📚 IGCSE OCR Chemistry: Alkanes Exam Essentials | IGCSE OCR 化学：烷烃 考点精讲
Alkanes are the simplest family of hydrocarbons, containing only single covalent bonds between carbon atoms. They are saturated compounds, meaning every carbon atom forms four single bonds, holding the maximum possible number of hydrogen atoms. In IGCSE OCR Chemistry, alkanes appear as a core topic under organic chemistry, and you need to master their general formula, naming rules, structural isomerism, physical trends, and their two major reactions: combustion and substitution with halogens.
烷烃是最简单的碳氢化合物家族，碳原子之间仅含有单共价键。它们是饱和化合物，意味着每个碳原子形成四个单键，连接了最大可能数目的氢原子。在 IGCSE OCR 化学中，烷烃是有机化学的核心主题，你需要掌握它们的通式、命名规则、结构异构、物理性质趋势，以及两个主要反应：燃烧和与卤素的取代反应。
1. What Are Alkanes? | 什么是烷烃？
Alkanes are a homologous series of hydrocarbons with the general formula CₙH₂ₙ₊₂. They contain only carbon and hydrogen atoms, and all carbon-carbon bonds are single covalent bonds. The term ‘saturated’ is key: because each carbon atom is bonded to the maximum number of atoms, alkanes cannot undergo addition reactions. Methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) are the first three members.
烷烃是一类同源系列的碳氢化合物，通式为 CₙH₂ₙ₊₂。它们只含碳和氢原子，所有碳-碳键均为单共价键。“饱和”这个术语很关键：由于每个碳原子都连接了最大数目的原子，烷烃不能发生加成反应。甲烷（CH₄）、乙烷（C₂H₆）和丙烷（C₃H₈）是前三个成员。
2. Homologous Series and General Formula | 同源系列与通式
A homologous series is a family of compounds with the same functional group, similar chemical properties, and a gradual change in physical properties. For alkanes, the functional group is essentially the C–C and C–H single bonds. Each successive member differs by a –CH₂– unit. The general formula CₙH₂ₙ₊₂ allows you to predict the molecular formula of any straight-chain or branched alkane provided you know the number of carbon atoms (n).
同源系列是一类具有相同官能团、相似化学性质且物理性质逐渐变化的化合物家族。对于烷烃，官能团本质上是 C–C 和 C–H 单键。每个后续成员相差一个 –CH₂– 单元。通式 CₙH₂ₙ₊₂ 使你能根据碳原子数（n）预测任何直链或支链烷烃的分子式。
3. Naming Straight-Chain Alkanes (IUPAC) | 直链烷烃的命名（IUPAC）
The names of straight-chain alkanes follow a simple prefix system based on the number of carbon atoms: meth- (1), eth- (2), prop- (3), but- (4), followed by the suffix -ane. For example, a 5-carbon alkane is pentane, 6 is hexane, 7 is heptane, and 8 is octane. You must be able to draw displayed formulae and write molecular formulae from these names.
直链烷烃的命名遵循基于碳原子数的简单前缀系统：meth- (1)、eth- (2)、prop- (3)、but- (4)，后接后缀 -ane。例如，5 个碳的烷烃是戊烷（pentane），6 个是己烷（hexane），7 个是庚烷（heptane），8 个是辛烷（octane）。你必须能够根据这些名称画出展示式并写出分子式。
4. Structural Isomerism in Alkanes | 烷烃的结构异构
Structural isomers are molecules that share the same molecular formula but have different structural arrangements of atoms. For alkanes, isomerism starts from butane (C₄H₁₀), which has two isomers: straight-chain butane and branched 2-methylpropane. As the carbon chain length increases, the number of possible isomers rises sharply. In exams, you may be asked to draw and name all isomers for a given molecular formula like C₅H₁₂ or C₆H₁₄.
结构异构体是指分子式相同但原子结构排列不同的分子。对于烷烃，异构现象从丁烷（C₄H₁₀）开始，它有两种异构体：直链丁烷和支链 2-甲基丙烷。随着碳链长度增加，可能的异构体数量急剧上升。考试中可能要求你画出并命名给定分子式（如 C₅H₁₂ 或 C₆H₁₄）的所有异构体。
5. Displayed Formulae and Skeletal Structures | 展示式与骨架结构
A displayed formula shows every atom and every bond in the molecule. For example, ethane is drawn as two carbon atoms each bonded to three hydrogens, with a single bond between the carbons. Skeletal formulae, which use lines to represent carbon-carbon bonds and omit hydrogen atoms, are not commonly required at IGCSE level, but you should recognise that each vertex represents a carbon atom with sufficient hydrogens to make four bonds.
展示式显示了分子中的每个原子和每个键。例如，乙烷画作两个碳原子各连接三个氢，碳原子间有一个单键。骨架式用线条表示碳-碳键并省略氢原子，在 IGCSE 阶段不常要求，但你应该认识到每个顶点代表一个碳原子，并带有足够的氢原子以满足四个键。
6. Physical Properties of Alkanes | 烷烃的物理性质
Alkanes are non-polar molecules because the electronegativity difference between carbon and hydrogen is very small. They are insoluble in water but dissolve in non-polar solvents. As the molecular size increases, the boiling point, melting point, and viscosity increase, while volatility decreases. Short-chain alkanes (1-4 carbons) are gases at room temperature, medium-chain (5-17 carbons) are liquids, and long-chain alkanes (18+ carbons) are waxy solids.
烷烃是非极性分子，因为碳和氢之间的电负性差异很小。它们不溶于水但可溶于非极性溶剂。随着分子增大，沸点、熔点和粘度升高，挥发性降低。短链烷烃（1-4 个碳）在室温下为气体，中链（5-17 个碳）为液体，长链烷烃（18+ 个碳）为蜡状固体。
7. Combustion Reactions | 燃烧反应
Alkanes burn in excess oxygen to produce carbon dioxide and water, releasing large amounts of energy. This complete combustion is exothermic. For example, the equation for propane combustion is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. In limited oxygen, incomplete combustion occurs, producing carbon monoxide (a toxic gas) or carbon (soot) along with water. You need to be able to write balanced symbol equations for both types of combustion.
烷烃在过量氧气中燃烧生成二氧化碳和水，释放大量能量。这种完全燃烧是放热反应。例如，丙烷燃烧的方程式为：C₃H₈ + 5O₂ → 3CO₂ + 4H₂O。在氧气不足时，发生不完全燃烧，生成一氧化碳（有毒气体）或碳（烟灰）以及水。你需要能够书写这两种燃烧类型的配平符号方程式。
8. Substitution Reaction with Halogens | 与卤素的取代反应
Alkanes react with halogens (chlorine or bromine) in the presence of ultraviolet light to form haloalkanes and hydrogen halides. This is a photochemical substitution reaction because a hydrogen atom is replaced by a halogen atom. For instance, methane reacts with chlorine: CH₄ + Cl₂ → CH₃Cl + HCl. The reaction can continue, replacing further hydrogens. The requirement for UV light is a key condition often tested.
烷烃在紫外光存在下与卤素（氯或溴）反应生成卤代烷和卤化氢。这是一个光化学取代反应，因为一个氢原子被卤素原子取代。例如，甲烷与氯气反应：CH₄ + Cl₂ → CH₃Cl + HCl。该反应可继续进行，取代更多的氢原子。需要紫外光这一条件是常考的关键点。
9. Cracking and Alkanes | 裂化与烷烃
Although cracking is a reaction of larger alkane molecules, it is essential to link it to your alkane knowledge. Cracking breaks long-chain alkanes into smaller, more useful alkanes and alkenes. The reaction requires heat and a catalyst. Alkenes formed are unsaturated and can be identified by bromine water turning from orange to colourless, whereas alkanes do not decolorise bromine water. This allows you to distinguish between an alkane and an alkene.
尽管裂化是较大烷烃分子的反应，但将其与你的烷烃知识联系起来至关重要。裂化将长链烷烃分解为更小、更有用的烷烃和烯烃。该反应需要加热和催化剂。生成的烯烃是不饱和的，可通过溴水从橙色变为无色来鉴别，而烷烃不能使溴水褪色。这使你能区分烷烃和烯烃。
10. Sources and Uses of Alkanes | 烷烃的来源和用途
Alkanes are obtained from crude oil and natural gas. Natural gas is mainly methane, used for heating and cooking. LPG (liquefied petroleum gas) contains propane and butane, used in camping gas and heating. Gasoline (petrol) contains alkanes from C₅ to C₁₀, used as fuel for internal combustion engines. Longer-chain alkanes are used in diesel, lubricating oils, and waxes. Being the main components of fossil fuels, alkanes are central to the energy industry.
烷烃来自原油和天然气。天然气主要是甲烷，用于取暖和烹饪。液化石油气（LPG）含有丙烷和丁烷，用于露营燃气和供暖。汽油含有 C₅ 至 C₁₀ 的烷烃，用作内燃机的燃料。长链烷烃用于柴油、润滑油和蜡。作为化石燃料的主要成分，烷烃在能源工业中处于核心地位。
11. Common Exam Pitfalls | 常见考试陷阱
Many students lose marks by not balancing combustion equations correctly, especially with larger alkanes like octane. Another mistake is writing the general formula as CₙH₂ₙ, confusing alkanes with alkenes. When naming isomers, missing the branch position number or failing to identify the longest continuous carbon chain can cost marks. Also, remember that the substitution reaction requires UV light; saying ‘light’ or ‘heat’ alone is not enough for full credit.
许多学生因未能正确配平燃烧方程式而失分，尤其是对于辛烷这样较大的烷烃。另一个错误是把通式写成 CₙH₂ₙ，混淆了烷烃和烯烃。在命名异构体时，遗漏支链位置编号或未能识别最长连续碳链也会导致失分。此外，请记住取代反应需要紫外光；仅写出“光照”或“加热”不足以获得满分。
12. Summary and Exam Tips | 总结与考试技巧
To master alkanes for your IGCSE OCR Chemistry exam, memorise the general formula, practice naming and drawing the first eight straight-chain alkanes, and be confident drawing structural isomers up to C₆H₁₄. Write balanced equations for complete and incomplete combustion, and describe the substitution mechanism with UV light. Use bromine water to distinguish alkanes from alkenes. Alkanes are saturated, undergo substitution, and are major fuels. Approach each question systematically, showing all your working steps.
为了在 IGCSE OCR 化学考试中掌握烷烃，请牢记通式，练习命名和画出前八个直链烷烃，并自信地画出 C₆H₁₄ 以内的结构异构体。书写完全和不完全燃烧的配平方程式，并描述在紫外光下的取代机理。使用溴水区分烷烃和烯烃。烷烃是饱和的，发生取代反应，是主要燃料。有条理地处理每道题，展示所有解题步骤。
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