📚 Alkanes: Comprehensive Exam-Focused Review | IB AQA 化学：烷烃 考点精讲
Alkanes are the simplest family of hydrocarbons, consisting only of carbon and hydrogen atoms linked by single bonds. Understanding their structure, nomenclature, physical properties, and reactivity is fundamental for IB and AQA Chemistry. This article provides an in-depth, exam-oriented review of alkanes, covering every key concept from IUPAC naming to free-radical substitution mechanisms and environmental implications. Each section pairs English explanations with precise Chinese translations to support bilingual learners.
烷烃是最简单的碳氢化合物家族，仅由碳和氢原子通过单键连接而成。理解烷烃的结构、命名、物理性质和反应活性是 IB 和 AQA 化学的基础。本文深入解析烷烃的考点，从 IUPAC 命名到自由基取代机理，再到环境影响，每个知识点都用中英双语对照讲解，助力双语学习者备考。
1. Introduction to Alkanes | 烷烃概述
Alkanes are saturated hydrocarbons with the general formula CnH2n+2. They contain only sigma (σ) bonds, which are strong and allow free rotation around the carbon–carbon bonds. Because all valence electrons are involved in strong σ bonds, alkanes are relatively unreactive. Their primary importance lies in their use as fuels and as starting materials for the chemical industry. The simplest alkane is methane (CH4), followed by ethane (C2H6), propane (C3H8), and butane (C4H10).
烷烃是饱和碳氢化合物，通式为 CnH2n+2。它们只含有西格玛（σ）键，键能强且允许碳碳单键自由旋转。由于所有价电子都参与强σ键形成，烷烃在常温下相对惰性。它们的主要用途是作为燃料和化工原料。最简单的烷烃是甲烷（CH4），其次是乙烷（C2H6）、丙烷（C3H8）和丁烷（C4H10）。
2. IUPAC Nomenclature of Alkanes | 烷烃的 IUPAC 命名
The systematic naming of alkanes follows clear IUPAC rules. First, identify the longest continuous carbon chain to determine the parent name (meth-, eth-, prop-, but-, pent-, hex-, etc.). Number the chain from the end nearest the first substituent. Name and number alkyl substituents (e.g., methyl, ethyl) and list them alphabetically. Use prefixes di-, tri-, and tetra- to indicate multiple identical substituents, but ignore these when alphabetising. For example, 3-ethyl-2,4-dimethylhexane means a six-carbon chain with an ethyl group on carbon 3 and methyl groups on carbons 2 and 4.
烷烃的系统命名遵循明确的 IUPAC 规则。首先，找出最长的连续碳链作为母体名称（甲、乙、丙、丁、戊、己等）。从最靠近第一个取代基的一端给链编号。命名烷基取代基（如甲基、乙基）并标出其位置，按字母顺序列出。用二、三、四等前缀表示相同取代基的个数，但在字母排序时忽略这些前缀。例如，3-乙基-2,4-二甲基己烷表示一个六碳链，碳 3 上有一个乙基，碳 2 和 4 上各有一个甲基。
A common exam mistake is failing to choose the longest chain or numbering incorrectly to give substituents the lowest possible locants. Always check that the sum of numbers is minimised. For cyclic alkanes, add the prefix 'cyclo-' (e.g., cyclohexane).
常见考试错误是未选对最长碳链，或编号未能使取代基位次最小。务必确保位次数字之和最小。对于环状烷烃，加前缀“环”（如环己烷）。
3. Structural Isomerism in Alkanes | 烷烃的结构异构
Alkanes with four or more carbon atoms exhibit structural isomerism. Structural isomers have the same molecular formula but different connectivity of atoms. For example, C4H10 has two isomers: butane (a straight chain) and 2-methylpropane (a branched chain). As the carbon number increases, the number of possible isomers grows rapidly. Isomers have different physical properties such as boiling point — branched isomers are more compact, have smaller surface area, and thus weaker London dispersion forces, leading to lower boiling points compared to their straight-chain counterparts.
含四个及以上碳原子的烷烃存在结构异构。结构异构体具有相同的分子式，但原子连接顺序不同。例如，C4H10 有两种异构体：丁烷（直链）和 2-甲基丙烷（支链）。随着碳原子数增加，异构体数量迅速增多。异构体的物理性质如沸点不同——支链异构体更紧凑，分子表面积较小，伦敦色散力较弱，因此沸点比对应的直链烷烃低。
When drawing isomers, ensure all carbon atoms form four bonds. Examiners often test the ability to deduce and draw all isomers of a given formula, especially for formulas like C5H12 or C6H14.
绘制异构体时，务必确保所有碳原子形成四个键。考官经常考查根据给定分子式推导并画出所有异构体的能力，尤其是对于 C5H12 或 C6H14 这类分子式。
4. Physical Properties of Alkanes | 烷烃的物理性质
The physical properties of alkanes show clear trends. Boiling points increase with increasing molecular mass because larger molecules have more electrons, leading to stronger London dispersion forces. Straight-chain alkanes have higher boiling points than their branched isomers. Melting points also generally increase with molecular mass, but symmetry plays a role — symmetrical molecules pack more efficiently in the solid state. Alkanes are non-polar and are thus insoluble in water, but dissolve in non-polar organic solvents. They have low density; liquid alkanes float on water.
烷烃的物理性质呈现明显规律。沸点随分子量增加而升高，因为分子越大电子越多，伦敦色散力越强。直链烷烃的沸点高于同分异构的支链烷烃。熔点通常也随分子量增加而升高，但对称性有影响——对称分子在固态中排列更紧密。烷烃是非极性分子，因此不溶于水，但溶于非极性有机溶剂。密度小，液态烷烃浮于水面。
In explaining these trends, always refer to the strength of intermolecular forces, not intramolecular bonds. 'Van der Waals' forces' is often an accepted term for London dispersion forces in this context.
解释这些趋势时，一定要指出与分子间作用力（而非分子内共价键）的强弱有关。在此语境下，范德华力常被用来泛指伦敦色散力。
5. Combustion of Alkanes | 烷烃的燃烧反应
Alkanes are excellent fuels and undergo combustion in excess oxygen to form carbon dioxide and water, releasing large amounts of heat. The general equation for complete combustion of an alkane is:
CnH2n+2 + (1.5n+0.5) O2 → n CO2 + (n+1) H2O
For example, propane: C3H8 + 5 O2 → 3 CO2 + 4 H2O. The large exothermic enthalpy change makes alkanes primary energy sources. However, incomplete combustion occurs when oxygen is limited, producing carbon monoxide (CO) and/or carbon (soot). CO is a toxic gas, and soot can clog engines and cause respiratory problems.
烷烃是优质燃料，在过量氧气中完全燃烧生成二氧化碳和水，并释放大量热量。烷烃完全燃烧的通式为：
CnH2n+2 + (1.5n+0.5) O2 → n CO2 + (n+1) H2O
例如丙烷：C3H8 + 5 O2 → 3 CO2 + 4 H2O。巨大的放热焓变使烷烃成为主要能源。然而，当氧气不足时发生不完全燃烧，产生一氧化碳 (CO) 和/或碳（炭黑）。CO 是有毒气体，炭黑会堵塞发动机并引起呼吸道问题。
In exams, you may be asked to balance combustion equations or to assess the environmental consequences of the products. Always show your working for stoichiometric coefficients.
考试中可能要求配平燃烧方程式或评估产物的环境影响。展示你的配平步骤以得到正确化学计量数。
6. Free-Radical Substitution with Halogens | 烷烃与卤素的自由基取代反应
Alkanes react with halogens (Cl2, Br2) in the presence of UV light or heat via a free-radical substitution mechanism. This is a photochemical chain reaction, initiated by the homolytic fission of the halogen molecule. For example, with chlorine:
Cl2 → 2 Cl·
The reaction results in the replacement of hydrogen atoms by halogen atoms, producing a mixture of halogenoalkanes and hydrogen halide. For methane, the overall reaction is:
CH4 + Cl2 → CH3Cl + HCl
Further substitution can occur, leading to CH2Cl2, CHCl3, and CCl4. The rate of reaction depends on the halogen: fluorine reacts explosively, chlorine moderately under UV, bromine more slowly, and iodine very slowly or not at all due to the bond energies involved.
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。这是一个光化学链反应，由卤素分子的均裂引发。例如，氯分子均裂：
Cl2 → 2 Cl·
反应结果是氢原子被卤原子取代，生成卤代烷混合物和卤化氢。对于甲烷，总反应为：
CH4 + Cl2 → CH3Cl + HCl
进一步取代会生成 CH2Cl2、CHCl3 和 CCl4。反应速率与卤素种类有关：氟反应剧烈，氯在紫外光下适度反应，溴较慢，碘因键能原因非常慢或不反应。
7. Mechanism of Free-Radical Substitution | 自由基取代反应机理
The mechanism consists of three stages: initiation, propagation, and termination.
Initiation: UV light provides energy to break the halogen–halogen bond homolytically, producing two halogen radicals.
Cl2 → 2 Cl·
Propagation: The halogen radical abstracts a hydrogen from the alkane, forming H–X and an alkyl radical. This alkyl radical then attacks another halogen molecule, generating the halogenoalkane and a new halogen radical, sustaining the chain.
CH4 + Cl· → ·CH3 + HCl
·CH3 + Cl2 → CH3Cl + Cl·
Termination: Two free radicals combine, removing radicals from the system. Possible termination steps include:
Cl· + Cl· → Cl2
·CH3 + Cl· → CH3Cl
·CH3 + ·CH3 → CH3CH3
Examiners frequently ask students to show the mechanism using curly half-arrows to indicate single-electron movements. The overall rate is controlled by the concentration of the halogen and the intensity of UV light.
该机理包含三个阶段：链引发、链增长和链终止。
链引发：紫外光提供能量使卤素–卤素键均裂，生成两个卤素自由基。
链增长：卤素自由基夺取烷烃中的氢原子，生成卤化氢和烷基自由基。该烷基自由基再进攻另一个卤素分子，生成卤代烷和一个新的卤素自由基，使链反应持续进行。
链终止：任意两个自由基结合，将自由基从体系中移除。可能的终止步骤包括 Cl· + Cl· → Cl2，·CH3 + Cl· → CH3Cl，以及 ·CH3 + ·CH3 → CH3CH3。
考官常要求用弯的半箭头表示单电子转移，展示机理。总反应速率受卤素浓度和紫外光强度控制。
8. Environmental Impact: Greenhouse Gases and Carbon Monoxide | 环境影响：温室气体与一氧化碳
Combustion of alkanes contributes significantly to environmental issues. Complete combustion produces CO2, a major greenhouse gas that enhances the natural greenhouse effect, leading to global warming. Unburnt alkanes themselves, such as methane, are even more potent greenhouse gases, with a global warming potential many times that of CO2 over a 20-year period. Methane emissions arise from natural gas leaks, agriculture, and incomplete combustion. In addition, carbon monoxide from incomplete combustion is a silent killer — it binds irreversibly to haemoglobin, preventing oxygen transport.
烷烃的燃烧对环境产生重大影响。完全燃烧生成 CO2，这是主要的温室气体，会加强自然温室效应，导致全球变暖。未燃烧的烷烃本身，比如甲烷，是更强的温室气体，其 20 年期的全球变暖潜势是 CO2 的许多倍。甲烷排放来自天然气泄漏、农业和不完全燃烧。此外，不完全燃烧产生的一氧化碳是一种无声杀手——它与血红蛋白不可逆结合，阻碍氧气运输。
The formation of nitrogen oxides (NOx) in car engines at high temperatures also contributes to acid rain and photochemical smog. Catalytic converters in vehicles help reduce CO and NOx emissions. Understanding these environmental consequences is a key part of the AQA and IB syllabuses.
汽车发动机高温下生成的氮氧化物 (NOx) 也会导致酸雨和光化学烟雾。车辆中的催化转化器有助于减少 CO 和 NOx 排放。理解这些环境后果是 AQA 和 IB 课程大纲的关键部分。
9. Catalytic Cracking and Reforming | 催化裂化与重整
Long-chain alkane fractions from crude oil have low demand and are broken into more useful shorter-chain alkanes and alkenes via catalytic cracking. This process uses a zeolite catalyst at around 500 °C and involves breaking C–C bonds to produce branched alkanes, cycloalkanes, and aromatic compounds suitable for gasoline. Reforming converts straight-chain alkanes into branched isomers or cyclic hydrocarbons to increase the octane number of petrol. Both processes are essential for matching the supply of hydrocarbon feedstocks to market demand.
来自原油的长链烷烃馏分需求量低，因此通过催化裂化将其分解为更有用的短链烷烃和烯烃。该过程使用沸石催化剂在约 500 °C 下进行，涉及 C–C 键断裂，生成适合汽油的支链烷烃、环烷烃和芳香烃。重整则将直链烷烃转化为支链异构体或环烃，以提高汽油的辛烷值。这两个工艺对于匹配烃类原料供应和市场需求至关重要。
Exam questions often ask for balanced equations showing the cracking of a larger alkane into an alkane and an alkene, e.g., C12H26 → C8H18 + C4H8. Pay attention to conserving the total number of carbon and hydrogen atoms.
考题常要求配平裂化方程式，将一个大烷烃分解为一个烷烃和一个烯烃，例如 C12H26 → C8H18 + C4H8。注意碳和氢原子总数守恒。
10. Key Exam Tips and Common Mistakes | 考试技巧与常见错误
To excel in alkane questions, remember these crucial points. Always count the longest continuous carbon chain for naming, not the drawn 'straight' line. For combustion, ensure you use O2 as reactant, not 'O'; balance carbon, then hydrogen, then oxygen. In free-radical substitution, never use ions or double-headed arrows; only half-headed curly arrows are correct. When drawing mechanisms, radicals must be clearly indicated with a single dot (·) on the atom. Do not forget that termination can produce an alkane from two alkyl radicals — this is a common trick question.
要想在烷烃问题上取得高分，请牢记这些关键点。命名时务必数最长的连续碳链，而不是纸上画的“直”链。燃烧方程式要使用 O2 作为反应物，而不是“O”；配平时先配碳，再配氢，最后配氧。自由基取代中绝不能用离子或全箭头，只能用半头弯箭头。画机理时，自由基必须在原子上用一个点（·）清晰标出。别忘了链终止可以由两个烷基自由基结合生成烷烃——这是常见的陷阱题。
If asked about the environmental effects of alkanes, structure your answer around CO2 (greenhouse gas), CO (toxic), unburnt hydrocarbons (smog, greenhouse), and NOx (acid rain). Always link the property or effect back to intermolecular forces, bond energies, or reaction conditions as appropriate.
若问到烷烃的环境影响，围绕 CO2（温室气体）、CO（有毒）、未燃烧烃（烟雾、温室效应）和 NOx（酸雨）组织答案。始终将性质或影响与分子间力、键能或反应条件等恰当联系起来。
Finally, in practical-related questions, recall that alkanes are generally unreactive towards bromine water in the dark, which can be used to distinguish them from alkenes — but under UV light, a substitution reaction occurs.
最后，在实验相关题目中，记住烷烃在黑暗处对溴水通常不反应，这可用于区分它们与烯烃—但在紫外光下却发生取代反应。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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