📚 Reactions of Alkanes | 烷烃的反应
This article covers the key reactions of alkanes required for Cambridge A-Level Chemistry: complete and incomplete combustion, free-radical substitution with halogens, and the limitations of radical substitution. Understanding the mechanism steps, conditions, and products is essential for short-answer and structured questions.
本文涵盖剑桥 A-Level 化学中烷烃的关键反应：完全与不完全燃烧、与卤素的自由基取代以及自由基取代的局限性。掌握反应机理步骤、反应条件和产物对于简答题和结构化题目至关重要。
1. Why Alkanes Are Relatively Unreactive | 烷烃为何相对不活泼
Alkanes contain only carbon-carbon and carbon-hydrogen sigma bonds. These bonds are strong, non-polar or only very weakly polar, and there is no functional group that can be attacked easily by electrophiles or nucleophiles.
烷烃只含碳-碳和碳-氢 σ 键。这些键较强、非极性或极性极弱，并且没有易于被亲电试剂或亲核试剂进攻的官能团。
As a result, alkanes are relatively unreactive. They do not react with acids, bases, oxidising agents or reducing agents under normal laboratory conditions. Their two major reaction types are combustion and free-radical substitution.
因此，烷烃相对不活泼。在常规实验室条件下，它们不与酸、碱、氧化剂或还原剂反应。它们的两类主要反应是燃烧和自由基取代。
2. Combustion of Alkanes: Complete Combustion | 烷烃的燃烧：完全燃烧
When an alkane burns in a plentiful supply of oxygen, complete combustion occurs. The only products are carbon dioxide and water, and the reaction is highly exothermic.
当烷烃在充足氧气中燃烧时，发生完全燃烧。唯一产物是二氧化碳和水，反应高度放热。
This is why alkanes are widely used as fuels. For example, the complete combustion of methane and propane can be represented as:
这就是烷烃被广泛用作燃料的原因。例如，甲烷和丙烷的完全燃烧可以表示如下：
CH₄ + 2O₂ → CO₂ + 2H₂O
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Complete combustion gives the maximum possible energy output per mole of fuel and produces no toxic carbon monoxide. In domestic and industrial heating, a blue flame usually indicates complete combustion.
完全燃烧释放每摩尔燃料可能的最大能量，并且不产生有毒的一氧化碳。在家庭和工业加热中，蓝色火焰通常表示燃烧完全。
3. Incomplete Combustion and Its Hazards | 不完全燃烧及其危害
If the oxygen supply is limited, incomplete combustion takes place. Instead of carbon dioxide, the products may be carbon monoxide, carbon particles, or a mixture of both, together with water.
如果氧气供应不足，则发生不完全燃烧。产物可能是一氧化碳、碳颗粒或两者的混合物，同时还生成水，而不是二氧化碳。
Carbon monoxide is particularly dangerous because it binds strongly to haemoglobin in red blood cells, preventing oxygen transport. Carbon particles appear as black soot and can cause respiratory problems.
一氧化碳特别危险，因为它与红细胞中的血红蛋白牢固结合，阻碍氧气运输。碳颗粒以黑色烟灰形式出现，可引起呼吸系统问题。
Examples of incomplete combustion equations include:
不完全燃烧方程的例子包括：
2CH₄ + 3O₂ → 2CO + 4H₂O
CH₄ + O₂ → C + 2H₂O
In exam questions, a yellow or sooty flame is often used as evidence of incomplete combustion, especially when oxygen is restricted.
在考试题目中，黄色或冒烟火焰通常被用作不完全燃烧的证据，尤其是在氧气受限时。
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