆H₁₄ 的结构异构体数目。C₅H₁₂ 有三种碳链异构体：戊烷、2-甲基丁烷和 2,2-二甲基丙烷。务必检查分子的对称性，避免重复画出同一个异构体。

4. Physical Properties and Trends | 物理性质及其变化规律

Boiling points of straight-chain alkanes increase as the chain length increases. This is because larger molecules have more electrons, leading to stronger instantaneous dipole–induced dipole (London) forces between molecules, which require more energy to overcome. Branching lowers boiling point: branched isomers have smaller surface area contact, so London forces are weaker.

直链烷烃的沸点随碳链增长而升高。这是因为分子越大，电子数越多，分子间瞬间偶极-诱导偶极力（伦敦力）越强，克服这些力需要更多能量。支链会降低沸点：支链异构体分子间接触表面积更小，因此伦敦力更弱。

Alkanes are insoluble in water but soluble in non-polar solvents like hexane or tetrachloromethane. Density increases with molecular mass but remains less than that of water (alkanes float). These solubility and density trends are common context questions in practical-based questions.

烷烃不溶于水，但可溶于非极性溶剂，如己烷或四氯甲烷。密度随相对分子质量增大而增大，但始终小于水（烷烃会浮在水面）。这些溶解性和密度变化趋势常常出现在实验背景题目中。

5. Chemical Reactivity of Alkanes | 烷烃的化学性质

Alkanes are generally unreactive under ordinary conditions because the C–C and C–H σ bonds are strong (average bond enthalpies: C–C 347 kJ mol⁻¹, C–H 413 kJ mol⁻¹) and have very low polarity. They do not react with acids, bases, oxidising agents, or reducing agents at room temperature. This inertness makes them useful as lubricants and non-polar solvents, but it also means they require extreme conditions or UV light to react.

烷烃在通常条件下化学性质不活泼，因为 C–C 和 C–H σ 键很强（平均键焓：C–C 347 kJ mol⁻¹，C–H 413 kJ mol⁻¹）且极性极低。它们在室温下不与酸、碱、氧化剂或还原剂反应。这种惰性使它们可用作润滑剂和非极性溶剂，但也意味着它们需要极端条件或紫外光照才能发生反应。

Two key reaction types for OCR are combustion (oxidation) and free-radical substitution with halogens. Both will be examined in detail, and you must be able to write overall equations, identify products, and explain mechanisms.

OCR 要求掌握的两大反应类型是燃烧（氧化）和与卤素的自由基取代反应。两者都会详细考查，你必须能够书写总反应方程式、识别产物并解释反应机理。

6. Combustion Reactions | 燃烧反应

Complete combustion occurs in excess oxygen, producing carbon dioxide and water. For example: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Incomplete combustion happens when oxygen supply is limited, producing carbon monoxide (CO) and/or carbon (soot) alongside water. These products have serious environmental and health implications.

完全燃烧在过量氧气中进行，生成二氧化碳和水。例如：C₃H₈ + 5O₂ → 3CO₂ + 4H₂O。当氧气供应不足时发生不完全燃烧，生成一氧化碳（CO）和/或碳（炭黑）以及水。这些产物对环境和健康有严重影响。

Incomplete combustion equation with CO: 2C₃H₈ + 7O₂ → 6CO + 8H₂O
不完全燃烧生成 CO 的方程式：2C₃H₈ + 7O₂ → 6CO + 8H₂O

OCR may ask you to write a balanced equation for incomplete combustion to form CO or C, given the alkane formula. Always check the balancing, especially for odd/even numbers of carbon atoms.

OCR 可能会要求你根据给出的烷烃分子式书写生成 CO 或 C 的不完全燃烧方程式。务必仔细配平，尤其是涉及奇数/偶数碳原子时。

7. Environmental Impact of Combustion Products | 燃烧产物对环境的影响

Carbon dioxide is a major greenhouse gas, contributing to climate change. Carbon monoxide is a toxic gas that binds irreversibly to haemoglobin, reducing the blood’s oxygen-carrying capacity. Unburnt hydrocarbons can cause photochemical smog. In vehicle engines, high temperatures also allow nitrogen and oxygen to react forming NO_x, which contributes to acid rain and respiratory problems. Particulates (soot) can exacerbate asthma and lung disease.

二氧化碳是主要的温室气体，导致气候变化。一氧化碳是有毒气体，会与血红蛋白不可逆结合，降低血液携氧能力。未燃烧的碳氢化合物能引起光化学烟雾。在汽车发动机中，高温还会使氮气和氧气反应生成氮氧化物 NO_x，导致酸雨和呼吸系统疾病。炭黑颗粒物会加重哮喘和肺部疾病。

Catalytic converters in vehicle exhausts use platinum, palladium, and rhodium catalysts to convert CO, NO_x, and unburnt hydrocarbons into less harmful CO₂, N₂, and H₂O. The key equations are: 2CO + 2NO → 2CO₂ + N₂, and hydrocarbons + NO → CO₂ + N₂ + H₂O (balanced according to the specific hydrocarbon).

汽车排气系统中的催化转换器使用铂、钯和铑作为催化剂，将 CO、NO_x 和未燃烧的碳氢化合物转化为危害较小的 CO₂、N₂ 和 H₂O。关键方程式为：2CO + 2NO → 2CO₂ + N₂，以及碳氢化合物 + NO → CO₂ + N₂ + H₂O（需根据具体碳氢化合物配平）。

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

Alkanes react with chlorine or bromine in the presence of UV light (or at high temperatures) via a free-radical substitution mechanism. This is a chain reaction with three stages: initiation, propagation, and termination. OCR requires you to describe each step using curly half-arrows showing the movement of single electrons.

烷烃在紫外光照射（或高温）下与氯气或溴蒸气发生自由基取代反应。这是一类连锁反应，分为引发、增长和终止三个阶段。OCR 要求用半箭头（弯钩箭头）表示单电子的移动，描述每一个步骤。

Initiation: Cl₂ → 2Cl• (UV light provides energy to break the Cl–Cl bond by homolytic fission, each chlorine atom taking one electron).
增长：Cl₂ → 2Cl•（UV 光提供能量使 Cl–Cl 键发生均裂，每个氯原子各得一个电子）。
Propagation 1: Cl• + CH₄ → •CH₃ + HCl
增长第一步：Cl• + CH₄ → •CH₃ + HCl
Propagation 2: •CH₃ + Cl₂ → CH₃Cl + Cl•
增长第二步：•CH₃ + Cl₂ → CH₃Cl + Cl•
Termination (two radicals combine): Cl• + Cl• → Cl₂, •CH₃ + •CH₃ → C₂H₆, Cl• + •CH₃ → CH₃Cl
终止（两个自由基结合）：Cl• + Cl• → Cl₂，•CH₃ + •CH₃ → C₂H₆，Cl• + •CH₃ → CH₃Cl

In termination, any two radicals can combine; examiners often reward recognising the possibility of ethane formation as evidence of the mechanism.

在终止阶段，任意两个自由基都可以结合；考官常常奖励那些认识到有乙烷生成并以此作为机理证据的答案。

9. Free Radical Substitution with Bromine and Overall Equation | 溴的自由基取代及总方程式

Bromine reacts more slowly and is more selective than chlorine because the Br–Br bond is weaker (193 kJ mol⁻¹ vs 242 kJ mol⁻¹ for Cl–Cl) and bromine radicals are less reactive. The overall equation for monosubstitution of methane by chlorine is: CH₄ + Cl₂ → CH₃Cl + HCl. However, further substitution can occur, producing CH₂Cl₂, CHCl₃, and CCl₄.

溴的反应比氯慢，选择性更高，因为 Br–Br 键较弱（193 kJ mol⁻¹，而 Cl–Cl 为 242 kJ mol⁻¹），且溴自由基反应活性更低。甲烷与氯气发生一取代反应的总体方程式为：CH₄ + Cl₂ → CH₃Cl + HCl。然而，进一步取代会发生，生成 CH₂Cl₂、CHCl₃ 和 CCl₄。

OCR expects you to write overall equations for both monohalogenation and further substitution. You must also understand why a mixture of products is obtained – because the propagation steps can attack both the original alkane and any halogenoalkane products.

OCR 要求你能书写一卤化和进一步卤化的总方程式。你还必须理解为什么产物是混合物——因为增长步骤既可以进攻原烷烃，也可以进攻任何卤代烷产物。

10. Limitations and Practical Implications | 反应的局限性与实际意义

The free-radical substitution mechanism has significant synthetic limitations. Because multiple substitutions occur, separation of the desired monohalogenated product requires fractional distillation, which is energy-intensive. Additionally, the reaction is not stereospecific nor regiospecific; radical intermediates are planar (trigonal planar) and can lead to racemic mixtures if a chiral centre is formed.

自由基取代机理在合成上存在明显的局限性。由于会发生多次取代，要想得到所需的一卤代产物就需要分馏分离，能耗较高。此外，该反应既无立体选择性也无区域选择性；自由基中间体为平面构型（平面三角形），若生成手性中心则会导致外消旋混合物。

In the laboratory, the reaction is initiated by UV lamp and stopped by removing the light source. The formation of white fumes of HCl can be confirmed with blue litmus paper turning red. When bromine is used, the red-brown colour fades – this is a useful visual indicator for the progress of substitution.

在实验室中，该反应通过紫外灯引发，移除光源即可停止。生成的 HCl 白雾可用蓝色石蕊试纸变红来确认。使用溴蒸汽时，红棕色褪去——这是指示取代反应进行的有用可视现象。

11. Common Exam Pitfalls and Top Tips | 常见失分点与高分技巧

Many students lose marks by mixing up homolytic and heterolytic fission. Remember: in free-radical chemistry, bonds break homolytically – one electron goes to each atom, drawn with a half-arrow (‘fish-hook’ arrow). Never use full-curly arrows for this mechanism. Also, ensure you label initiation, propagation, and termination clearly in written questions.

许多学生因混淆均裂和异裂而失分。记住：在自由基化学中，键以均裂方式断裂——每个原子得到一个电子，必须用半箭头（“鱼钩箭头”）表示。千万不要在此机理中使用全弯箭头。同时，在书写题中务必清楚标出引发、增长和终止阶段。

Balancing combustion equations for large alkanes can be tricky. A useful tip is to balance C first, then H, and finally O; for incomplete combustion producing CO, treat CO as the carbon-containing product. In mechanisms, always show the regeneration of the radical in the second propagation step to demonstrate the chain character.

大型烷烃燃烧方程式的配平可能很棘手。一个有用的技巧是：先配平 C，再配平 H，最后配平 O；对于生成 CO 的不完全燃烧，视 CO 为含碳产物。在机理题中，务必在第二个增长步骤中显示自由基的再生，以体现连锁反应的特征。

Lastly, when discussing environmental effects, link each pollutant to a specific consequence: CO – toxicity to humans, CO₂ – greenhouse gas, NOₓ – acid rain and photochemical smog, unburnt hydrocarbons – smog. Structured answers that make these explicit links score highly.

最后，在讨论环境影响时，要将每种污染物与其具体后果对应起来：CO——对人体的毒性，CO₂——温室效应，NOₓ——酸雨和光化学烟雾，未燃烧的碳氢化合物——光化学烟雾。能明确建立这些联系的结构化作答往往得分很高。

12. Summary and Exam Check | 考点总结与应试自查

Alkanes may appear straightforward, but OCR examiners probe deep understanding of structure–property relationships, reaction mechanisms, and environmental chemistry. Be prepared to draw dot-and-cross diagrams to show bonding, explain boiling point trends in terms of intermolecular forces, apply free-radical substitution to unfamiliar alkanes, and evaluate the efficiency of catalytic converters.

烷烃看似简单，但 OCR 考官会深入考查结构-性质关系、反应机理和环境的化学理解。做好准备，能够画点叉图表示成键，用分子间作用力解释沸点变化趋势，将自由基取代机理应用于陌生烷烃，并评价催化转换器的效能。

Keep revisiting the free-radical substitution mechanism – it is one of the most heavily assessed organic mechanisms in the AS papers. Memorise the steps but, more importantly, understand the logic so you can adapt to any given alkane and halogen combination.

反复回顾自由基取代机理——它是 AS 考卷中考查最频繁的有机机理之一。记住每一步骤，但更重要的是理解其中的逻辑，以便能应对任何给定的烷烃和卤素组合。

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A-Level OCR Chemistry: Alkanes – Key Exam Points | A-Level OCR 化学：烷烃 考点精讲