📚 Alkanes in IB Chemistry | Key Concepts and Exam Tips | IB 化学：烷烃 考点精讲
Alkanes are the simplest saturated hydrocarbons, yet they form the backbone of organic chemistry in the IB syllabus. Understanding their structure, nomenclature, physical properties, and key reactions (especially combustion and free-radical substitution) is essential for both Standard Level and Higher Level students. This article systematically breaks down the core concepts, common pitfalls, and exam-focused strategies to help you master alkanes.
烷烃是最简单的饱和烃，却是 IB 有机化学的基础。掌握其结构、命名、物理性质以及关键反应（尤其是燃烧和自由基取代），对 Standard Level 和 Higher Level 学生都至关重要。本文将系统拆解核心概念、常见误区与应试策略，助你彻底攻克烷烃考点。
1. Structure and Saturation | 结构与饱和性
Alkanes have the general formula CₙH₂ₙ₊₂ and consist only of carbon and hydrogen atoms connected exclusively by single covalent bonds (σ bonds). Each carbon atom is sp³ hybridised, leading to a tetrahedral geometry with bond angles of approximately 109.5°. Because alkanes contain no multiple bonds, they are described as saturated hydrocarbons.
烷烃通式为 CₙH₂ₙ₊₂，仅由碳氢原子以单共价键（σ 键）连接。每个碳原子为 sp³ 杂化，呈正四面体构型，键角约 109.5°。因不含重键，烷烃被称作饱和烃。
- Every C atom forms four single bonds, making alkanes relatively unreactive compared to alkenes.
- 每个碳原子都形成四根单键，使烷烃与烯烃相比相对不活泼。
- Molecular shape is determined by the tetrahedral arrangement; long-chain alkanes adopt zigzag conformations.
- 分子形状由四面体排布决定；长链烷烃采取锯齿状构象。
2. Homologous Series and General Trends | 同系物与一般变化规律
Alkanes form a homologous series in which each successive member differs by a –CH₂– unit. As the molecular mass increases, the physical properties change gradually: boiling points, melting points, and viscosity increase while volatility decreases. This arises from stronger London dispersion forces between larger molecules.
烷烃形成同系列，相邻成员相差一个 –CH₂– 单元。随着分子量增大，物理性质逐渐变化：沸点、熔点、黏度升高，挥发性降低。这是因为分子间伦敦色散力随分子增大而增强。
- Methane (CH₄) to butane (C₄H₁₀) are gases; C₅H₁₂ to C₁₆H₃₄ are liquids; higher alkanes are waxy solids at room temperature.
- 甲烷至丁烷为气体；C₅H₁₂ 至 C₁₆H₃₄ 为液体；更高级烷烃室温下为蜡状固体。
- Branching lowers boiling point because the molecules become more compact, reducing surface area and intermolecular contact.
- 支链会降低沸点，因为分子更紧凑，表面积和分子间接触减少。
3. IUPAC Nomenclature | IUPAC 命名法
Correct systematic naming is a core skill examined repeatedly. The basic steps: identify the longest continuous carbon chain (parent name), number the chain to give the substituents the lowest possible locants, name alkyl branches, and assemble the name alphabetically with prefixes (di-, tri-, etc.) ignored in alphabetising but used to indicate multiple identical groups.
正确的系统命名是反复考查的核心技能。基本步骤：确定最长的连续碳链（母体名）；编号使取代基位次最小；命名烷基支链；按字母顺序组合名称，前缀（二、三等）不参与字母排序，但用于表示多个相同基团。
| Alkyl group | Structure | Name |
| –CH₃ | methyl | 甲基 |
| –C₂H₅ | ethyl | 乙基 |
| –C₃H₇ | propyl | 丙基 |
Example: 2,3-dimethylpentane. The longest chain is 5 carbons (pentane); two methyl groups are attached at positions 2 and 3.
例如：2,3-二甲基戊烷。最长链为 5 个碳（戊烷）；两个甲基连在 2 和 3 位。
Common mistake: failing to recognise that numbering must start from the end nearest the first substituent, not necessarily from left to right.
常见错误：未从最靠近第一个取代基的一端开始编号，而习惯性地从左向右读。
4. Conformational Isomerism | 构象异构
Free rotation around C–C single bonds allows alkanes to adopt different conformations. The two extreme conformers are staggered and eclipsed. Staggered conformation is more stable due to minimal electron repulsion between C–H bonds on adjacent carbons; eclipsed conformation represents an energy maximum. IB often focuses on the concept of torsional strain, not requiring detailed energy profiles for simple alkanes but using Newman projections to illustrate stability.
C–C 单键可自由旋转，使烷烃可采取不同构象。两种极限构象为交叉式和重叠式。交叉式构象因相邻碳上 C–H 键间电子排斥最小而更稳定；重叠式对应能量极大值。IB 常聚焦扭转张力的概念，不要求简单烷烃的详细能量曲线，但会使用纽曼投影表示稳定性。
For ethane, the staggered conformation is preferred; the energy barrier between staggered and eclipsed is about 12 kJ mol⁻¹.
对乙烷而言，交叉式更占优势；交叉式与重叠式之间的能垒约 12 kJ mol⁻¹。
5. Physical Properties and Solubility | 物理性质与溶解性
Alkanes are non-polar molecules; their only intermolecular forces are London dispersion forces. These forces become stronger with increasing molecular size and surface area. As a result, higher alkanes have higher melting and boiling points. Alkanes are insoluble in water (a polar solvent) but dissolve readily in non-polar solvents such as hexane or benzene.
烷烃为非极性分子，分子间仅存在伦敦色散力。该作用力随分子尺寸和表面积增大而增强，因此高级烷烃的熔、沸点更高。烷烃不溶于水（极性溶剂），但易溶于己烷或苯等非极性溶剂。
Branching reduces the effectiveness of intermolecular contacts, so branched-chain isomers have lower boiling points than their straight-chain counterparts.
支链会削弱分子间接触效果，因此支链异构体的沸点低于直链异构体。
6. Combustion Reactions | 燃烧反应
Alkanes undergo complete combustion to produce carbon dioxide and water, releasing large amounts of energy. This is a major exam topic: writing balanced equations, interpreting enthalpy changes, and comparing fuel efficiencies.
烷烃可完全燃烧生成二氧化碳和水，同时释放大量能量。这是主要考点：书写配平方程式、解释焓变、比较燃料效率。
CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion occurs with limited oxygen, yielding carbon monoxide and/or carbon (soot). Carbon monoxide is toxic; soot reduces efficiency and causes respiratory problems.
氧气不足时发生不完全燃烧，生成一氧化碳和（或）碳（黑烟）。一氧化碳有毒；黑烟降低效率并引发呼吸系统问题。
IB may ask you to calculate the CO₂ emissions per unit energy released or to discuss environmental impacts such as global warming and acid rain (though SO₂ is not produced from pure alkanes, the term is often used in broader fossil fuel contexts).
IB 可能要求计算单位能量释放的 CO₂ 排放量，或讨论全球变暖等环境影响（尽管纯烷烃不产生 SO₂，但在化石燃料语境下常被提及）。
7. Free-Radical Substitution (Halogenation) | 自由基取代反应（卤代）
This is the key reaction mechanism for alkanes. It involves three stages — initiation, propagation, and termination — and is triggered by UV light or heat. The classic example is chlorination of methane.
这是烷烃的关键反应机理，包含链引发、链增长、链终止三个阶段，由紫外光或加热引发。经典实例为甲烷的氯化。
Initiation: Cl₂ → 2Cl• (homolytic fission)
链引发：Cl₂ → 2Cl•（均裂）
Propagation: Cl• + CH₄ → HCl + •CH₃, then •CH₃ + Cl₂ → CH₃Cl + Cl•
链增长：Cl• + CH₄ → HCl + •CH₃，接着 •CH₃ + Cl₂ → CH₃Cl + Cl•
Termination: possible combinations: 2Cl• → Cl₂, 2•CH₃ → C₂H₆, Cl• + •CH₃ → CH₃Cl
链终止：可能的结合：2Cl• → Cl₂，2•CH₃ → C₂H₆，Cl• + •CH₃ → CH₃Cl
Note that the product is a mixture of mono-, di-, and tri-substituted alkanes due to further substitution. IB questions often ask why the reaction gives a mixture and how the reactivity of different hydrogen atoms (primary, secondary, tertiary) affects product distribution.
注意，产物为包含一取代、二取代、三取代在内的混合物，因为反应会继续进行。IB 常问及为何得到混合物，以及不同氢原子（伯、仲、叔）的反应活性如何影响产物分布。
8. Reactivity Patterns and Selectivity | 反应活性规律与选择性
In radical substitution, bromination is more selective than chlorination. Bromine radicals are less reactive and therefore more discriminating, favouring substitution at the most stable radical intermediate. The relative stability of alkyl radicals follows the trend: tertiary > secondary > primary > methyl.
在自由基取代中，溴代比氯代选择性更高。溴自由基活性较低，辨别力更强，倾向在最稳定的自由基中间体上取代。烷基自由基的相对稳定性顺序为：叔＞仲＞伯＞甲基。
This explains why 2-bromopropane is the major product in bromination of propane, while chlorination gives a more even mixture. You should be able to use bond dissociation enthalpy data to rationalise selectivity.
这解释了为何丙烷溴代主产物为 2-溴丙烷，而氯代产物比例更均匀。你应能运用键解离焓数据合理解释选择性。
9. Environmental and Health Aspects | 环境与健康层面
Alkanes, especially methane, are potent greenhouse gases. Unburnt hydrocarbons contribute to photochemical smog and ground-level ozone formation. Complete combustion reduces these issues but generates CO₂, a major climate forcer. IB syllabi ask you to evaluate the use of alkane fuels and their alternatives (e.g., hydrogen, biofuels) based on energy density, renewability, and environmental impact.
烷烃（尤其是甲烷）是强效温室气体。未燃烧的碳氢化合物参与光化学烟雾和地面臭氧的形成。完全燃烧可减少此类问题，但产生的 CO₂ 是主要的气候强迫因子。IB 大纲要求基于能量密度、可再生性和环境影响评价烷烃燃料及其替代品（如氢能、生物燃料）。
Additionally, liquid alkanes pose aspiration hazards and can cause respiratory damage if inhaled. The safe handling and storage of alkanes are linked to their volatility and flammability.
此外，液态烷烃有吸入性毒性风险，误吸可致呼吸系统损伤。安全运输和储存与其挥发性和易燃性密切相关。
10. Experimental Work and Data Analysis | 实验操作与数据分析
Typical IB practical tasks include measuring heats of combustion of alcohols and alkanes, investigating the effect of chain length on boiling point, or studying the rate of radical bromination. You must be adept at using calorimetry data to calculate enthalpy changes and at evaluating errors (heat loss, incomplete combustion, etc.).
典型 IB 实验包括测量醇与烷烃的燃烧热、探究链长对沸点的影响，或研究自由基溴代速率。你应熟练使用量热数据计算焓变，并评估误差（热损耗、不完全燃烧等）。
Graphing boiling points against molar mass for straight-chain versus branched isomers is a common data-based question. Expect to interpret trends and relate them to intermolecular forces.
绘制直链与支链异构体沸点-摩尔质量关系图是常见的数据分析题。预期需解读趋势并结合分子间作用力阐述。
11. Common Misconceptions and Exam Tips | 常见误区与应试技巧
Many students confuse 'saturated' with 'saturated solution' or think alkanes react via addition like alkenes. Remember: alkanes undergo substitution, not addition. Another common error is drawing curly arrows in radical mechanisms — IB generally only uses curly arrows for polar mechanisms; radical mechanisms use fishhook arrows (half-headed). Stick to IB's preferred representation.
许多学生将“饱和”与“饱和溶液”混淆，或误认为烷烃可像烯烃一样发生加成反应。牢记：烷烃发生取代反应，而非加成。另一常见错误是在自由基机理中画卷曲箭头——IB 通常曲线箭头仅用于极性机理，自由基机理使用鱼钩箭头（单箭头）。请遵循 IB 偏好的表达方式。
When naming, always check for the longest chain, even if it is not drawn linearly. Use molecular model kits or visualisation to avoid misreading condensed formulas.
命名时务必检查最长碳链，即便它未画成直线。使用分子模型或形象化手段避免误解缩简式。
12. Practice and Past Paper Analysis | 真题演练与备考指南
IB past papers often include multiple-choice questions on homologous series trends, structural isomers of C₅H₁₂, and the mechanism of methane chlorination. Data-based questions require you to analyse boiling point tables and deduce the effect of branching. Interpretation of mass spectra for alkanes (fragmentation patterns) is also common at HL.
IB 历年真题常包含同系物递变规律、C₅H₁₂ 结构异构以及甲烷氯化机理的选择题。数据分析题要求分析沸点表格并推断支链效应。HL 层次还常见烷烃质谱碎裂规律解读。
For extended response questions, ensure you can write balanced combustion equations for any given alkane, explain the free-radical mechanism stepwise with correct fishhook arrows, and discuss the relative stability of radicals using hyperconjugation or bond dissociation energies.
对于问答题，确保能为任意给定烷烃书写配平燃烧方程式，用正确的鱼钩箭头逐步解释自由基机理，并借助超共轭或键解离能讨论自由基的相对稳定性。
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