Reactions of Alkanes | 烷烃的反应

📚 Reactions of Alkanes | 烷烃的反应

Alkanes are generally considered unreactive due to the strength and non‑polar nature of their C–C and C–H bonds. However, under specific conditions they undergo characteristic reactions such as combustion, free‑radical substitution, and cracking, which are vital both industrially and environmentally. This article explores all the major reaction pathways of alkanes required for A‑Level Cambridge Chemistry, with clear mechanisms and real‑world context.

由于C–C键和C–H键很强且为非极性键，烷烃通常被认为是惰性的。然而在特定条件下，它们能发生燃烧、自由基取代和裂化等特征反应，这些反应在工业和环境中都至关重要。本文系统剖析剑桥A‑Level化学要求的烷烃主要反应路径，提供清晰的机理和实际应用联系。

---

1. Introduction to Alkane Reactivity | 烷烃反应性简介

Alkanes consist only of carbon–carbon single bonds and carbon–hydrogen bonds, both of which have high bond enthalpies and very low polarity. This makes alkanes exceptionally inert towards polar reagents, acids, bases, and oxidising agents at room temperature. They are often used as non‑polar solvents and are major components of crude oil that survive geological time scales without change.

烷烃仅由碳–碳单键和碳–氢单键组成，这两种键的键焓高、极性极低。因此烷烃在室温下对极性试剂、酸、碱和氧化剂极其稳定。它们常用作非极性溶剂，也是原油的主要成分，能在地质时间尺度上保持不变。

The key to alkane reactivity is the input of energy (heat or ultraviolet light) that can break bonds homolytically, generating highly reactive free radicals. Combustion proceeds by a free‑radical chain mechanism once initiated, and halogenation requires UV light to produce halogen atoms. Cracking uses high temperatures and catalysts to break strong C–C bonds.

烷烃反应的关键在于输入能量（热或紫外光）使键发生均裂，生成高反应活性的自由基。燃烧一旦引发，就按自由基链式机理进行；卤代反应需要紫外光产生卤原子；裂化则利用高温和催化剂断裂牢固的C–C键。

---

2. Combustion of Alkanes | 烷烃的燃烧

Alkanes burn readily in air or oxygen to release considerable energy, making them the world’s most important fuels. In an excess of oxygen, complete combustion produces carbon dioxide and water vapour. For instance, the combustion of methane is: CH₄ + 2O₂ → CO₂ + 2H₂O.

烷烃在空气或氧气中容易燃烧并释放大量能量，因此是全球最重要的燃料。在过量氧气中，完全燃烧生成二氧化碳和水蒸气。例如甲烷的燃烧：CH₄ + 2O₂ → CO₂ + 2H₂O。

If the oxygen supply is limited, incomplete combustion occurs, generating carbon monoxide (a toxic gas) and sometimes carbon particulates (soot). The equation for incomplete combustion of propane may be written as: C₃H₈ + 3½O₂ → 3CO + 4H₂O, or even C₃H₈ + 2O₂ → 3C + 4H₂O. The CO produced binds irreversibly to haemoglobin, reducing the blood’s oxygen‑carrying capacity.

若氧气供应不足，则发生不完全燃烧，生成有毒气体一氧化碳（CO），有时还会产生碳微粒（烟灰）。丙烷不完全燃烧的方程式可写作：C₃H₈ + 3½O₂ → 3CO + 4H₂O，甚至 C₃H₈ + 2O₂ → 3C + 4H₂O。生成的一氧化碳与血红蛋白不可逆结合，降低血液的携氧能力。

Soot and unburnt hydrocarbons from incomplete combustion also contribute to air pollution. In vehicle engines, the combination of high temperature and pressure can cause nitrogen and oxygen to react, producing nitrogen oxides (NOₓ) as additional pollutants.

不完全燃烧产生的烟灰和未燃烧烃类还会加剧空气污染。在汽车发动机中，高温高压条件促使氮气与氧气反应，生成另一类污染物——氮氧化物（NOₓ）。

---

3. Free Radical Substitution: An Overview | 自由基取代反应概述

In the presence of ultraviolet (UV) light or strong heating, alkanes react with halogens (Cl₂, Br₂) via a free‑radical substitution mechanism. The overall reaction replaces one or more hydrogen atoms with halogen atoms, producing a mixture of halogenoalkanes and hydrogen halide. For methane and chlorine: CH₄ + Cl₂ → CH₃Cl + HCl.

在紫外光或强热条件下，烷烃与卤素（Cl₂、Br₂）发生自由基取代反应。总反应是一个或多个氢原子被卤原子取代，生成卤代烷和卤化氢的混合物。以甲烷和氯为例：CH₄ + Cl₂ → CH₃Cl + HCl。

This reaction is not useful for producing a single pure product because further substitution can occur, giving CH₂Cl₂, CHCl₃, and CCl₄. Nevertheless, its mechanism is a classic example of a radical chain reaction, taught to illustrate initiation, propagation, and termination steps.

该反应不适用于制备单一纯产物，因为会继续取代生成CH₂Cl₂、CHCl₃和CCl₄。尽管如此，其机理仍是自由基链式反应的经典范例，用来阐明链引发、链增长和链终止三个步骤。

---

4. Mechanism: Initiation Step | 反应机理：链引发

Initiation begins when a halogen molecule absorbs UV light of sufficient energy to break the halogen–halogen bond homolytically. Homolytic fission means that each atom retains one of the bonding electrons, producing two highly reactive halogen free radicals. For chlorine: Cl₂ → 2Cl•.

引发步骤始于卤素分子吸收足够能量的紫外光，导致卤素–卤素键发生均裂。均裂指每个原子各保留一个成键电子，生成两个高反应活性的卤素自由基。以氯为例：Cl₂ → 2Cl•。

The dot (•) represents the unpaired electron carried by the radical. Because radicals are electron‑deficient, they attack C–H bonds very readily. The initiation step is the only step that generates radicals without consuming any, establishing the chain.

圆点（•）代表自由基携带的未成对电子。自由基因缺电子而极易进攻C–H键。引发步骤是唯一在不消耗自由基的情况下产生自由基的步骤，从而建立链式反应。

---

5. Mechanism: Propagation Steps | 反应机理：链增长

Propagation consists of two alternating steps that sustain the chain. First, a chlorine radical abstracts a hydrogen atom from methane, forming hydrogen chloride and a methyl radical: CH₄ + Cl• → •CH₃ + HCl.

链增长由两个交替进行的步骤维系。第一步，氯自由基从甲烷夺取一个氢原子，生成氯化氢和甲基自由基：CH₄ + Cl• → •CH₃ + HCl。

Second, the methyl radical reacts with a chlorine molecule, producing chloromethane and regenerating a chlorine radical: •CH₃ + Cl₂ → CH₃Cl + Cl•. The chlorine radical can then initiate another propagation cycle, allowing the reaction to continue as long as reactants are present.

第二步，甲基自由基与氯分子反应，生成氯甲烷并再生氯自由基：•CH₃ + Cl₂ → CH₃Cl + Cl•。该氯自由基又能开启下一个增长循环，只要反应物存在，反应就能持续进行。

The overall enthalpy change for propagation is negative for chlorination, meaning the reaction is exothermic. Many thousands of molecules can be converted by a single initiating photon, giving a high quantum yield.

对于氯代反应，增长步骤的总焓变为负值，反应放热。一个引发光子可转化成千上万个分子，量子产率很高。

---

6. Mechanism: Termination Steps | 反应机理：链终止

Termination occurs when two radicals combine to form a stable molecule, thereby removing radicals from the system without producing new ones. Any pair of radicals present can undergo termination. For a methane–chlorine system, the possible combinations are: Cl• + Cl• → Cl₂; •CH₃ + Cl• → CH₃Cl; •CH₃ + •CH₃ → C₂H₆.

当两个自由基结合生成稳定分子，且不产生新的自由基时，即发生链终止。任何一对存在的自由基都能终止。在甲烷–氯体系中，可能的组合有：Cl• + Cl• → Cl₂；•CH₃ + Cl• → CH₃Cl；•CH₃ + •CH₃ → C₂H₆。

Because the concentration of radicals is very low at any moment, termination is relatively infrequent, which is why the chain can proceed through many cycles. However, termination inevitably happens and limits the overall chain length.

由于任意时刻自由基浓度都很低，终止反应相对罕见，这正是链反应能进行许多循环的原因。但终止最终会发生，并限制总链长。

---

7. Multiple Substitution and Product Distribution | 多取代与产物分布

Once some chloromethane (CH₃Cl) forms, its remaining C–H bonds can also be attacked by chlorine radicals. Sequential substitutions give dichloromethane CH₂Cl₂, trichloromethane CHCl₃, and tetrachloromethane CCl₄. The product mixture is difficult to control; even with a large excess of methane, some polysubstitution occurs.

一旦生成了氯甲烷（CH₃Cl），其余C–H键也可能被氯自由基进攻。依次取代生成二氯甲烷CH₂Cl₂、三氯甲烷CHCl₃和四氯甲烷CCl₄。产物混合物难以控制；即使使用过量甲烷，仍会发生部分多取代。

Because of this, free‑radical halogenation is seldom used for laboratory synthesis of a specific halogenoalkane. Instead, alternative methods such as alcohol halogenation with PCl₅ or SOCl₂ are preferred. The mechanism highlights the non‑selective nature of radical chemistry.

因此，自由基卤代很少用于实验室合成某种特定卤代烷，更倾向用醇与PCl₅或SOCl₂等试剂反应。该机理凸显了自由基化学的非选择性特征。

---

8. Cracking of Alkanes | 烷烃的裂化

Cracking converts long‑chain, less useful alkanes into shorter, more valuable hydrocarbons: the smaller alkanes used as fuels and the alkenes used as feedstocks for polymers. Thermal cracking uses high temperature (≈700–1200 K) and pressure (up to 70 atm) to break C–C bonds homolytically, producing a mixture of alkanes and alkenes.

裂化将长链、用途较少的烷烃转化为短链、更有价值的烃类：短链烷烃用作燃料，烯烃用作聚合物原料。热裂化在高温（约700–1200 K）和高压（最高70 atm）下使C–C键均裂，生成烷烃和烯烃的混合物。

An example is the cracking of decane: C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀. Catalytic cracking uses a zeolite catalyst at a lower temperature (≈720 K) and lower pressure, producing a higher proportion of branched alkanes and cycloalkanes, which have better octane ratings, as well as aromatic hydrocarbons.

例如癸烷裂化：C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀。催化裂化使用沸石催化剂，在较低温度（约720 K）和较低压力下进行，产生更多支链烷烃和环烷烃（具有更高辛烷值），以及芳香烃。

The alkenes produced are essential for the petrochemical industry, particularly ethene and propene. Catalytic cracking also reduces the production of unwanted gaseous products and promotes isomerisation.

生成的烯烃对石化工业至关重要，尤其是乙烯和丙烯。催化裂化还能减少不必要的气态产物，并促进异构化。

---

9. Catalytic Reforming | 催化重整

Reforming is a process that rearranges the carbon skeleton of alkanes without changing the number of carbon atoms, aiming to produce aromatic compounds and branched alkanes for high‑octane petrol. Under platinum or platinum–rhenium catalysts at about 770 K, straight‑chain alkanes can undergo cyclisation and dehydrogenation.

重整是在不改变碳原子数的前提下重排烷烃碳骨架的过程，旨在生产用于高辛烷值汽油的芳香烃和支链烷烃。在铂或铂‑铼催化剂、约770 K条件下，直链烷烃可发生环化与脱氢。

For example, hexane can be converted into benzene: C₆H₁₄ → C₆H₆ + 4H₂. The hydrogen produced is a valuable by‑product used in the Haber process and in hydrodesulfurisation. Isomerisation, a milder reforming reaction, simply converts straight‑chain alkanes to branched isomers with higher octane numbers.

例如，己烷可转化为苯：C₆H₁₄ → C₆H₆ + 4H₂。产生的氢气是宝贵的副产品，可用于哈伯法和加氢脱硫。异构化是一种较温和的重整反应，仅将直链烷烃转化为辛烷值更高的支链异构体。

---

10. Environmental and Industrial Considerations | 环境与工业意义

The combustion of alkanes, while essential for energy, is the leading anthropogenic source of CO₂, a greenhouse gas. Unburnt hydrocarbons and NOₓ from vehicle exhausts contribute to photochemical smog. Catalytic converters on automobiles catalyse the oxidation of CO and unburnt hydrocarbons to CO₂, as well as the reduction of NOₓ to N₂.

烷烃燃烧虽是能量主要来源，却是人为CO₂（温室气体）的最大排放源。汽车尾气中的未燃烧烃和NOₓ会引发光化学烟雾。汽车上的催化转化器可催化CO和未燃烧烃氧化为CO₂，并将NOₓ还原为N₂。

Cracking and reforming make the petrochemical industry possible, turning crude oil into fuels, plastics, solvents, and pharmaceuticals. However, these high‑temperature processes are energy‑intensive and contribute to carbon emissions. Ongoing research explores greener catalytic routes and the use of renewable feedstocks.

裂化和重整使石化工业成为可能，将原油转化为燃料、塑料、溶剂和药品。但这些高温过程耗能高，并带来碳排放。当前研究正探索更绿色的催化途径及可再生原料的利用。

---

11. Summary of Key Reactions | 关键反应总结

The table below summarises the main reaction types of alkanes, their conditions, and the significant features tested at A‑Level.

下表总结了烷烃的主要反应类型、条件及A‑Level考查的重要特征。

Reaction	反应	Conditions	条件	Key features	关键特征
Complete combustion	完全燃烧	Excess O₂	过量氧气	CO₂ + H₂O; exothermic	生成CO₂和H₂O；放热
Incomplete combustion	不完全燃烧	Limited O₂	有限氧气	CO, C soot; toxic products	生成CO和碳烟；产物有毒
Free‑radical substitution	自由基取代	UV light or heat, halogen (Cl₂, Br₂)	紫外光或加热，卤素	Chain mechanism; mixture of halogenoalkanes	链式机理；卤代烷混合物
Thermal cracking	热裂化	~700–1200 K, high pressure	高温高压	Mainly alkenes + shorter alkanes	主要生成烯烃和短链烷烃
Catalytic cracking	催化裂化	Zeolite catalyst, ~720 K	沸石催化剂，约720 K	Branched alkanes, aromatics, more alkenes	支链烷烃、芳烃、更多烯烃
Reforming	重整	Pt/Al₂O₃ catalyst, moderate T, H₂ atmosphere	铂催化剂、中温、氢气气氛	Aromatics, branched isomers; H₂ by‑product	芳烃、异构体；副产氢气

---

12. Exam Tips for Alkane Reactions | 关于烷烃反应的考试技巧

When writing free‑radical substitution mechanisms, always use curly half‑arrow or describe homolytic fission clearly, and draw dot‑formulae for radicals. Do not forget to label the steps as initiation, propagation, and termination. Balancing equations for complete and incomplete combustion is a common numerical task.

书写自由基取代机理时，要清晰地使用弯钩单箭头或描述均裂，并用点式表示自由基。切勿忘记标注引发、增长和终止步骤。配平完全与不完全燃烧方程式是常见的计算任务。

In cracking, identify the two products that add up to the molecular formula of the original alkane, ensuring one is an alkene and the other an alkane. For environmental questions, connect incomplete combustion to CO and soot, and link NOₓ formation to high‑temperature engines rather than fuel composition. Radical chain mechanisms may be examined with unfamiliar alkanes; the same principles apply.

在裂化反应中，要确定两种产物的分子式相加等于原烷烃，且一种为烯烃、另一种为烷烃。回答环境问题时，要把不完全燃烧与CO和烟尘联系起来，并指出NOₓ的形成源于发动机高温而非燃料组成。考试可能用陌生的烷烃考查自由基链式机理，原理完全相同。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）