📚 Alkanes in IB Chemistry | IB 化学：烷烃 考点精讲
Alkanes form the simplest family of hydrocarbons, consisting entirely of carbon and hydrogen atoms linked by single bonds. In IB Chemistry, they serve as the foundation for understanding organic nomenclature, isomerism, physical trends, and fundamental reaction mechanisms such as free‑radical substitution. Mastery of alkanes is essential for tackling more complex topics, from polymerisation to biochemistry, and appears regularly in both Paper 1 and Paper 2 examinations.
烷烃是最简单的碳氢化合物家族，完全由碳和氢以单键连接而成。在 IB 化学中，烷烃是掌握有机命名、异构现象、物理性质趋势以及基本反应机理（如自由基取代）的基础。学好烷烃对于攻克更复杂的主题（从聚合到生物化学）至关重要，并且在试卷一和试卷二中经常出现。
1. Introduction to Alkanes | 烷烃简介
Alkanes are saturated hydrocarbons, meaning every carbon atom forms four single covalent bonds, achieving a tetrahedral geometry with bond angles of approximately 109.5°. They are relatively unreactive compared to other organic families, which is why they were historically called paraffins (from Latin parum affinis, meaning 'little affinity'). Their main commercial importance lies in their use as fuels and as starting materials for the chemical industry.
烷烃是饱和烃，意味着每个碳原子都形成四个单共价键，呈四面体构型，键角约为 109.5°。与其他有机家族相比，它们的反应活性较低，因此在历史上被称为石蜡烃（源自拉丁语 parum affinis，意为“亲和力小”）。烷烃的主要商业价值在于用作燃料和化学工业的原料。
2. General Formula and Homologous Series | 通式与同系物
The alkanes form a homologous series described by the general formula CnH2n+2, where n is the number of carbon atoms. A homologous series is a family of compounds in which each member differs from the next by a –CH₂– unit. Members of the series share similar chemical properties and show a gradual trend in physical properties, such as boiling points and viscosity.
烷烃构成一个同系物系列，通式为 CnH2n+2，其中 n 代表碳原子数。同系物是这样一族化合物：每相邻两个成员相差一个 –CH₂– 单元。系列成员具有相似的化学性质，并在物理性质（如沸点和黏度）上呈现渐变趋势。
The first ten straight‑chain alkanes are essential knowledge for IUPAC naming. Methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂), hexane (C₆H₁₄), heptane (C₇H₁₆), octane (C₈H₁₈), nonane (C₉H₂₀), and decane (C₁₀H₂₂). You should be able to write their molecular, structural and skeletal formulae.
前十种直链烷烃是 IUPAC 命名的基础知识。甲烷 (CH₄)、乙烷 (C₂H₆)、丙烷 (C₃H₈)、丁烷 (C₄H₁₀)、戊烷 (C₅H₁₂)、己烷 (C₆H₁₄)、庚烷 (C₇H₁₆)、辛烷 (C₈H₁₈)、壬烷 (C₉H₂₀) 和癸烷 (C₁₀H₂₂)。同学应能够书写它们的分子式、结构式和骨架式。
| Name (英文名) | 中文名 | Molecular formula | Condensed formula |
|---|---|---|---|
| Methane | 甲烷 | CH₄ | CH₄ |
| Ethane | 乙烷 | C₂H₆ | CH₃CH₃ |
| Propane | 丙烷 | C₃H₈ | CH₃CH₂CH₃ |
| Butane | 丁烷 | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
| Pentane | 戊烷 | C₅H₁₂ | CH₃(CH₂)₃CH₃ |
3. Nomenclature of Alkanes | 烷烃的命名
IUPAC rules for branched alkanes require identifying the longest continuous carbon chain as the parent name. Number the chain from the end nearest the first branch to give substituents the lowest possible locants. Branches are named as alkyl groups (e.g., methyl –CH₃, ethyl –C₂H₅) and listed alphabetically, with prefixes di‑, tri‑, tetra‑ used for multiple identical substituents. Commas separate numbers, hyphens separate numbers from words.
支链烷烃的 IUPAC 命名规则要求找出最长的连续碳链作为母体名称。从最靠近第一个支链的一端开始给主链编号，以使取代基具有尽可能小的位次。支链命名为烷基（如甲基 –CH₃、乙基 –C₂H₅），并按字母顺序列出；当有多个相同取代基时使用前缀 di‑、tri‑、tetra‑。逗号分隔数字，连字符分隔数字与文字。
For example, the isomer of hexane with a methyl group on the second carbon is named 2‑methylpentane, not 3‑methylpentane, because numbering gives a lower locant (2 versus 3). Common naming pitfalls include mistaking the longest chain or ignoring alphabetical order when listing ethyl before methyl.
例如，己烷的一个异构体在第二个碳上带有一个甲基，命名为 2‑甲基戊烷，而不是 3‑甲基戊烷，因为这样编号位次更小（2 与 3 相比）。常见的命名错误包括找错最长链，或未按字母顺序排列（如乙基应排在甲基之前）。
4. Structural Isomerism in Alkanes | 烷烃的结构异构
Alkanes exhibit structural (constitutional) isomerism from butane (C₄H₁₀) onwards. Structural isomers have the same molecular formula but different connectivity of atoms. For C₄H₁₀, two isomers exist: straight‑chain butane and branched 2‑methylpropane (isobutane). The number of possible isomers increases rapidly with chain length; pentane (C₅H₁₂) has three isomers – pentane, 2‑methylbutane and 2,2‑dimethylpropane.
烷烃从丁烷 (C₄H₁₀) 开始出现结构（构造）异构现象。结构异构体具有相同的分子式，但原子的连接方式不同。对于 C₄H₁₀，存在两种异构体：直链丁烷和带支链的 2‑甲基丙烷（异丁烷）。可能的异构体数目随碳链增长而迅速增加；戊烷 (C₅H₁₂) 有三种异构体——戊烷、2‑甲基丁烷和 2,2‑二甲基丙烷。
IB exam questions frequently ask you to draw and name all isomers of a given alkane formula. You must ensure each structure is unique – rotated or flipped versions of the same skeleton are not different isomers. Pay attention to the fact that isomers have distinct physical properties (e.g., boiling points) because of differences in molecular shape and packing.
IB 考题常要求画出并命名给定烷烃分子式的全部异构体。必须确保每个结构都是独一无二的——同一骨架的旋转或翻转版本并不算不同的异构体。注意，由于分子形状和堆积方式的不同，异构体具有不同的物理性质（如沸点）。
5. Physical Properties of Alkanes | 烷烃的物理性质
The physical properties of alkanes are determined by van der Waals' (London dispersion) forces. As the relative molecular mass increases, the number of electrons increases, strengthening these temporary‑dipole‑induced‑dipole interactions. Consequently, boiling point, melting point and viscosity rise smoothly with chain length. Methane to butane are gases at room temperature; pentane to hexadecane are liquids; higher alkanes are waxy solids.
烷烃的物理性质由范德华力（伦敦色散力）决定。随着相对分子质量增加，电子数增多，这些瞬时偶极‑诱导偶极相互作用增强。因此，沸点、熔点和黏度随链长平稳升高。甲烷至丁烷在室温下为气体；戊烷至十六烷为液体；更高级的烷烃为蜡状固体。
Branching lowers the boiling point because branched molecules have a more spherical shape with reduced surface area for intermolecular contact, weakening the van der Waals forces. For instance, pentane (straight) boils at 36 °C, while 2,2‑dimethylpropane (highly branched) boils at only 9.5 °C. Alkanes are non‑polar and insoluble in water but dissolve readily in non‑polar solvents such as benzene and hexane.
支链化会降低沸点，因为支链分子形状更接近球形，分子间接触的表面积减小，削弱了范德华力。例如，直链戊烷的沸点为 36 °C，而高度支链化的 2,2‑二甲基丙烷沸点仅为 9.5 °C。烷烃是非极性分子，不溶于水，但易溶于苯和己烷等非极性溶剂。
6. Sources and Fractional Distillation | 来源与分馏
Alkanes are primarily obtained from crude oil and natural gas. Crude oil is a complex mixture of hydrocarbons that is separated into useful fractions by fractional distillation in a tall column with a temperature gradient. Smaller, lighter alkanes condense near the top of the column (cooler), while larger, heavier molecules condense lower down (hotter). Key fractions include refinery gas, gasoline, naphtha, kerosene, diesel, lubricating oil and bitumen.
烷烃主要来自原油和天然气。原油是一种复杂的碳氢化合物混合物，通过分馏塔（具有温度梯度的柱）分离为有用的馏分。较小、较轻的烷烃在塔顶（较冷处）冷凝，而较大、较重的分子在较低处（较热处）冷凝。重要的馏分包括炼厂气、汽油、石脑油、煤油、柴油、润滑油和沥青。
Understanding fractional distillation explains why different alkanes are used for different purposes: lighter fractions for bottled gas, gasoline for internal combustion engines, kerosene for aviation, and diesel for heavy vehicles. The process relies on differences in boiling point ranges, not chemical reactions.
理解分馏可以解释为什么不同的烷烃用于不同用途：较轻的馏分用于瓶装燃气，汽油用于内燃机，煤油用于航空，柴油用于重型车辆。该过程依靠沸点范围的差异，而非化学反应。
7. Combustion Reactions | 燃烧反应
Alkanes undergo complete combustion in a plentiful supply of oxygen, producing carbon dioxide and water vapour while releasing large amounts of energy. The general equation for complete combustion can be written as: 2CnH2n+2 n+2) H₂O. For example, propane burns completely according to: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.
烷烃在充足的氧气中发生完全燃烧，生成二氧化碳和水蒸气，同时释放大量能量。完全燃烧的通式可写为：2CnH2n+2 + (3n+1) O₂ → 2n CO₂ + (2n+2) H₂O。例如，丙烷的完全燃烧：C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O。
ation depends on the conditions; for propane, a possible incomplete combustion is: C₃H₈ + 3½ O₂ → 3 CO + 4 H₂O, although mixtures of CO and C are often produced. Incomplete combustion significantly reduces energy output and creates serious safety hazards.
不完全燃烧发生在氧气供应不足时，产生有毒的一氧化碳 (CO) 和碳黑形式的未燃碳。方程式随条件而异；对丙烷而言，一种可能的不完全燃烧为：C₃H₈ + 3½ O₂ → 3 CO + 4 H₂O，尽管通常同时产生 CO 和 C 的混合物。不完全燃烧会显著降低能量输出，并带来严重的安全隐患。
8. Free Radical Substitution (Halogenation) | 自由基取代反应（卤代）
. This is a substitution reaction in which a hydrogen atom is replaced by a halogen atom, forming a haloalkane and a hydrogen halide. For instance, methane reacts with chlorine under UV light to form chloromethane and hydrogen chloride: CH₄ + Cl₂ → CH₃Cl + HCl (with UV light).
烷烃在紫外光或高温下与卤素（氯或溴）发生自由基链式反应。这是一种取代反应，氢原子被卤素原子取代，生成卤代烷和卤化氢。例如，甲烷在紫外光下与氯气反应生成氯甲烷和氯化氢：CH₄ + Cl₂ → CH₃Cl + HCl（需紫外光）。
The reaction does not stop at monosubstitution; further substitution can produce dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃) and tetrachloromethane (CCl₄). To favour monohalogenation, an excess of alkane is used. Bromine reacts more slowly and selectively than chlorine, preferentially substituting tertiary hydrogens over secondary and primary hydrogens.
反应不会停留在单取代阶段；进一步取代可生成二氯甲烷 (CH₂Cl₂)、三氯甲烷 (CHCl₃) 和四氯甲烷 (CCl₄)。为有利于一卤代，需使用过量的烷烃。溴的反应比氯慢，且选择性更高，优先取代叔氢而非仲氢和伯氢。
The overall reaction can be summarised as: R–H + X₂ → R– + HX, where X = Cl, Br. IB questions may ask you to write equations for the possible multi‑ oducts of a given alkane or to explain why UV light is necessary to initiate the reaction.
总反应可概括为：R–H + X₂ → R–X + HX，其中 X = Cl, Br。IB 试题可能要求写出给定烷烃可能的多取代产物方程式，或解释为何需要紫外光来引发反应。
9. Mechanism of Free Radical Substitution | 自由基取代机理
The radical chain mechanism comprises three stages: initiation, propagation and termination. Initiation ght, producing two highly reactive halogen atoms (free radicals). For chlorine: Cl₂ → 2 Cl•. Each chlorine atom has an unpaired electron, making it an electrophilic radical.
自由基链式机理包括三个阶段：链引发、链增长和链终止。链引发：卤素分子在紫外光下发生均裂，生成两个高反应活性的卤素原子（自由基）。以氯气为例：Cl₂ → 2 Cl•。每个氯原子带有一个未成对电子，成为亲电自由基。
Propagation acts a hydrogen atom from the alkane, producing a hydrogen halide and an alkyl radical: Cl• + CH₄ → • l attacks a chlorine molecule, forming the haloalkane and regenerating a chlorine radical: •CH₃ + Cl₂ → CH₃Cl + Cl•. These two steps repeat many times.
链增长步骤是一对使链式反应持续进行而不净消耗自由基的反应。首先，一个氯自由基从烷烃中夺取一个氢原子，生成卤化氢和一个烷基自由基：Cl• + CH₄ → •CH₃ + HCl。然后，甲基自由基攻击一个氯分子，生成卤代烷并再生一个氯自由基：•CH₃ + Cl₂ → CH₃Cl + Cl•。这两个步骤可多次循环。
Termination ule, thereby removing radicals from the system. Possible termination reactions include: Cl• + Cl• → Cl₂; •CH₃ + Cl• → CH₃Cl; and •CH₃ + • is a relatively rare event compared to propagation because radical concentrations are low.
链终止发生在两个自由基结合形成稳定分子时，从而将自由基从体系中移除。可能的终止反应包括：Cl• + Cl• → Cl₂； •CH₃ + Cl• → CH₃Cl；以及 •CH₃ + •CH₃ → C₂H₆（乙烷）。与链增长相比，链终止较为少见，因为自由基浓度较低。
IB Higher Level students must be able to write the full mechanism for methane or ethane halogenation, using curly half‑arrows (or fish‑hooks) to show single‑electron movements. Practice distinguishing between homolytic fission and heterolytic fission, as alkanes only undergo homolytic bond breaking in these reactions.
IB 高水平学生必须会书写甲烷或乙烷卤代的完整机理，使用弯半箭头（鱼钩箭头）表示单电子转移。练习区分均裂与异裂，因为烷烃在这些反应中只发生均裂断键。
10. Environmental Considerations | 环境考量
The combustion of alkanes as fossil fuels has profound environmental consequences. Complete combustion produces carbon dioxide, a major greenhouse gas contributing to global warming and climate change. Incomplete combustion generates carbon monoxide, which binds irreversibly to haemoglobin, reducing the blood's oxygen‑carrying capacity, and unburnt carbon particulates that contribute to respiratory illnesses and smog.
烷烃作为化石燃料燃烧对环境具有深远影响。完全燃烧产生二氧化碳，这是一种主要温室气体，导致全球变暖和气候变化。不完全燃烧生成一氧化碳（与血红蛋白不可逆结合，降低血液携氧能力）和未燃碳颗粒物（导致呼吸系统疾病和烟雾）。
In addition, the release of unburned volatile hydrocarbons from vehicle exhausts and industrial processes contributes to photochemical smog formation in the troposphere. Oxides of nitrogen, formed at high engine temperatures, react with hydrocarbons in the presence of sunlight to produce ground‑level ozone, peroxyacetyl nitrate (PAN) and other harmful oxidants. Catalytic converters in cars oxidise CO and unburnt hydrocarbons to CO₂ and water while reducing NOₓ to N₂.
此外，车辆尾气和工业过程释放的未燃烧挥发性烃类参与了对流层光化学烟雾的形成。在发动机高温下生成的氮氧化物，在阳光下与烃类反应，产生地面臭氧、过氧乙酰硝酸酯 (PAN) 和其他有害氧化剂。汽车尾气催化转化器将 CO 和未燃烃氧化为 CO₂ 和水，同时将 NOₓ 还原为 N₂。
IB exam questions often link alkane chemistry to green chemistry principles, asking you to evaluate the advantages and disadvantages of using alkanes as fuels and to suggest ways of minimising their environmental impact, such as using catalytic converters, improving combustion efficiency, and developing renewable alternatives.
IB 考试常将烷烃化学与绿色化学原理联系起来，要求你评价使用烷烃作为燃料的优缺点，并提出减少其环境影响的方法，例如使用催化转化器、提高燃烧效率以及开发可再生替代品。
11. Summary of Key Points | 考点总结
- General formula: CnH2n+2; saturated; tetrahedral geometry. 通式：CnH2n+2；饱和烃；四面体构型。
- Nomenclature: Longest chain rule; lowest locants; alphabetical listing of alkyl substituents. 命名：最长链规则；最低位次；烷基取代基按字母顺序排列。
- Isomerism: Structural isomers from C₄H₁₀; physical properties differ due to van der Waals forces and branching. 异构现象：从 C₄H₁₀ 起出现结构异构；因范德华力和支链化不同导致物理性质差异。
- Boiling points: Increase with chain length, decrease with branching. 沸点：随链长升高，随支链化降低。
- Combustion: Complete → CO₂ + H₂O; incomplete → CO/C + H₂O. 燃烧：完全燃烧生成 CO₂ + H₂O；不完全燃烧生成 CO/C + H₂O。
- Free‑radical substitution: UV light; initiation, propagation, termination steps; mixture of products. 自由基取代：需紫外光；链引发、链增长、链终止步骤；产物为混合物。
- Mechanism: Homolytic fission; Cl₂ → 2 Cl•; propagation loops; termination by radical combination. 机理：均裂；Cl₂ → 2 Cl•；链增长循环；自由基结合终止。
- Environmental issues: CO₂ as greenhouse gas, CO toxicity, photochemical smog from unburnt hydrocarbons. 环境问题：CO₂ 为温室气体，CO 的毒性，未燃烃导致光化学烟雾。
By integrating the facts, patterns and reaction mechanisms above, you will be well prepared to tackle any IB Chemistry question on alkanes, whether it tests recall, application or synthesis of concepts.
通过整合上述事实、规律和反应机理，你将能够从容应对任何关于烷烃的 IB 化学题目，无论考查的是记忆、应用还是概念的综合。
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