A-Level Chemistry: Typical Chemical Reactions of Alkanes | A-Level 化学：烷烃的典型化学反应

📚 A-Level Chemistry: Typical Chemical Reactions of Alkanes | A-Level 化学：烷烃的典型化学反应

Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They are often described as ‘chemically inert’ because they react with very few reagents under normal conditions. However, their typical reactions—combustion, free radical substitution, cracking, and reforming—are central to both industrial chemistry and A-Level examinations. This article provides a comprehensive and exam-focused review of these characteristic reactions, complete with mechanisms, conditions, and common pitfalls.

烷烃是通式为 CₙH₂ₙ₊₂ 的饱和烃。由于在常温下能与烷烃反应的试剂极少，它们常被描述为”化学惰性”。然而，其典型反应——燃烧、自由基取代、裂化和重整——既是工业化学的核心，也是 A-Level 考试的重点。本文将全面且紧扣考点地梳理这些特征反应，包括机理、反应条件和常见易错点。

1. Structure and Bonding in Alkanes | 烷烃的结构与成键

Each carbon atom in an alkane is sp³ hybridised and forms four sigma (σ) bonds arranged tetrahedrally around the carbon centre. The C–C and C–H bonds are both single sigma bonds. The electronegativity difference between carbon (2.5) and hydrogen (2.1) is very small (about 0.4), so the C–H bond has very low polarity.

烷烃中每个碳原子均为 sp³ 杂化，形成四个呈四面体排列的 σ 键。C–C 键和 C–H 键都是单键（σ 键）。碳（电负性 2.5）与氢（电负性 2.1）的电负性差异极小（约 0.4），因此 C–H 键的极性非常低。

The C–C bond enthalpy is about 348 kJ mol⁻¹ and the C–H bond enthalpy is about 412 kJ mol⁻¹. These strong, non-polar, localised sigma bonds mean that alkanes are not attacked by nucleophiles, electrophiles, acids, or bases under normal conditions. No π electrons are available to interact with approaching reagents.

C–C 键能约为 348 kJ mol⁻¹，C–H 键能约为 412 kJ mol⁻¹。这些强而无非极性且定域的 σ 键意味着，在常温条件下，烷烃不会被亲核试剂、亲电试剂、酸或碱攻击，因为烷烃没有可供进攻物种相互作用的 π 电子。

2. General Unreactivity: Why Alkanes Are ‘Inert’ | 为何烷烃”惰性”

• Strong C–C and C–H sigma bonds are difficult to break; the activation energy for bond fission is very high.

  C–C 和 C–H 强 σ 键难以断裂；键断裂所需的活化能非常高。

• Low polarity: there are no obvious electron-rich or electron-poor sites on the molecule to attract attacking species.

  极性低：分子上没有明显的电子富集区或电子贫乏区来吸引进攻物种。

• No π bonds are present, so addition reactions (typical of alkenes) are impossible.

  不存在 π 键，因此无法发生烯烃特有的加成反应。

• Alkanes have no lone pairs and no vacant orbitals, so they cannot act as ligands or Lewis acids/bases.

  烷烃既无孤对电子，也无空轨道，因此不能充当配体或路易斯酸碱。

As a result, the reactions that alkanes do undergo typically require extreme conditions—high temperatures, UV light, or a catalyst. The most important of these are combustion and free radical substitution.

因此，烷烃能发生的反应通常需要极端条件——高温、紫外光或催化剂。其中最重要的是燃烧反应和自由基取代反应。

3. Complete Combustion | 完全燃烧

Alkanes burn readily in excess oxygen to produce carbon dioxide and water. This is a highly exothermic process, which is exactly why alkanes are important fuels in domestic and industrial settings.

烷烃在过量氧气中燃烧生成二氧化碳和水。该过程放出大量热，这正是烷烃在家庭和工业中成为重要燃料的原因。

CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O

For example, the complete combustion of methane is CH₄ + 2O₂ → CO₂ + 2H₂O; for octane, 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O.

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