📚 AS Chemistry: Alkanes – Essential Exam Points | AS 化学：烷烃 考点精讲
Alkanes are the simplest family of hydrocarbons, containing only single covalent bonds between carbon atoms and between carbon and hydrogen. They are saturated compounds and serve as the foundation for understanding organic chemistry mechanisms, isomerism, and industrial processes. This article covers the core AS-level exam topics, including structure, naming, physical trends, combustion, free-radical substitution, cracking, and environmental aspects.
烷烃是最简单的烃类家族，分子中只含有碳-碳单键和碳-氢单键，属于饱和化合物。它们是理解有机化学机理、同分异构现象和工业过程的基础。本文涵盖 AS 级别的核心考点，包括结构、命名、物理性质递变、燃烧反应、自由基取代、裂化反应以及环境影响等内容。
1. Understanding Alkanes – Saturated Hydrocarbons | 认识烷烃——饱和烃
Alkanes have the general formula CnH2n+2. Each carbon atom forms four single bonds, adopting a tetrahedral geometry with bond angles of approximately 109.5°. The carbon-carbon sigma bonds are strong and allow free rotation, which leads to a variety of conformations.
烷烃的通式为 CnH2n+2。每个碳原子形成四个单键，呈四面体构型，键角约为 109.5°。碳-碳 σ 键强度高且可自由旋转，使得分子可以采取不同的构象。
The term ‘saturated’ signifies that the molecule contains the maximum possible number of hydrogen atoms per carbon. Because there are no π bonds, alkanes are relatively unreactive compared to alkenes, but they do undergo combustion and free-radical substitution under specific conditions.
“饱和”一词意味着分子中每个碳原子连接了尽可能多的氢原子。由于不存在 π 键，烷烃相对于烯烃反应性较低，但在特定条件下仍可发生燃烧反应和自由基取代反应。
2. Nomenclature and Structural Isomerism | 命名与构造异构
IUPAC names for straight-chain alkanes follow the prefix (meth-, eth-, prop-, but-, pent-, hex-, etc.) plus the -ane ending. Branched alkanes are named by identifying the longest continuous carbon chain and numbering the substituents to give the lowest set of locants. Substituent groups such as methyl, ethyl, or halogen atoms are listed alphabetically as prefixes.
直链烷烃的 IUPAC 命名规则：采用前缀（甲、乙、丙、丁、戊、己等）加后缀“烷”。支链烷烃需找出最长连续碳链作为母体，从最靠近支链的一端开始编号，使取代基位次之和最小。甲基、乙基或卤素等取代基按字母顺序列为前缀。
Structural isomerism arises when alkanes with four or more carbon atoms can arrange the same molecular formula in different ways. For example, C4H10 has two isomers: butane and 2-methylpropane. Isomers have different physical properties – branched isomers have lower boiling points due to reduced surface contact and weaker van der Waals forces.
含四个或以上碳原子的烷烃可出现构造异构现象，即相同的分子式对应不同的原子连接方式。例如 C4H10 有两种异构体：丁烷和 2-甲基丙烷。异构体物理性质不同——支链异构体因分子接触面积减小、范德华力变弱而具有更低的沸点。
3. Physical Properties – Boiling Points and Solubility | 物理性质——沸点与溶解度
Alkanes are non-polar molecules. The only intermolecular forces are instantaneous dipole–induced dipole (London dispersion) forces, which increase with the number of electrons and the surface area of the molecule. As a result, the boiling points of straight-chain alkanes rise with increasing chain length.
烷烃是非极性分子，分子间只存在瞬时偶极-诱导偶极力（伦敦色散力）。分子所含电子数越多、分子表面积越大，这种力就越强。因此，直链烷烃的沸点随碳链增长而升高。
| Alkane | Formula | Boiling Point / °C |
|---|---|---|
| Methane | CH₄ | -162 |
| Ethane | C₂H₆ | -89 |
| Propane | C₃H₈ | -42 |
| Butane | C₄H₁₀ | -0.5 |
| Pentane | C₅H₁₂ | 36 |
Branching lowers the boiling point because the more spherical shape of branched isomers offers less surface area for intermolecular contact, weakening the van der Waals forces. For the same reason, branched alkanes are less viscous and have lower melting points, although melting point trends can be complicated by symmetry factors.
支链化会降低沸点，因为支链异构体更接近球形，分子间接触面积减小，范德华力减弱。同理，支链烷烃的粘度较低，熔点一般也较低，但对称性因素可能使熔点变化复杂。
Alkanes are insoluble in water due to their inability to form hydrogen bonds, but they dissolve readily in other non-polar solvents such as cyclohexane or tetrachloromethane.
烷烃不溶于水，因为无法与水形成氢键，但它们易溶于环己烷或四氯化碳等非极性溶剂。
4. Combustion Chemistry | 燃烧化学
Alkanes are excellent fuels. Complete combustion in excess oxygen produces carbon dioxide and water, releasing a large amount of energy. The general equation for complete combustion is:
烷烃是优良的燃料，在过量氧气中完全燃烧生成二氧化碳和水，同时释放大量热能。完全燃烧的通式为：
CnH2n+2 + (1.5n + 0.5)O₂ → nCO₂ + (n+1)H₂O
For methane, the equation simplifies to:
对于甲烷，方程式可简化为：
CH₄ + 2O₂ → CO₂ + 2H₂O ΔHc = -890 kJ mol⁻¹
Incomplete combustion occurs when the oxygen supply is limited, producing toxic carbon monoxide (CO) or even elemental carbon (soot). CO binds irreversibly to haemoglobin, reducing the blood’s oxygen-carrying capacity.
当氧气供应不足时，会发生不完全燃烧，生成有毒的一氧化碳(CO)，甚至析出碳微粒(炭黑)。CO 会与血红蛋白不可逆结合，降低血液的载氧能力。
The environmental consequence of burning alkanes includes the emission of greenhouse gases (CO₂) and pollutants like CO and unburned hydrocarbons, which contribute to photochemical smog and global warming.
燃烧烷烃的环境后果包括排放温室气体(CO₂)以及 CO 和未燃烧烃类等污染物，这些污染物会加剧光化学烟雾和全球变暖。
5. Free Radical Substitution – The Halogenation Mechanism | 自由基取代——卤代机理
Alkanes react with halogens (Cl₂, Br₂) in the presence of ultraviolet light or high temperature to form haloalkanes. This reaction proceeds via a free radical substitution mechanism, starting with the homolytic fission of the halogen molecule.
烷烃在紫外光或高温下与卤素（Cl₂、Br₂）反应生成卤代烷。该反应属于自由基取代机理，首先发生卤素分子的均裂。
A free radical is a species with an unpaired electron, making it highly reactive. The chlorine radical is denoted as Cl· and the methyl radical as ·CH₃. The unpaired electron is shown as a dot.
自由基是一种带有未成对电子的物种，反应活性极高。氯自由基用 Cl· 表示，甲基自由基用 ·CH₃ 表示，黑点代表未成对电子。
This mechanism is a chain reaction, meaning a single initiation event can lead to many propagation cycles. It is essential to clearly identify and label the three stages: initiation, propagation, and termination.
该机理为链式反应，一次引发即可引发多次增长循环。在考试中，必须清晰识别和标注反应的三个阶段：链引发、链增长和链终止。
6. Initiation, Propagation, and Termination Steps | 链引发、链增长与链终止步骤
Initiation: The halogen molecule undergoes homolytic fission when exposed to UV light. Two halogen radicals are formed.
链引发：卤素分子在紫外光照射下发生均裂，生成两个卤素自由基。
Cl₂ → 2Cl· (UV light)
Propagation: A two-step cycle where radicals are regenerated. First, a chlorine radical abstracts a hydrogen atom from an alkane, forming HCl and an alkyl radical. Then, the alkyl radical reacts with a chlorine molecule to produce the haloalkane and a new chlorine radical, which can continue the cycle.
链增长：这是一个两步循环，每一步都重新生成自由基。首先，氯自由基从烷烃分子中夺走一个氢原子，生成 HCl 和一个烷基自由基。随后，该烷基自由基与一个氯分子反应，生成卤代烷并再生一个氯自由基，循环得以继续。
CH₄ + Cl· → ·CH₃ + HCl
·CH₃ + Cl₂ → CH₃Cl + Cl·
Termination: Any two radicals combine to form a stable molecule, removing radicals from the system and slowing the reaction. Possible termination steps include:
链终止：任意两个自由基相互结合形成稳定分子，消耗自由基从而使反应减速。可能的终止步骤包括：
2Cl· → Cl₂
2·CH₃ → C₂H₆
·CH₃ + Cl· → CH₃Cl
These termination steps are statistically less likely than propagation because radical concentrations are low. However, they must be mentioned in complete mechanistic explanations.
这些终止步骤在统计上不如增长步骤频繁，因为自由基浓度较低。但在完整的机理阐述中必须提到。
7. Chain Reaction Outcomes and Product Mixtures | 链式反应结果与产物混合物
During the chlorination of methane, further substitution can occur as chlorine radicals attack the chloromethane product, leading to a mixture of CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄. A large excess of methane can minimise multiple substitution, but the reaction is rarely clean.
在甲烷的氯化过程中，氯自由基会继续进攻产物氯甲烷，导致生成 CH₃Cl、CH₂Cl₂、CHCl₃ 和 CCl₄ 的混合物。使用大过量的甲烷可减少多次取代，但反应通常难以得到单一产物。
With higher alkanes, substitution can occur at different carbon positions, giving isomeric haloalkanes. For example, chlorination of propane produces both 1-chloropropane and 2-chloropropane, with the secondary radical being more stable and thus the 2-substituted product being favoured.
对于更高级的烷烃，取代可发生在不同碳位，生成卤代烷的异构体。例如，丙烷氯化同时生成 1-氯丙烷和 2-氯丙烷，其中二级自由基更稳定，因此 2-位取代产物占优势。
The relative stability of radicals follows the order: tertiary > secondary > primary > methyl. This is explained by the hyperconjugation and inductive effects of neighbouring alkyl groups, which help delocalise the unpaired electron.
自由基的相对稳定性顺序为：三级 > 二级 > 一级 > 甲基。邻近烷基的超共轭效应和诱导效应有助于分散未成对电子，从而解释了这一稳定性趋势。
8. Cracking: Thermal and Catalytic | 裂化：热裂化与催化裂化
Cracking converts long-chain alkanes from crude oil fractions into shorter, more useful hydrocarbons, including alkenes and branched alkanes. This is essential because the demand for petrol and short-chain alkenes is far higher than that for heavy residues.
裂化将原油分馏得到的长链烷烃转化为更短、更有用的烃类，包括烯烃和支链烷烃。由于市场对汽油和短链烯烃的需求远高于对重质残余物的需求，裂化工艺至关重要。
Thermal cracking uses high temperatures (700–1200 K) and high pressures to break C–C bonds homolytically, producing a high proportion of alkenes. Catalytic cracking employs a zeolite catalyst at lower temperatures (about 700 K) and modest pressure, favouring the formation of branched alkanes for high-octane petrol and cycloalkanes.
热裂化采用高温（700–1200 K）和高压，使 C–C 键发生均裂，产物中烯烃比例较高。催化裂化使用沸石催化剂，温度较低（约 700 K），压力适中，更有利于生成带分支的烷烃（用于高辛烷值汽油）和环烷烃。
The general equation for cracking a long-chain alkane can be represented as:
长链烷烃裂化的通式可表示为：
C₁₂H₂₆ → C₆H₁₄ + C₄H₈ + C₂H₄ (simplified example)
Because free radicals are intermediates, cracking also follows a radical mechanism, but at AS level the focus is on conditions, products, and the economic significance.
由于涉及自由基中间体，裂化同样遵循自由基机理，但在 AS 阶段主要关注反应条件、产物及其经济意义。
9. Environmental Impact of Alkane Use | 烷烃使用的环境影响
Burning alkane-based fuels releases CO₂, a major greenhouse gas that enhances the natural greenhouse effect and drives climate change. In addition, incomplete combustion emits carbon monoxide, which is a toxic air pollutant, and particulate carbon (soot) that contributes to respiratory problems and black carbon deposits.
燃烧烷烃燃料会释放 CO₂，这是一种主要温室气体，会加强天然温室效应并推动气候变化。此外，不完全燃烧会排放有毒空气污染物一氧化碳和颗粒碳（炭黑），引发呼吸系统疾病并造成黑色碳沉积。
Oxides of nitrogen (NOx) can form at the high temperatures of internal combustion engines when nitrogen and oxygen in the air react. These gases, together with unburned hydrocarbons, contribute to the formation of photochemical smog and acid rain.
内燃机的高温会使空气中的氮气和氧气反应生成氮氧化物 (NOx)。这些气体与未燃烧烃类共同参与光化学烟雾和酸雨的形成。
Catalytic converters on vehicles help reduce these emissions by oxidising CO to CO₂, reducing NOx to N₂, and converting unburned hydrocarbons to CO₂ and H₂O. A full understanding of these processes links alkane chemistry to real-world applications.
汽车上的催化转化器可将 CO 氧化为 CO₂、将 NOx 还原为 N₂、并将未燃烧烃转化为 CO₂ 和 H₂O，从而减少有害排放。充分理解这些过程有助于将烷烃化学与实际应用联系起来。
10. Exam-Style Tips and Common Mistakes | 考试技巧与常见错误
Tip 1 – Never confuse free-radical substitution with electrophilic addition. Alkanes undergo substitution because they are saturated; alkenes undergo addition because they possess a π bond. Students often write an addition mechanism for halogenation of methane, which is incorrect.
技巧一：切勿将自由基取代与亲电加成混淆。烷烃因饱和而进行取代反应；烯烃因含有 π 键而进行加成反应。学生常将甲烷的卤代写为加成机理，这是错误的。
Tip 2 – Always write UV light or heat above the arrow. The free-radical mechanism requires energy to break the halogen bond. Omitting the reaction condition is a common mark-loser.
技巧二：务必在箭头之上标明紫外光或加热。自由基机理需要能量来断裂卤素键。遗漏反应条件是常见的失分点。
Tip 3 – Show the single electron movement with half-headed (fishhook) arrows. In initiation, draw an arrow from the bond to each halogen atom. In propagation, show the halogen radical taking a hydrogen atom with a half-arrow, and the alkyl radical attacking the halogen molecule.
技巧三：用单钩箭头（半箭头）表示单电子转移。链引发步骤中，画一个箭头从共价键指向每个卤原子；链增长步骤中，用半箭头表示卤自由基夺取氢原子，再用半箭头表示烷基自由基进攻卤素分子。
Tip 4 – Be explicit about termination steps. Write as many possible radical combination equations as you can think of, and state that they remove radicals, stopping the chain.
技巧四：明确写出链终止步骤。尽可能写出所有可能的自由基结合方程式，并说明它们消耗了自由基，使链反应终止。
Tip 5 – Link properties to intermolecular forces. When explaining boiling point trends, always refer to van der Waals forces and molecular surface contact. Compare straight-chain and branched isomers using the same molecular formula.
技巧五：将性质与分子间力关联。解释沸点变化时，务必提及范德华力和分子接触面积。使用相同分子式的直链与支链异构体进行比较。
Tip 6 – Practice balancing combustion equations. Complete combustion questions are straightforward but often involve larger alkanes. Remember the general formula and double-check the oxygen coefficient.
技巧六：练习配平燃烧方程式。完全燃烧问题相对简单，但常涉及较高级烷烃。牢记通式并仔细核对氧气的系数。
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