Alkanes | 烷烃

📚 Alkanes | 烷烃

Alkanes are the simplest family of hydrocarbons, consisting of only carbon and hydrogen atoms joined by single covalent bonds. They form a homologous series with the general formula CₙH₂ₙ₊₂, and their chemistry is dominated by their relative inertness and by free-radical substitution reactions.

烷烃是最简单的烃类家族，仅由碳原子和氢原子通过单共价键连接而成。它们构成通式为 CₙH₂ₙ₊₂ 的同系物系列，其化学性质以相对惰性和自由基取代反应为主要特征。

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1. Structure and Bonding | 结构与成键

Each carbon atom in an alkane is sp³ hybridised, forming four sigma (σ) bonds arranged tetrahedrally around the carbon centre. The bond angle is approximately 109.5°, which maximises the distance between electron pairs and minimises repulsion.

烷烃中的每个碳原子均为 sp³ 杂化，形成四个 σ 键，围绕碳中心呈四面体排列。键角约为 109.5°，这一角度可使电子对之间的距离最大化，从而将排斥作用降至最低。

The carbon–hydrogen bond is essentially non-polar because the electronegativity difference between carbon (2.5) and hydrogen (2.1) is very small. As a result, alkanes are non-polar molecules with weak intermolecular forces.

碳氢键本质上是非极性的，因为碳（电负性 2.5）与氢（2.1）之间的电负性差异非常小。因此，烷烃是非极性分子，分子间作用力较弱。

The first four members of the homologous series are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀). Each successive member differs by a CH₂ unit.

同系物系列的前四个成员依次为甲烷（CH₄）、乙烷（C₂H₆）、丙烷（C₃H₈）和丁烷（C₄H₁₀）。相邻成员之间相差一个 CH₂ 单元。

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2. Nomenclature | 命名规则

Alkanes are named according to IUPAC rules. The longest continuous carbon chain determines the root name (meth-, eth-, prop-, but-), and the suffix ‘-ane’ indicates a saturated hydrocarbon. Branches are named as alkyl groups such as methyl (CH₃–) and ethyl (C₂H₅–).

烷烃依照 IUPAC 规则命名。最长连续碳链决定名称词根（甲、乙、丙、丁），后缀“烷”表示饱和烃。支链以烷基命名，例如甲基（CH₃–）和乙基（C₂H₅–）。

When numbering the parent chain, the substituents must be given the lowest possible locants. For example, 2-methylbutane is correct, whereas 3-methylbutane is not, because numbering from the other end gives a lower number.

在给主链编号时，取代基必须获得尽可能低的位次编号。例如，2-甲基丁烷是正确的，而3-甲基丁烷不正确，因为从另一端编号可得到更小的数字。

For more complex molecules, multiple substituents are listed alphabetically, and prefixes such as di-, tri- and tetra- are used to indicate identical groups. Commas separate numbers, and hyphens separate numbers from letters.

对于更复杂的分子，多个取代基按字母顺序排列，并使用二、三、四等前缀表示相同基团的数目。数字之间用逗号分隔，数字与字母之间用短横线连接。

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3. Physical Properties: Boiling Point Trends | 物理性质：沸点趋势

The boiling points of alkanes increase as the chain length increases. This is because longer chains have a larger surface area, which strengthens the instantaneous dipole–induced dipole (London) forces between molecules. More energy is therefore required to overcome these forces.

烷烃的沸点随碳链增长而升高。这是因为较长的碳链具有更大的表面积，从而增强了分子间的瞬时偶极–诱导偶极（伦敦色散力）作用。因此需要更多能量来克服这些作用力。

Branching reduces the boiling point for a given molecular formula. A branched isomer has a more compact, spherical shape, which reduces the contact area between molecules and weakens the London forces.

对于相同的分子式，支链化会降低沸点。支链异构体的形状更紧凑、更接近球形，分子间接触面积减小，从而削弱伦敦色散力。

Alkane	Formula	Mr	Boiling point / °C
Methane	CH₄	16	−164
Ethane	C₂H₆	30	−89
Propane	C₃H₈	44	−42
Butane	C₄H₁₀	58	−0.5

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4. Solubility and Density | 溶解度与密度

Alkanes are non-polar molecules, so they are insoluble in water, which is a polar solvent. They do not form hydrogen bonds with water molecules, and the disruption of water’s hydrogen-bonded network is energetically unfavourable. Instead, alkanes dissolve readily in non-polar organic solvents such as cyclohexane.

烷烃是非极性分子，因此不溶于水这一极性溶剂。它们无法与水分子形成氢键，破坏水的氢键网络在能量上是不利的。相反，烷烃易溶于环己烷等非极性有机溶剂。

Because alkanes are less dense than water, they float on the surface of water. This property has significant environmental consequences in oil spills, where an alkane layer spreads across the water surface and prevents oxygen exchange.

由于烷烃密度小于水，它们会漂浮在水面上。这一性质在石油泄漏中具有重要的环境后果：烷烃层会在水面扩散，阻碍氧气交换。

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5. Chemical Inertness | 化学惰性

Alkanes are generally unreactive under standard conditions. The C–C and C–H bonds are strong and non-polar, and there are no empty orbitals or lone pairs on the carbon atoms to attract nucleophiles or electrophiles. This makes alkanes excellent lubricants and solvents.

烷烃在标准条件下通常不活泼。C–C 键和 C–H 键既强又非极性，碳原子上没有空轨道或孤对电子来吸引亲核试剂或亲电试剂。这使得烷烃成为优良的润滑剂和溶剂。

However, alkanes do react under certain conditions, most notably in combustion and in free-radical substitution reactions. Both of these reactions involve highly reactive species: oxygen at high temperature, or halogen atoms under UV radiation.

然而，烷烃在特定条件下确实会发生反应，最典型的是燃烧反应和自由基取代反应。这两种反应都涉及高活性物种：高温下的氧气，或紫外辐射下的卤素原子。

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6. Combustion Reactions | 燃烧反应

The complete combustion of an alkane produces carbon dioxide and water, releasing a large amount of energy. For example, the combustion of methane is represented by the following equation:

烷烃的完全燃烧生成二氧化碳和水，并释放大量能量。例如，甲烷的燃烧可用以下方程式表示：

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)　ΔH° = −890 kJ mol⁻¹

In a limited supply of oxygen, incomplete combustion occurs, producing carbon monoxide (CO) and/or carbon (soot). Carbon monoxide is toxic because it binds to haemoglobin more strongly than oxygen, reducing the blood’s ability to transport oxygen.

在氧气供应不足时，会发生不完全燃烧，生成一氧化碳（CO）和/或碳（烟灰）。一氧化碳具有毒性，因为它与血红蛋白的结合能力比氧气强得多，从而降低血液输送氧气的能力。

Incomplete combustion can be represented by the equation for methane:

甲烷的不完全燃烧可用以下方程式表示：

2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(l)

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7. Free-Radical Substitution | 自由基取代反应

When alkanes are mixed with halogens such as chlorine or bromine and exposed to UV radiation, a substitution reaction occurs in which a hydrogen atom is replaced by a halogen atom. For methane reacting with chlorine, the overall reaction is:

当烷烃与氯气或溴等卤素混合并暴露于紫外辐射时，会发生取代反应，一个氢原子被卤素原子替换。甲烷与氯气反应的总反应式为：

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

The reaction proceeds by a free-radical mechanism, which is divided into three stages: chain initiation, chain propagation and chain termination. Free radicals are species with an unpaired electron, such as Cl• and CH₃•.

该反应通过自由基机理进行，分为三个阶段：链引发、链增长和链终止。自由基是带有一个未成对电子的物种，例如 Cl• 和 CH₃•。

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8. Chain Initiation | 链引发

In the initiation step, a chlorine molecule absorbs UV radiation and undergoes homolytic fission. Each chlorine atom retains one electron from the shared pair, producing two chlorine free radicals:

在引发步骤中，氯分子吸收紫外辐射并发生均裂。每个氯原子保留共用电子对中的一个电子，生成两个氯自由基：

Cl₂ → 2Cl•

Homolytic fission is the breaking of a covalent bond where each atom takes one electron from the shared pair. This requires energy, which is supplied by UV radiation. The chlorine free radical is extremely reactive because it has an unpaired electron.

均裂是共价键断裂时每个原子从共用电子对中各取一个电子的过程。这需要能量，由紫外辐射提供。氯自由基因带有一个未成对电子而极其活泼。

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9. Chain Propagation | 链增长

In the propagation steps, free radicals react with neutral molecules to produce new free radicals. The first propagation step involves a chlorine radical abstracting a hydrogen atom from methane:

在链增长步骤中，自由基与中性分子反应生成新的自由基。第一步链增长是氯自由基从甲烷中夺取一个氢原子：

Cl• + CH₄ → HCl + CH₃•

A methyl radical is formed, which then reacts with a chlorine molecule to give the substituted product and regenerate a chlorine radical:

生成的甲基自由基随后与氯分子反应，得到取代产物并再生一个氯自由基：

CH₃• + Cl₂ → CH₃Cl + Cl•

The chlorine radical produced here can attack another methane molecule, so the chain reaction continues. Both propagation steps must balance in terms of atoms and electrons.

此处产生的氯自由基可以继续攻击另一个甲烷分子，因此链反应得以持续。两步链增长反应在原子和电子数目上都必须保持平衡。

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10. Chain Termination | 链终止

The reaction stops when two free radicals collide and combine, forming a stable molecule without producing new radicals. Possible termination steps include:

当两个自由基碰撞并结合生成稳定分子且不再产生新自由基时，反应终止。可能的终止步骤包括：

• Cl• + Cl• → Cl₂

• CH₃• + Cl• → CH₃Cl

• CH₃• + CH₃• → C₂H₆

The combination of two methyl radicals to form ethane is particularly important in exam questions, as it demonstrates that the product distribution of free-radical substitution is not perfectly controlled and multiple organic products can form.

两个甲基自由基结合生成乙烷的反应在考题中尤为重要，因为它说明自由基取代的产物分布并非完全可控，可能生成多种有机产物。

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11. Limitations of the Mechanism | 反应机理的局限

The free-radical substitution of methane with chlorine does not produce only chloromethane. Because the reaction is non-selective, further substitution occurs to form dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃) and tetrachloromethane (CCl₄).

甲烷与氯气的自由基取代不会仅生成氯甲烷。由于反应没有选择性，会继续发生取代，生成二氯甲烷（CH₂Cl₂）、三氯甲烷（CHCl₃）和四氯化碳（CCl₄）。

Trace amounts of ethane are also observed due to the termination step CH₃• + CH₃• → C₂H₆. This means that the free-radical substitution mechanism has limited synthetic value in the laboratory, as the product mixture is difficult to separate.

由于终止步骤 CH₃• + CH₃• → C₂H₆ 的存在，还能观察到微量乙烷。这意味着自由基取代机理在实验室中的合成价值有限，因为产物混合物难以分离。

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12. Environmental Significance: CFCs and the Ozone Layer | 环境意义：氯氟烃与臭氧层

Chlorofluorocarbons (CFCs) are compounds containing carbon, chlorine and fluorine, such as CCl₂F₂. They were widely used as refrigerants and aerosol propellants. In the stratosphere, UV radiation causes C–Cl bonds in CFCs to undergo homolytic fission, producing chlorine radicals:

氯氟烃（CFCs）是含碳、氯和氟的化合物，例如 CCl₂F₂。它们曾被广泛用作制冷剂和气雾剂推进剂。在平流层中，紫外辐射使 CFC 中的 C–Cl 键发生均裂，产生氯自由基：

CCl₂F₂ → Cl• + •CClF₂

These chlorine radicals catalyse the breakdown of ozone (O₃). In the first step, a chlorine radical reacts with ozone:

这些氯自由基催化臭氧（O₃）的分解。第一步，氯自由基与臭氧反应：

Cl• + O₃ → ClO• + O₂

In the second step, the chlorine monoxide radical reacts with a free oxygen atom to regenerate the chlorine radical:

第二步，一氧化氯自由基与游离氧原子反应，再生活性氯自由基：

ClO• + O• → Cl• + O₂

Because the chlorine radical is regenerated, a single CFC molecule can destroy many thousands of ozone molecules before being removed from the stratosphere. The depletion of the ozone layer allows more harmful UV-B radiation to reach Earth’s surface, increasing the risks of skin cancer and cataracts. This environmental concern led to the Montreal Protocol, which phased out the production of CFCs.

由于氯自由基能够再生，单个 CFC 分子在被清除出平流层之前可破坏成千上万个臭氧分子。臭氧层损耗使更多有害的 UV-B 辐射到达地球表面，增加皮肤癌和白内障的风险。这一环境问题促成了《蒙特利尔议定书》的签署，逐步淘汰了 CFC 的生产。

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