AS Chemistry 4.2 Alkanes: Core Principles | AS化学4.2 烷烃核心原理

📚 AS Chemistry 4.2 Alkanes: Core Principles | AS化学4.2 烷烃核心原理

Alkanes are the simplest family of hydrocarbons, containing only carbon and hydrogen atoms bonded by single covalent bonds. They are saturated compounds with the general formula CₙH₂ₙ₊₂. This unit explores their structure, naming, physical trends, and the characteristic chemical reaction – free radical substitution – which underpins much of organic synthesis. Understanding alkanes lays the foundation for grasping more complex organic chemistry at AS level.

烷烃是最简单的烃类家族，仅由碳和氢原子以单共价键结合而成。它们是饱和化合物，通式为CₙH₂ₙ₊₂。本单元探讨烷烃的结构、命名、物理性质变化趋势，以及标志性的化学反应——自由基取代反应，这是有机合成的重要基础。理解烷烃为掌握AS阶段更复杂的有机化学奠定了根基。

1. Introduction to Alkanes | 烷烃简介

Alkanes are aliphatic saturated hydrocarbons. The term ‘saturated’ means they contain only C–C and C–H single bonds, with no double or triple bonds. They form an homologous series, where each member differs from the next by a –CH₂– group. Methane (CH₄) is the simplest alkane, followed by ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). They occur naturally in crude oil and natural gas and serve as fuels and chemical feedstocks.

烷烃是脂肪族饱和烃。“饱和”意味着它们只含有C–C和C–H单键，没有双键或三键。它们构成一个同系列，每个成员之间相差一个–CH₂–基团。甲烷（CH₄）是最简单的烷烃，其次是乙烷（C₂H₆）、丙烷（C₃H₈）和丁烷（C₄H₁₀）。它们天然存在于原油和天然气中，用作燃料和化工原料。

2. General Formula and Homologous Series | 通式与同系列

The general molecular formula for straight-chain alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms (n ≥ 1). Cycloalkanes, which are ring structures, have the general formula CₙH₂ₙ. In an homologous series, physical properties change gradually with chain length, while chemical properties remain similar because of the same functional group – for alkanes, the saturated C–C and C–H bonds.

直链烷烃的通式为CₙH₂ₙ₊₂，其中n为碳原子数（n ≥ 1）。环烷烃是环状结构，通式为CₙH₂ₙ。在同系列中，物理性质随链长逐渐变化，而化学性质因具有相同的官能团（烷烃的饱和C–C和C–H键）而保持相似。

3. Nomenclature of Alkanes | 烷烃的命名

IUPAC rules are used to name alkanes. Identify the longest continuous carbon chain to assign the parent name (e.g., meth-, eth-, prop-, but-, pent-, hex-). Number the chain to give substituents the lowest possible locants. Prefixes (methyl, ethyl, etc.) and numbers indicate side chains. Multiple identical substituents use di-, tri-, tetra-. For example, 2,2-dimethylbutane has a four-carbon main chain with two methyl groups on carbon-2.

烷烃采用IUPAC命名法。找出最长的连续碳链作为母体名称（如甲、乙、丙、丁、戊、己）。将链编号，使取代基获得最小的位次。用前缀（甲基、乙基等）和数字标明支链。多个相同取代基使用二、三、四等词头。例如，2,2-二甲基丁烷的主链为四个碳，2号碳上有两个甲基。

4. Structural Isomerism | 结构异构

Alkanes with four or more carbon atoms exhibit chain isomerism. Butane (C₄H₁₀) has two isomers: straight-chain n-butane and branched 2-methylpropane (isobutane). Pentane (C₅H₁₂) has three. As the carbon number increases, the number of possible isomers grows rapidly. Isomers have different physical properties (e.g., boiling point) but similar chemical behaviour because the functional group – saturated C–C/C–H bonds – remains the same.

含四个或以上碳原子的烷烃会出现链异构现象。丁烷（C₄H₁₀）有两种异构体：直链正丁烷和支链2-甲基丙烷（异丁烷）。戊烷（C₅H₁₂）有三种。随着碳数增加，可能的异构体数目迅速增长。异构体的物理性质（如沸点）不同，但由于官能团（饱和C–C/C–H键）一致，化学行为相似。

5. Physical Properties of Alkanes | 烷烃的物理性质

Alkanes are non-polar molecules, so only weak van der Waals’ forces exist between them. Boiling point and melting point increase with molecular size because larger molecules have more electrons and stronger instantaneous dipole–induced dipole attractions. Branching lowers boiling point: branched isomers have less surface contact, resulting in weaker intermolecular forces than their straight-chain counterparts. Alkanes are insoluble in water but soluble in non-polar organic solvents.

烷烃是非极性分子，分子间仅存在微弱的范德华力。沸点和熔点随分子增大而升高，因为更大的分子拥有更多电子，瞬时偶极–诱导偶极作用更强。支链会降低沸点：支链异构体的分子接触面积较小，分子间力弱于相应的直链异构体。烷烃不溶于水，但可溶于非极性有机溶剂。

6. Chemical Reactivity: General Features | 化学活性：总体特点

Alkanes are generally unreactive due to the strength and non-polar nature of C–C and C–H bonds. They do not react with acids, bases, oxidising agents, or reducing agents under ordinary conditions. However, they undergo two main types of reactions: combustion (oxidation) in excess oxygen, and substitution reactions with halogens under ultraviolet light. The high bond enthalpy and low polarity make the C–H bond very stable.

烷烃通常化学性质不活泼，因为C–C和C–H键的键能高且为非极性键。在通常条件下，它们不与酸、碱、氧化剂或还原剂反应。但它们能进行两种主要类型的反应：在过量氧气中燃烧（氧化），以及在紫外光照下与卤素发生取代反应。高键能和低极性使C–H键十分稳定。

7. Combustion Reactions | 燃烧反应

Complete combustion of alkanes in excess oxygen produces carbon dioxide and water, releasing a large amount of energy. This makes them excellent fuels.

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion occurs when oxygen supply is limited, producing carbon monoxide (CO) or even solid carbon (soot). Carbon monoxide is a toxic, odourless gas that binds haemoglobin. Soot causes respiratory problems and reduces efficiency of engines. The general equation for complete combustion of an alkane is:

CₙH₂ₙ₊₂ + ( 3n+1 / 2 ) O₂ → nCO₂ + (n+1)H₂O

烷烃在过量氧气中完全燃烧生成二氧化碳和水，并释放大量能量，因此它们是优良的燃料。

CH₄ + 2O₂ → CO₂ + 2H₂O

当氧气供应有限时发生不完全燃烧，产生一氧化碳（CO），甚至固体碳（烟灰）。一氧化碳是一种有毒无味的气体，能与血红蛋白结合。烟灰会引起呼吸系统问题，并降低发动机效率。烷烃完全燃烧的通式为：

CₙH₂ₙ₊₂ + ( 3n+1 / 2 ) O₂ → nCO₂ + (n+1)H₂O

8. Free Radical Substitution Reaction | 自由基取代反应

Alkanes react with halogens (chiefly Cl₂ and Br₂) in the presence of ultraviolet (UV) light or heat, undergoing substitution of hydrogen atoms by halogen atoms. This is a photochemical reaction. The overall equation for methane chlorination is:

CH₄ + Cl₂ → CH₃Cl + HCl

The reaction proceeds by a free radical mechanism, which involves three stages: initiation, propagation, and termination. It is a chain reaction because the propagation steps regenerate the reactive radical that sustains the cycle.

烷烃在紫外光或加热条件下与卤素（主要是Cl₂和Br₂）反应，氢原子被卤素原子取代。这是一个光化学反应。甲烷氯化的总反应式为：

CH₄ + Cl₂ → CH₃Cl + HCl

反应通过自由基机理进行，包括三个阶段：引发、增长和终止。它是一个连锁反应，因为增长步骤会再生出维持循环的活性自由基。

9. Mechanism of Halogenation | 卤代反应机理

Initiation: UV light provides energy to break the Cl–Cl bond homolytically, generating two chlorine radicals. Each atom takes one electron from the bond pair.

Cl₂ → 2Cl·

Propagation: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical (CH₃·). The methyl radical then reacts with a Cl₂ molecule, producing chloromethane and another Cl· radical, which continues the chain.

CH₄ + Cl· → ·CH₃ + HCl

·CH₃ + Cl₂ → CH₃Cl + Cl·

Termination: Two radicals combine to form a stable molecule, ending the chain. Possible termination steps for methane chlorination include:

Cl· + Cl· → Cl₂

·CH₃ + Cl· → CH₃Cl

·CH₃ + ·CH₃ → C₂H₆

引发：紫外光提供能量使Cl–Cl键发生均裂，生成两个氯自由基。每个原子从键对中取得一个电子。

Cl₂ → 2Cl·

增长：一个氯自由基从甲烷中夺取一个氢原子，生成HCl和甲基自由基（·CH₃）。甲基自由基再与Cl₂分子反应，生成氯甲烷和另一个Cl·自由基，使链反应继续。

CH₄ + Cl· → ·CH₃ + HCl

·CH₃ + Cl₂ → CH₃Cl + Cl·

终止：两个自由基结合形成稳定分子，使链终止。甲烷氯化的可能终止步骤包括：

Cl· + Cl· → Cl₂

·CH₃ + Cl· → CH₃Cl

·CH₃ + ·CH₃ → C₂H₆

10. Problems with Free Radical Substitution | 自由基取代反应的问题

Further substitution can occur because the product chloromethane still contains C–H bonds that can be attacked by Cl· radicals. This leads to a mixture of mono-, di-, tri-, and tetra-substituted products (e.g., CH₂Cl₂, CHCl₃, CCl₄). The reaction is difficult to control when a single chlorinated product is desired. Additionally, with larger alkanes, substitution may occur at different positions, producing structural isomers. Because of this, radical substitution is less useful for targeted synthesis, though it remains an important industrial process for making chlorinated solvents.

由于产物氯甲烷仍含有可被Cl·自由基进攻的C–H键，可能发生进一步取代。这导致生成一取代、二取代、三取代和四取代产物的混合物（如CH₂Cl₂、CHCl₃、CCl₄）。当希望获得单一氯化产物时反应难以控制。此外，对于较大烷烃，取代可发生在不同位置，生成结构异构体。因此，自由基取代在定向合成中用处有限，但它仍是制造氯化溶剂的重要工业过程。

11. Uses and Environmental Aspects | 用途与环境影响

Alkanes are vital as fuels – methane (natural gas), propane and butane (LPG), and petrol (a mixture of liquid alkanes) power domestic heating, cooking, and internal combustion engines. They are also used as lubricants, solvents, and as starting materials for cracking to produce alkenes. However, combustion releases CO₂, a greenhouse gas, and incomplete combustion produces toxic CO and particulates. The extraction and transport of fossil fuels also carry environmental risks, such as oil spills and methane leakage, which is a potent greenhouse gas.

烷烃作为燃料至关重要——甲烷（天然气）、丙烷和丁烷（液化石油气）以及汽油（液态烷烃混合物）用于家庭取暖、烹饪和内燃机。它们也用作润滑油、溶剂，并作为裂解生产烯烃的原料。然而，燃烧会释放温室气体CO₂，不完全燃烧还会产生有毒的CO和颗粒物。化石燃料的开采与运输也带有环境风险，如石油泄漏和甲烷泄漏，后者是一种强烈的温室气体。

12. Summary and Revision Focus | 总结与复习要点

• General formula: CₙH₂ₙ₊₂; saturated hydrocarbons; homologous series.
  通式：CₙH₂ₙ₊₂；饱和烃；同系列。
• Naming: IUPAC rules, longest chain, lowest locant numbers, prefixes.
  命名：IUPAC规则，最长链，最低位次，前缀。
• Isomerism: Chain isomers from C₄ onwards; differing boiling points.
  异构：从丁烷开始的链异构；不同的沸点。
• Physical trends: Boiling point increases with chain length; branching lowers boiling point; insoluble in water.
  物理性质趋势：沸点随链长增加；支链降低沸点；不溶于水。
• Chemical reactions: Combustion (complete to CO₂ + H₂O, incomplete to CO, C); free radical substitution with Cl₂/Br₂ in UV light.
  化学反应：燃烧（完全燃烧生成CO₂和H₂O，不完全燃烧生成CO和C）；在紫外光下与Cl₂/Br₂发生自由基取代。
• Free radical mechanism: Initiation (homolytic fission), propagation (two steps regenerating the Cl· radical), termination (radical combination). Draw stepwise mechanisms clearly.
  自由基机理：引发（均裂），增长（再生Cl·自由基的两步），终止（自由基结合）。清晰画出分步机理。
• Limitations: Mixture of products; lack of selective substitution; environmental concerns from combustion and fuel extraction.
  局限性：产物混合物；缺乏选择性取代；燃烧和燃料提取带来的环境问题。

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