📚 Alkanes: CIE A-Level Chemistry Exam-Focused Notes | 烷烃：CIE A-Level 化学考点精讲
Alkanes are the simplest family of hydrocarbons, consisting solely of carbon and hydrogen atoms linked by single bonds. In CIE A-Level Chemistry, a thorough understanding of their structure, nomenclature, isomerism, physical trends, and characteristic reactions – especially the free‑radical substitution mechanism – is essential. This revision guide walks you through every core concept and common exam pitfall, using clear explanations and side‑by‑side bilingual paragraphs.
烷烃是最简单的烃类家族，仅由碳氢原子以单键连接而成。在 CIE A‑Level 化学中，深入掌握烷烃的结构、命名、同分异构、物理性质趋势及其特征反应——尤其是自由基取代机理——至关重要。本考点精讲通过清晰的双语解说，带你梳理每一个核心概念和常见考试失分点。
1. General Formula and Homologous Series | 通式与同系物
Alkanes form a homologous series with the general molecular formula CₙH₂ₙ₊₂, where n is the number of carbon atoms. Each member differs from the next by a –CH₂– unit, leading to a gradual trend in physical properties. Being a homologous series means all alkanes share similar chemical behaviour, although reaction rates may vary with chain length.
烷烃构成同系物，通式为 CₙH₂ₙ₊₂，其中 n 为碳原子数。每相邻两个成员相差一个 –CH₂– 单元，导致物理性质呈现渐变趋势。同系物意味着所有烷烃具有相似的化学行为，但反应速率可能因碳链长度而异。
For straight‑chain alkanes, the first five members are methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), and pentane (C₅H₁₂). The bonding consists entirely of σ (sigma) bonds derived from sp³ hybridised carbon atoms, giving a tetrahedral geometry with bond angles of about 109.5°.
直链烷烃的前五个成员为甲烷 (CH₄)、乙烷 (C₂H₆)、丙烷 (C₃H₈)、丁烷 (C₄H₁₀) 和戊烷 (C₅H₁₂)。键合完全由 sp³ 杂化碳原子形成的 σ 键组成，呈四面体构型，键角约为 109.5°。
| n | Name / 名称 | Molecular formula | Structural formula / 结构简式 |
|---|---|---|---|
| 1 | Methane / 甲烷 | CH₄ | CH₄ |
| 2 | Ethane / 乙烷 | C₂H₆ | CH₃CH₃ |
| 3 | Propane / 丙烷 | C₃H₈ | CH₃CH₂CH₃ |
| 4 | Butane / 丁烷 | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
| 5 | Pentane / 戊烷 | C₅H₁₂ | CH₃(CH₂)₃CH₃ |
2. Naming Alkanes (IUPAC) | 烷烃命名（IUPAC）
IUPAC nomenclature for alkanes follows a systematic set of rules: identify the longest continuous carbon chain as the parent name, number the chain to give the lowest possible locants to substituents, and name alkyl side groups as prefixes. Common alkyl substituents include methyl (–CH₃), ethyl (–C₂H₅), propyl, and isopropyl. Multiple identical groups are indicated by prefixes like di‑, tri‑, tetra‑.
烷烃的 IUPAC 命名遵循系统规则：找出最长的连续碳链作为母体名称，给主链编号，使取代基获得最小的位次，并以烷基侧基作为前缀。常见烷基取代基包括甲基 (–CH₃)、乙基 (–C₂H₅)、丙基和异丙基。多个相同基团用二、三、四等前缀表示。
Exam tip: When two different substituents could be assigned the same number from either end, give priority to the group that comes first alphabetically (e.g., ethyl before methyl). Always write the name as a single word, using hyphens to separate numbers from words and commas between numbers.
考试提示：当两个不同取代基从两端编号可能得到相同位次时，优先使字母顺序靠前的基团编号较小（如 ethyl 先于 methyl）。名称整体为一个单词，数字与单词之间用连字符分隔，数字间用逗号。
For example, the molecule (CH₃)₂CHCH₂CH(CH₃)₂ is named 2,4‑dimethylpentane – not 2,4‑dimethylpentane with wrong numbering. Careful numbering ensures locants are as low as possible.
例如，分子 (CH₃)₂CHCH₂CH(CH₃)₂ 命名为 2,4‑二甲基戊烷，而不是编号错误的名称。仔细编号可确保位次尽可能低。
3. Isomerism in Alkanes | 烷烃的同分异构
Alkanes exhibit structural (constitutional) isomerism from butane (C₄H₁₀) onwards. The number of possible isomers increases rapidly with carbon count: butane has two isomers (n‑butane and isobutane), pentane has three, and hexane has five. Isomers differ in the arrangement of the carbon skeleton and thus have distinct physical properties, such as boiling points.
烷烃从丁烷 (C₄H₁₀) 起出现结构异构现象。可能的异构体数目随碳原子数迅速增加：丁烷有两种异构体（正丁烷和异丁烷），戊烷有三种，己烷有五种。异构体因碳骨架排列不同而具有不同的物理性质，如沸点。
Branched isomers tend to have lower boiling points than their straight‑chain counterparts because branching reduces the surface area for intermolecular contact, weakening van der Waals’ forces. In CIE exams, you may be asked to draw all constitutional isomers of a given molecular formula and name them systematically.
支链异构体的沸点通常低于直链异构体，因为支链减小了分子间接触的表面积，削弱了范德华力。在 CIE 考试中，你可能需要画出一个给定分子式的所有构造异构体，并系统命名。
Remember that cycloalkanes (CₙH₂ₙ) are not isomeric with alkanes of the same carbon count because they have a different general formula. Only compounds sharing the same molecular formula are considered isomers.
注意，环烷烃 (CₙH₂ₙ) 与同碳数烷烃并不互为异构体，因为通式不同。只有分子式相同的化合物才被称为异构体。
4. Physical Properties: Trends | 物理性质变化规律
The physical properties of alkanes are governed almost entirely by van der Waals’ (dispersion) forces. As the number of electrons increases with molecular size, the strength of these instantaneous dipole‑induced dipole interactions grows. This explains why boiling points, melting points, and viscosity rise smoothly along the homologous series from gases (C₁–C₄) through liquids (C₅–C₁₇) to solids (C₁₈ and above).
烷烃的物理性质几乎完全由范德华力（色散力）决定。随着分子尺寸增大、电子数增多，瞬时偶极‑诱导偶极作用的强度增强。这解释了为何沸点、熔点和粘度沿同系物平稳上升——从气态 (C₁–C₄) 到液态 (C₅–C₁₇)，再到固态 (C₁₈及以上)。
Branching reduces the boiling point because branched molecules cannot pack as closely as linear chains, decreasing the contact area and thus the strength of van der Waals forces. For example, 2,2‑dimethylpropane (neopentane) has a boiling point of about 9.5 °C, while n‑pentane boils at 36 °C.
支链降低沸点，因为支链分子无法像直链那样紧密堆积，接触面积减小，从而削弱了范德华力。例如，2,2‑二甲基丙烷（新戊烷）的沸点约为 9.5°C，而正戊烷在 36°C 沸腾。
Alkanes are essentially non‑polar and therefore insoluble in water but dissolve readily in non‑polar organic solvents such as hexane or benzene. Density is low (less than 1 g cm⁻³), making alkanes float on water.
烷烃基本上是非极性分子，因此不溶于水，但易溶于非极性有机溶剂，如己烷或苯。密度低（小于 1 g cm⁻³），故烷烃会浮在水面上。
5. Chemical Reactivity and Combustion | 化学性质与燃烧
Alkanes are relatively unreactive due to the strength of C–C and C–H σ bonds and their non‑polar nature. Their most important reactions are combustion and free‑radical substitution. Complete combustion in excess oxygen produces carbon dioxide and water, releasing a large amount of energy; this makes alkanes excellent fuels.
烷烃因 C–C 与 C–H σ 键的键能高且分子无极性，性质相对稳定。最重要的反应是燃烧和自由基取代。在过量氧气中完全燃烧生成二氧化碳和水，同时释放大量热量，使烷烃成为优质燃料。
CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O
Incomplete combustion, which occurs when the oxygen supply is limited, produces carbon monoxide (CO) and/or carbon (soot) along with water. CO is a toxic gas that binds irreversibly to haemoglobin, reducing the blood’s oxygen‑carrying capacity. Soot particles can cause respiratory problems and contribute to global dimming.
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。CO 是有毒气体，能与血红蛋白不可逆地结合，降低血液携氧能力。炭黑颗粒则会引起呼吸系统问题，并加剧全球变暗效应。
You should be able to write balanced equations for both complete and incomplete combustion. CIE exam questions often ask for the environmental consequences of the products formed.
你应能配平完全与不完全燃烧的方程式。CIE 考试常要求阐述产物的环境影响。
6. Free Radical Substitution: Chlorination of Methane | 自由基取代：甲烷氯化
The reaction between methane and chlorine, initiated by ultraviolet (UV) light or strong heating, is the classic example of a free‑radical substitution mechanism. The overall equation is:
甲烷与氯气在紫外光或强热引发下发生的反应，是自由基取代机理的经典范例。总反应方程式为：
CH₄ + Cl₂ → CH₃Cl + HCl (UV light)
The reaction proceeds through a chain mechanism involving homolytic bond fission – the breaking of a covalent bond where each atom takes one electron, forming two free radicals. Free radicals are highly reactive species carrying an unpaired electron.
该反应通过链式机理进行，涉及均裂——共价键断裂时每个原子各带走一个电子，形成两个自由基。自由基是带有未成对电子的高活性物种。
Although this is a substitution reaction, further chlorination can occur, giving a mixture of CH₂Cl₂, CHCl₃, and CCl₄ depending on the methane‑to‑chlorine ratio. Excess methane favours the monosubstituted product, while excess chlorine pushes the reaction towards polychlorinated products.
尽管这是取代反应，但会继续氯化，根据甲烷与氯气的比例生成 CH₂Cl₂、CHCl₃ 和 CCl₄ 的混合物。甲烷过量有利于单取代产物，氯气过量则推动反应向多氯代产物进行。
The mechanism is not restricted to chlorine; bromine can also be used, though its reaction is slower and more selective owing to the higher activation energy for the propagation step with Br₂.
该机理不仅适用于氯气，溴也可使用，但因 Br₂ 的增长步骤活化能较高，反应更慢且选择性更强。
7. Mechanism Steps: Initiation, Propagation, Termination | 机理步骤：引发、增长、终止
The free‑radical substitution mechanism consists of three distinct stages, which must be clearly distinguished in exam answers:
自由基取代机理包含三个不同阶段，考试作答时须明确区分：
Initiation: Homolytic fission of a chlorine molecule by UV light produces two chlorine atoms (chlorine free radicals).
引发：氯分子在紫外光下均裂，产生两个氯原子（氯自由基）。
Cl₂ → 2 Cl· (UV light)
Propagation: Two repeating steps that consume and regenerate free radicals, sustaining the chain. (i) A Cl· abstracts a hydrogen from methane, forming HCl and a methyl radical. (ii) The methyl radical attacks a Cl₂ molecule, producing chloromethane and another Cl·.
增长：两个反复进行的步骤，消耗并再生自由基，维持链反应。(i) Cl· 从甲烷中夺取一个氢原子，生成 HCl 和甲基自由基。(ii) 甲基自由基进攻 Cl₂ 分子，生成氯甲烷和另一个 Cl·。
CH₄ + Cl· → ·CH₃ + HCl
·CH₃ + Cl₂ → CH₃Cl + Cl·
Termination: Any two free radicals combine, terminating the chain. Possible terminations include the coupling of two Cl·, two methyl radicals, or one of each.
终止：任意两个自由基结合，使链反应停止。可能的终止方式包括两个 Cl· 结合、两个甲基自由基结合，或一个甲基自由基与一个 Cl· 结合。
Cl· + Cl· → Cl₂
·CH₃ + ·CH₃ → C₂H₆
·CH₃ + Cl· → CH₃Cl
Exam marks are commonly awarded for using the correct curly‑arrow representation is not required for free‑radical steps; instead, single‑barbed ‘half‑arrow’ or simply a dot representation is acceptable. Make sure to show the unpaired electron on the radical clearly using a dot.
考试中，自由基步骤不需使用卷曲箭头，用单钩“半箭头”或仅用点表示即可。务必用点清晰标出自由基上的未成对电子。
8. Further Substitution and Product Distribution | 进一步取代与产物分布
Once chloromethane is formed, it can undergo further substitution because its remaining C–H bonds are still susceptible to attack by chlorine radicals. Successive substitution leads to dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃, chloroform), and tetrachloromethane (CCl₄, carbon tetrachloride). The product distribution depends on the relative concentrations of the alkane and halogen.
氯甲烷一旦生成，其剩余的 C–H 键仍可被氯自由基进攻，从而发生进一步取代。逐步取代依次生成二氯甲烷 (CH₂Cl₂)、三氯甲烷 (CHCl₃，氯仿) 和四氯甲烷 (CCl₄，四氯化碳)。产物分布取决于烷烃与卤素的相对浓度。
For ethane or higher alkanes, substitution can occur at different carbon positions, leading to a mixture of structural isomers. The relative proportion of isomers is determined by a combination of statistical probability and the stability of the intermediate radical. Tertiary radicals are most stable, followed by secondary and primary; hence chlorination of longer alkanes often favours the more substituted product despite statistical factors.
对于乙烷及更高级烷烃，取代可发生在不同碳位上，产生结构异构体的混合物。异构体的相对比例由统计概率和中间体自由基的稳定性共同决定。叔自由基最稳定，仲自由基次之，伯自由基最不稳定；因此长链烷烃的氯化通常有利于取代程度更高的产物，尽管统计因素可能略有不符。
When answering questions about organic synthesis, it is important to recognise that free‑radical substitution is inherently poor for making a single pure product because of these competing pathways. Other methods, such as alkene addition, are preferred for halogenoalkane synthesis.
在回答有机合成相关问题时，应认识到由于存在竞争反应路径，自由基取代本质上不适合制取单一纯品。卤代烷的合成更宜采用烯烃加成等方法。
9. Cracking of Alkanes | 烷烃裂解
Cracking is the thermal decomposition of longer‑chain alkanes into shorter, more useful hydrocarbons. CIE distinguishes between thermal cracking (high temperature, high pressure, produces a high proportion of alkenes) and catalytic cracking (lower temperature and pressure using a zeolite catalyst, yields branched alkanes and aromatic compounds suitable for petrol blending).
裂解是将长链烷烃热分解为更短、更有用的烃类的过程。CIE 区分热裂解（高温高压，烯烃比例高）与催化裂解（使用沸石催化剂，较低的温度和压力，生成支链烷烃和芳香族化合物，适用于汽油调配）。
The general equation for cracking can be written as:
裂解的一般方程式可写为：
CₙH₂ₙ₊₂ → CₐH₂ₐ₊₂ + C₆H₂₆ (alkene)
Cracking is essential for matching the supply of petroleum fractions with market demand: long‑chain fractions such as fuel oil are cracked to produce shorter‑chain alkanes for petrol and alkenes for the chemical industry. Alkenes are particularly valuable as feedstocks for polymers, alcohols, and other chemicals.
裂解对于调节石油馏分的供需至关重要：将燃料油等长链馏分裂解，以生产用作汽油的短链烷烃和用于化工行业的烯烃。烯烃作为聚合物、醇类及其他化学品的原料，具有极高的附加值。
Catalytic cracking is favoured in industry because it uses less energy, produces higher‑octane branched hydrocarbons, and the catalyst can be regenerated. Conditions: approx. 450–500 °C and slight pressure, with a zeolite catalyst.
工业上倾向采用催化裂解，因为它能耗更低、生成高辛烷值的支链烃，且催化剂可再生。反应条件：约 450–500 °C，轻微加压，使用沸石催化剂。
10. Environmental and Safety Aspects | 环境与安全
Alkanes themselves, as components of crude oil and natural gas, pose environmental risks largely through their combustion products and through accidental spills. Carbon dioxide, the main product of complete combustion, is a major greenhouse gas contributing to climate change. Incomplete combustion releases carbon monoxide and particulate carbon, both dangerous to health and the environment.
烷烃作为原油和天然气的组成部分，其环境风险主要来自燃烧产物和意外泄漏。完全燃烧的主要产物二氧化碳是导致气候变化的主要温室气体。不完全燃烧释放的一氧化碳和炭黑颗粒对健康和环境均有危害。
Methane, the primary component of natural gas, is itself a potent greenhouse gas with a global warming potential many times that of CO₂ over a 20‑year period. Leaks from pipelines and flaring at oil wells contribute significantly to atmospheric methane levels.
天然气的主要成分甲烷本身是一种强效温室气体，其 20 年尺度的全球增温潜势远高于 CO₂。管道泄漏和油井燃烧废气向大气排放大量甲烷。
Safety aspects: Alkanes with low molecular mass are highly flammable and form explosive mixtures with air. Adequate ventilation, careful storage away from ignition sources, and the use of flame arrestors are necessary safety measures. In the laboratory, reactions of halogens with alkanes should be carried out in a fume cupboard because the halogen vapour and the hydrogen halide produced are toxic and corrosive.
安全方面：低分子量烷烃高度易燃，与空气形成爆炸性混合物。必须采取充分通风、远离点火源储存、使用阻火器等安全措施。在实验室中，卤素与烷烃的反应应在通风橱内进行，因为卤素蒸气及生成的卤化氢均有毒且具有腐蚀性。
CIE may ask you to evaluate the environmental impact of using alkanes as fuels and to suggest ways of minimising harm, such as catalytic converters for vehicle exhausts (oxidising CO to CO₂) or switching to renewable sources.
CIE 可能要求你评估使用烷烃作为燃料的环境影响，并提出减少危害的方法，例如为汽车尾气安装催化转化器（将 CO 氧化为 CO₂），或转向使用可再生能源。
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