Alkanes | 烷烃考点精讲

📚 Alkanes | 烷烃考点精讲

Alkanes are a fundamental topic in IGCSE CCEA Chemistry. They are the simplest family of hydrocarbons, containing only carbon and hydrogen atoms joined by single covalent bonds. Understanding alkanes is essential for grasping organic chemistry, as they form the basis for fuels, plastics, and many other important substances. In this revision guide, we will cover the key points you need to know about alkanes for your exam, including their structure, properties, reactions, and uses.

烷烃是IGCSE CCEA化学中的基础主题。它们是最简单的碳氢化合物家族，仅由碳和氢原子通过单键共价结合而成。理解烷烃对于掌握有机化学至关重要，因为它们是燃料、塑料及许多其他重要物质的基础。在本复习指南中，我们将涵盖烷烃的考试重点，包括其结构、性质、反应和用途。

1. What are Alkanes? | 什么是烷烃？

Alkanes are organic compounds made up exclusively of carbon and hydrogen atoms. They are called saturated hydrocarbons because all the bonds between carbon atoms are single covalent bonds, and each carbon atom is bonded to the maximum possible number of hydrogen atoms. This saturation makes alkanes relatively unreactive compared to alkenes or other functional groups, although they do undergo combustion and substitution reactions under suitable conditions. Methane (CH₄) is the simplest alkane, followed by ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).

烷烃是完全由碳和氢原子组成的有机化合物。它们被称为饱和烃，因为碳原子之间的所有键都是单键共价键，每个碳原子结合了尽可能多的氢原子。这种饱和特性使得烷烃相对于烯烃或其他官能团而言较不活泼，但在合适条件下它们仍可发生燃烧和取代反应。甲烷 (CH₄) 是最简单的烷烃，其次是乙烷 (C₂H₆)、丙烷 (C₃H₈) 和丁烷 (C₄H₁₀)。

2. Homologous Series and General Formula | 同系物与通式

Alkanes form a homologous series, a family of compounds with the same general formula, similar chemical properties, and a gradual trend in physical properties. The general formula for alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms. This means that for each additional carbon atom, two extra hydrogen atoms are added to the molecular structure. As members of the same homologous series, all alkanes share typical reactions such as combustion and substitution with halogens. The regular increase in molecular size leads to predictable changes in boiling point, viscosity and flammability.

烷烃构成一个同系物系列，这是一组具有相同通式、相似化学性质以及物理性质呈现规律性变化的化合物家族。烷烃的通式为 CₙH₂ₙ₊₂，其中 n 代表碳原子数。这意味着每增加一个碳原子，分子结构中便增加两个氢原子。作为同一同系物的成员，所有烷烃共享典型的反应，例如燃烧和与卤素的取代反应。分子大小的规律增大导致沸点、粘度和可燃性发生可预测的变化。

3. Naming and Molecular Formulae of the First Four Alkanes | 前四种烷烃的命名与分子式

The IGCSE CCEA syllabus requires you to know the names, molecular formulae and structures of the first four straight-chain alkanes. Methane is CH₄, ethane is C₂H₆, propane is C₃H₈ and butane is C₄H₁₀. The naming follows the prefix indicating the number of carbons (meth- = 1, eth- = 2, prop- = 3, but- = 4) followed by the suffix ‘-ane’ for saturated hydrocarbons. You must be able to write these molecular formulae correctly and derive the molecular formula of any alkane given the number of carbon atoms by using the general formula CₙH₂ₙ₊₂.

IGCSE CCEA 大纲要求你掌握前四种直链烷烃的名称、分子式和结构。甲烷为 CH₄，乙烷为 C₂H₆，丙烷为 C₃H₈，丁烷为 C₄H₁₀。命名遵循表示碳原子数的前缀（meth- 表示1，eth- 表示2，prop- 表示3，but- 表示4），后跟表示饱和烃的后缀 ‘-ane’。你必须能够正确书写这些分子式，并能利用通式 CₙH₂ₙ₊₂ 根据碳原子数推导出任何烷烃的分子式。

4. Displayed and Structural Formulae | 展示式与结构式

A displayed formula shows every atom and every bond in a molecule. For alkanes, this involves drawing all C–H and C–C single bonds. For example, the displayed formula of ethane shows two carbon atoms linked by a single bond, with three hydrogen atoms bonded to each carbon. A structural formula is a simplified representation that omits the C–H bonds and writes them as groups, e.g. CH₃–CH₃ for ethane and CH₃–CH₂–CH₃ for propane. In CCEA exams, you may be asked to draw or interpret both displayed and structural formulae, so practice drawing these for methane, ethane, propane and butane.

展示式显示分子中的每一个原子和每一个键。对于烷烃，这涉及到画出所有 C–H 和 C–C 单键。例如，乙烷的展示式显示两个碳原子通过一个单键连接，每个碳原子结合三个氢原子。结构式是一种简化表示，省略了 C–H 键并将它们写成基团，例如乙烷写作 CH₃–CH₃，丙烷写作 CH₃–CH₂–CH₃。在 CCEA 考试中，你可能需要绘制或解释展示式和结构式，因此请练习绘制甲烷、乙烷、丙烷和丁烷的这两种式子。

5. Structural Isomerism in Alkanes | 烷烃的结构异构

Structural isomers are molecules that have the same molecular formula but different structural arrangements of atoms. For alkanes, isomerism becomes possible when there are four or more carbon atoms. Butane (C₄H₁₀) has two isomers: the straight-chain butane (CH₃CH₂CH₂CH₃) and the branched isomer called methylpropane, with the structural formula CH₃CH(CH₃)CH₃. These isomers have different physical properties such as boiling points because the shape of the molecule affects the strength of intermolecular forces. The straight-chain isomer has a higher boiling point than the branched isomer due to greater surface contact and stronger London dispersion forces.

结构异构体是指分子式相同但原子的结构排列方式不同的分子。对于烷烃，当碳原子数达到四个或更多时，异构现象成为可能。丁烷 (C₄H₁₀) 有两种异构体：直链丁烷 (CH₃CH₂CH₂CH₃) 和带支链的异构体甲基丙烷，结构式为 CH₃CH(CH₃)CH₃。这些异构体具有不同的物理性质，例如沸点，因为分子的形状会影响分子间作用力的强度。直链异构体由于分子间接触面更大、伦敦色散力更强，其沸点高于支链异构体。

6. Physical Properties and Trends | 物理性质及变化规律

As you move up the homologous series from methane to butane and beyond, the physical properties of alkanes change gradually. The boiling points and melting points increase with increasing molecular mass because larger molecules have stronger London (instantaneous dipole-induced dipole) forces between them, requiring more energy to separate. At room temperature, methane, ethane, propane and butane are all colourless gases. Alkanes with longer chains (more than four carbons) become volatile liquids, and those with very long chains are waxy solids. Viscosity also increases with chain length, while flammability decreases because the percentage of carbon by mass rises, making complete combustion more difficult.

随着同系物从甲烷向上到丁烷乃至更长碳链，烷烃的物理性质呈渐变趋势。沸点和熔点随分子质量增加而升高，因为较大的分子之间具有更强的伦敦力（瞬时偶极-诱导偶极力），分离它们需要更多能量。在室温下，甲烷、乙烷、丙烷和丁烷都是无色气体。具有更长碳链（超过四个碳）的烷烃变成挥发性液体，而碳链很长的烷烃则为蜡状固体。粘度也随链长增加而增大，而可燃性则下降，因为碳的质量百分比升高，使完全燃烧变得更加困难。

7. Alkanes as Fuels – Complete Combustion | 作为燃料 – 完全燃烧

Alkanes are widely used as fuels because they release a large amount of energy when burned in oxygen – a reaction called complete combustion. In a plentiful supply of oxygen, alkanes burn to produce carbon dioxide and water. The general word equation is: alkane + oxygen → carbon dioxide + water. For example, the balanced chemical equation for the complete combustion of methane is:

烷烃被广泛用作燃料，因为它们在氧气中燃烧时释放大量能量——该反应称为完全燃烧。在氧气供应充足的情况下，烷烃燃烧生成二氧化碳和水。一般文字方程式为：烷烃 + 氧气 → 二氧化碳 + 水。例如，甲烷完全燃烧的配平化学方程式为：

CH₄ + 2O₂ → CO₂ + 2H₂O

Similarly, the complete combustion of propane follows:

类似地，丙烷的完全燃烧方程式为：

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

You should be able to write and balance complete combustion equations for any given alkane up to butane.

你应该能够为给定烷烃（碳数不超过丁烷）书写并配平完全燃烧方程式。

8. Incomplete Combustion and Its Hazards | 不完全燃烧及其危害

Incomplete combustion occurs when there is a limited or insufficient supply of oxygen. Under these conditions, alkanes may produce carbon monoxide (CO) and/or carbon (soot) in addition to carbon dioxide and water. Carbon monoxide is a colourless, odourless and highly toxic gas that binds to haemoglobin in red blood cells, preventing them from carrying oxygen around the body. This can lead to asphyxiation and death. Soot consists of unburnt carbon particles that can cause respiratory problems and blacken surfaces. The incomplete combustion of methane can be represented as:

当氧气供应受限或不足时，发生不完全燃烧。在此条件下，烷烃除了生成二氧化碳和水外，还可能产生一氧化碳 (CO) 和/或碳（烟灰）。一氧化碳是一种无色、无味且剧毒的气体，能与红细胞中的血红蛋白结合，阻止其向全身输送氧气，从而导致窒息和死亡。烟灰由未燃烧的碳微粒组成，可引发呼吸系统问题并使表面变黑。甲烷的不完全燃烧可表示为：

2CH₄ + 3O₂ → 2CO + 4H₂O

or with even less oxygen:

或氧气更少时：

CH₄ + O₂ → C + 2H₂O

In the exam, you may be asked to identify the poisonous gas produced during incomplete combustion, so always remember CO.

在考试中，你可能会被要求指出不完全燃烧产生的有毒气体，因此务必记住 CO。

9. Substitution Reaction with Halogens | 与卤素的取代反应

Alkanes react with halogens such as chlorine and bromine in the presence of ultraviolet (UV) light. In this substitution reaction, one or more hydrogen atoms in the alkane are replaced by halogen atoms. For methane and chlorine, the overall reaction producing chloromethane is:

烷烃在紫外光 (UV) 存在下与氯、溴等卤素反应。在这种取代反应中，烷烃中的一个或多个氢原子被卤素原子取代。对于甲烷和氯气，生成氯甲烷的总反应为：

CH₄ + Cl₂ → CH₃Cl + HCl

The reaction does not stop at this stage; further substitution can replace more hydrogen atoms, yielding dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃) and tetrachloromethane (CCl₄). A similar reaction occurs with bromine, forming bromomethane and hydrogen bromide. In the laboratory, we can show the need for UV light by keeping one test tube in the dark and another in sunlight; only the mixture exposed to UV light shows a colour change as the halogen is consumed. This reaction is typical of saturated hydrocarbons and contrasts with the addition reaction of alkenes.

该反应不会在此阶段停止；进一步的取代可替换更多的氢原子，生成二氯甲烷 (CH₂Cl₂)、三氯甲烷 (CHCl₃) 和四氯甲烷 (CCl₄)。溴也发生类似反应，生成溴甲烷和溴化氢。在实验室中，可通过将一支试管避光放置、另一支置于阳光下来展示对紫外光的需要；只有暴露在紫外光下的混合物会因卤素被消耗而褪色。该反应是饱和烃的典型反应，与烯烃的加成反应形成对比。

10. Reaction Conditions and Mechanism Overview | 反应条件与机理概述

It is important to remember that the substitution reaction of alkanes with halogens requires ultraviolet light to initiate. Without UV light, even long exposure in the dark does not cause a noticeable reaction at room temperature. The CCEA specification does not require detailed radical mechanism knowledge at IGCSE level, but you should be aware that the UV light provides the energy to break the halogen–halogen bond homolytically, forming highly reactive halogen atoms that drive the chain reaction. The overall process is a photochemical substitution. The hydrogen halide gas produced turns blue litmus paper red, indicating its acidic nature.

务必记住，烷烃与卤素的取代反应需要紫外光引发。没有紫外光，即使在暗处长时间暴露，室温下也不会引发明显反应。CCEA 大纲在 IGCSE 阶段不要求详细的自由基机理知识，但你应该知道紫外光提供能量使卤素-卤素键发生均裂，形成高活性的卤素原子，从而驱动链式反应。整个过程是光化学取代。产生的卤化氢气体可使蓝色石蕊试纸变红，表明其酸性。

11. Sources of Alkanes: Fractional Distillation | 烷烃的来源：分馏

The main natural source of alkanes is crude oil (petroleum), a finite resource formed over millions of years from the remains of marine organisms. Crude oil is a complex mixture of hydrocarbons, mostly alkanes of various chain lengths. It is separated into useful fractions by fractional distillation in a fractionating column. The column is hot at the bottom and cooler at the top. Smaller alkanes with low boiling points, such as methane, ethane, propane and butane, rise to the top where they condense as gases (refinery gases). Liquid fractions like petrol, kerosene and diesel are collected at lower levels in the column, while heavy fuel oil and bitumen, which have long-chain alkanes, are drawn off near the bottom. Each fraction has a specific range of carbon chain lengths and boiling points, and is used for different purposes, from domestic heating to transportation fuels.

烷烃的主要天然来源是原油（石油），这是一种经过数百万年由海洋生物遗骸形成的不可再生资源。原油是碳氢化合物的复杂混合物，其中大部分为不同链长的烷烃。通过分馏塔中的分馏，原油被分离成有用的馏分。塔底部较热，顶部较冷。沸点低的小分子烷烃，如甲烷、乙烷、丙烷和丁烷，升至塔顶冷凝为气体（炼厂气）。汽油、煤油和柴油等液体馏分在塔内较低位置收集，而含长链烷烃的重燃料油和沥青则在接近底部处引出。每种馏分具有特定的碳链长度和沸点范围，用于从家庭取暖到运输燃料的不同用途。

12. Environmental Issues Related to Alkane Use | 与烷烃使用相关的环境问题

The combustion of alkanes as fuels contributes to several environmental problems. Complete combustion produces carbon dioxide (CO₂), a greenhouse gas that contributes to global warming and climate change. Incomplete combustion releases carbon monoxide, which is toxic to humans and animals, and unburnt hydrocarbons that participate in the formation of photochemical smog. Impurities in fossil fuels can also lead to the release of sulfur dioxide, causing acid rain. Furthermore, the extraction, transportation and accidental spillage of crude oil can cause devastating pollution of marine and terrestrial ecosystems. As oil is a finite resource, there is growing pressure to develop renewable energy sources and improve fuel efficiency. In the CCEA exam, you should be able to discuss these environmental impacts and propose measures such as using catalytic converters, developing alternative fuels and promoting conservation.

烷烃作为燃料的燃烧导致若干环境问题。完全燃烧产生二氧化碳 (CO₂)，这是一种导致全球变暖和气候变化的温室气体。不完全燃烧释放有毒的一氧化碳，以及参与形成光化学烟雾的未燃烧碳氢化合物。化石燃料中的杂质还可能导致二氧化硫的排放，从而引起酸雨。此外，原油的开采、运输和意外泄漏可对海洋和陆地生态系统造成毁灭性污染。由于石油是不可再生资源，开发可再生能源和提高燃料效率的压力日益增大。在 CCEA 考试中，你应能够讨论这些环境影响，并提出诸如使用催化转化器、开发替代燃料和提倡节能等措施。

Published by TutorHao | Chemistry Revision Series | aleveler.com

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