Hydrocarbons | 碳氢化合物

📚 Hydrocarbons | 碳氢化合物

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They form the backbone of organic chemistry and are the fundamental components of fossil fuels such as natural gas, petroleum, and coal. From the simplest alkane, methane (CH₄), to complex polycyclic aromatic structures, hydrocarbons exhibit a rich variety in bonding, structure, and reactivity. Understanding their classification, nomenclature, and characteristic reactions is essential for A-Level Chemistry, as it lays the groundwork for more advanced topics in synthesis, mechanism, and industrial applications.

碳氢化合物是仅由碳和氢原子组成的有机化合物。它们构成了有机化学的骨架，是天然气、石油和煤等化石燃料的基本成分。从最简单的烷烃甲烷 (CH₄) 到复杂的多环芳香结构，碳氢化合物在键合、结构和反应性方面展现出丰富的多样性。理解它们的分类、命名和特征反应对于 A-Level 化学至关重要，因为它为合成、反应机理和工业应用等更高阶主题奠定了基础。

1. Classification of Hydrocarbons | 碳氢化合物的分类

Hydrocarbons are broadly divided into aliphatic and aromatic categories. Aliphatic hydrocarbons include alkanes (single bonds only), alkenes (at least one C=C double bond), and alkynes (at least one C≡C triple bond). These can be straight-chain, branched, or cyclic. Aromatic hydrocarbons contain one or more benzene rings, a special cyclic structure with delocalised π electrons that confer unusual stability. This classification is not merely academic—it directly determines the types of reactions a hydrocarbon will undergo. Alkanes are relatively inert and react mainly by free-radical substitution, whereas alkenes and alkynes are electron-rich and undergo electrophilic addition. Aromatic compounds favour electrophilic substitution to preserve the stable aromatic system.

碳氢化合物主要分为脂肪烃和芳香烃两大类。脂肪烃包括烷烃（只含单键）、烯烃（至少一个 C=C 双键）和炔烃（至少一个 C≡C 三键）。它们可以是直链、支链或环状的。芳香烃含有一个或多个苯环，这是一种具有离域 π 电子的特殊环状结构，赋予了它们不同寻常的稳定性。这种分类并非纸上谈兵——它直接决定了碳氢化合物将发生何种类型的反应。烷烃相对惰性，主要通过自由基取代反应；而烯烃和炔烃富电子，发生亲电加成反应。芳香化合物则倾向于亲电取代以保持稳定的芳香体系。

2. Alkanes: Structure and Nomenclature | 烷烃：结构与命名

Alkanes have the general formula CₙH₂ₙ₊₂ for non-cyclic structures. Each carbon atom is sp³ hybridised, giving a tetrahedral geometry with bond angles of approximately 109.5°. The IUPAC naming system selects the longest continuous carbon chain as the parent name (methane, ethane, propane, butane, pentane, hexane, etc.) and treats alkyl branches as substituents with the suffix -yl. When numbering the chain, the lowest possible numbers are given to the substituents. For example, 2-methylbutane indicates a four-carbon parent chain with a methyl group on carbon 2. Cycloalkanes (CₙH₂ₙ) are named by adding the prefix ‘cyclo-‘ to the alkane with the same number of carbons; they exhibit geometric isomerism in substituted forms, such as cis and trans isomers in 1,2-dimethylcyclopropane.

烷烃的非环状结构通式为 CₙH₂ₙ₊₂。每个碳原子均为 sp³ 杂化，呈四面体几何构型，键角约 109.5°。IUPAC 命名系统选择最长的连续碳链作为母体名称（甲烷、乙烷、丙烷、丁烷、戊烷、己烷等），并将烷基支链视为取代基，以 -yl 结尾。给主链编号时，应使取代基获得尽可能小的位次。例如，2-甲基丁烷表示一个四碳母链，在 2 号碳上有一个甲基。环烷烃 (CₙH₂ₙ) 的命名是在相同碳数的烷烃前加前缀“环”；取代环烷烃会表现出几何异构现象，如 1,2-二甲基环丙烷的顺反异构体。

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

Alkanes are non-polar molecules; the only intermolecular forces present are weak London dispersion forces (instantaneous dipole–induced dipole attractions). As a result, they have relatively low melting and boiling points compared to polar compounds of similar molecular mass. Within a homologous series, boiling points increase with increasing chain length because larger molecules have more electrons and a greater surface area for contact, leading to stronger dispersion forces. Branching lowers the boiling point by reducing the area of intermolecular contact. Alkanes are insoluble in water but dissolve in non-polar organic solvents. The following table illustrates the trend in boiling points for straight-chain alkanes:

烷烃是非极性分子；分子间仅存在微弱的伦敦色散力（瞬时偶极–诱导偶极吸引力）。因此，与相似分子质量的极性化合物相比，它们的熔点和沸点相对较低。在同系物中，沸点随碳链增长而升高，因为较大的分子拥有更多的电子和更大的接触表面积，导致色散力增强。支链会降低沸点，因为分子间接触面积减小。烷烃不溶于水，但可溶于非极性有机溶剂。下表展示了直链烷烃的沸点变化趋势：

Alkane	Molecular Formula	Boiling Point / °C
Methane	CH₄	-162
Ethane	C₂H₆	-89
Propane	C₃H₈	-42
Butane	C₄H₁₀	-0.5
Pentane	C₅H₁₂	36
Hexane	C₆H₁₄	69

For isomeric alkanes, the more highly branched the structure, the lower the boiling point—compare butane (bp -0.5 °C) and 2-methylpropane (bp -12 °C).

对于异构烷烃，支链越多，沸点越低——比较丁烷（沸点 -0.5 °C）和 2-甲基丙烷（沸点 -12 °C）。

4. Chemical Reactions of Alkanes | 烷烃的化学反应

Alkanes are often described as paraffins (Latin: parum affinis, little affinity) because of their low reactivity. They are saturated hydrocarbons and can only undergo two main types of reactions under typical A-Level conditions: combustion and free-radical substitution. Combustion is an exothermic oxidation reaction. Complete combustion produces carbon dioxide and water, while incomplete combustion can yield carbon monoxide or carbon (soot). The balanced equation for the complete combustion of propane is:

烷烃常被称为石蜡烃（源自拉丁语 parum affinis，意为亲和力低），因其反应活性低。它们是饱和烃，在 A-Level 典型条件下主要发生两类反应：燃烧和自由基取代。燃烧是放热的氧化反应。完全燃烧生成二氧化碳和水，而不完全燃烧则会生成一氧化碳或碳（烟灰）。丙烷完全燃烧的配平方程式为：

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Free-radical substitution occurs when alkanes react with halogens (Cl₂ or Br₂) in the presence of ultraviolet light or heat. The mechanism proceeds via three stages: initiation, propagation, and termination. For example, in the chlorination of methane:

当烷烃与卤素（Cl₂ 或 Br₂）在紫外光或加热条件下反应时，发生自由基取代。其机理分三个阶段：引发、增长和终止。以甲烷的氯代反应为例：

Initiation: Cl₂ → 2Cl• (homolytic fission)
Propagation: CH₄ + Cl• → CH₃• + HCl; CH₃• + Cl₂ → CH₃Cl + Cl•
Termination: Cl• + Cl• → Cl₂; CH₃• + Cl• → CH₃Cl; CH₃• + CH₃• → C₂H₆

The reaction can lead to a mixture of chloromethane, dichloromethane, trichloromethane, and tetrachloromethane due to further substitution. Similar reactivity is observed with bromine, though bromination is slower and more selective.

由于进一步取代，反应可能得到氯甲烷、二氯甲烷、三氯甲烷和四氯甲烷的混合物。溴的反应类似，但溴代反应更慢且选择性更高。

5. Alkenes: Structure, Bonding, and Isomerism | 烯烃：结构、键合与异构现象

Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond, with the general formula CₙH₂ₙ. The double bond consists of one σ bond (formed by head-on overlap of sp² hybrid orbitals) and one π bond (formed by sideways overlap of unhybridised p orbitals). This π bond restricts rotation, leading to geometric (E/Z) isomerism when each carbon of the double bond has two different groups attached. The Cahn–Ingold–Prelog (CIP) rules assign priority based on atomic number: if the higher priority groups are on the same side, it is the Z isomer (zusammen); if opposite, the E isomer (entgegen). For example, but-2-ene exists as both E-but-2-ene and Z-but-2-ene. Alkenes also exhibit structural isomerism from chain branching or movement of the double bond, such as but-1-ene and but-2-ene.

烯烃是不饱和烃，含至少一个碳碳双键，通式为 CₙH₂ₙ。双键由一个 σ 键（sp² 杂化轨道的头对头重叠）和一个 π 键（未杂化 p 轨道的肩并肩重叠）组成。π 键限制了旋转，因此当双键两端碳原子上各连有两个不同基团时，会产生几何 (E/Z) 异构。Cahn–Ingold–Prelog (CIP) 规则根据原子序数决定优先次序：若优先基团在双键同侧为 Z 异构体（zusammen），在异侧则为 E 异构体（entgegen）。例如，丁-2-烯存在 E-丁-2-烯和 Z-丁-2-烯。烯烃也会因链的支化或双键位置不同而产生构造异构体，如丁-1-烯和丁-2-烯。

6. Electrophilic Addition Reactions of Alkenes | 烯烃的亲电加成反应

The high electron density of the π bond makes alkenes susceptible to attack by electrophiles. Electrophilic addition is the most characteristic reaction of alkenes. The general mechanism involves the electrophile accepting a pair of electrons from the π bond to form a carbocation intermediate (or a cyclic intermediate in some cases), followed by rapid combination with a nucleophile. Markovnikov’s rule states that in the addition of HX (or other unsymmetrical reagents) to an unsymmetrical alkene, the hydrogen attaches to the carbon with the greater number of hydrogen atoms (i.e., the more stable carbocation is formed). Key reactions include:

π 键的高电子密度使烯烃容易受到亲电试剂的进攻。亲电加成是烯烃最具特征的反应。一般机理为亲电试剂从 π 键接受一对电子，形成碳正离子中间体（或在某些情况下形成环状中间体），然后迅速与亲核试剂结合。马尔科夫尼科夫规则指出，在 HX（或其他不对称试剂）与不对称烯烃的加成中，氢加在含氢较多的碳原子上（即生成更稳定的碳正离子）。主要反应包括：

Addition of hydrogen halides: C₂H₄ + HBr → C₂H₅Br

卤化氢加成： C₂H₄ + HBr → C₂H₅Br
Hydration (direct addition of water catalysed by concentrated H₂SO₄ or phosphoric acid): C₂H₄ + H₂O → C₂H₅OH

水化（浓硫酸或磷酸催化的直接加水）： C₂H₄ + H₂O → C₂H₅OH
Halogenation (e.g., bromine water test): C₂H₄ + Br₂ → CH₂Br–CH₂Br. The rapid decolourisation of orange bromine water is a diagnostic test for unsaturation.

卤素加成（如溴水试验）： C₂H₄ + Br₂ → CH₂Br–CH₂Br。橙色溴水迅速褪色是不饱和键的特征检验。
Hydrogenation (addition of H₂ with a metal catalyst such as Ni, Pt or Pd): C₂H₄ + H₂ → C₂H₆, converting alkenes to alkanes.

加氢（在金属催化剂如 Ni、Pt 或 Pd 存在下加 H₂）： C₂H₄ + H₂ → C₂H₆，将烯烃转变为烷烃。

The carbocation intermediate can undergo rearrangement (methyl or hydride shifts) to form a more stable carbocation, leading to unexpected products in some cases. Alkenes also undergo oxidation with cold, dilute KMnO₄ to form diols (the Baeyer test).

碳正离子中间体可能经历重排（甲基或氢负离子迁移）生成更稳定的碳正离子，有时会导致意外产物。烯烃也可被冷的稀高锰酸钾氧化生成二醇（Baeyer 试验）。

7. Polymerisation of Alkenes | 烯烃的聚合反应

Alkenes can undergo addition polymerisation, where many monomer molecules join together to form a long saturated polymer chain. The reaction is initiated by a free radical, a cation, or a coordination catalyst. Poly(ethene) (polythene) is produced from ethene under high pressure and temperature or using Ziegler–Natta catalysts. Poly(propene) is another important polymer used in packaging and textiles. The general representation is:

烯烃可发生加成聚合，许多单体分子连接在一起形成长的饱和聚合物链。反应可由自由基、阳离子或配位催化剂引发。聚乙烯（聚乙烷）是通过乙烯在高压高温下或使用齐格勒–纳塔催化剂制得的。聚丙烯是另一种用于包装和纺织品的重要聚合物。一般表示形式为：

n CH₂=CHR → –(–CH₂–CHR–)ₙ–

The properties of the polymer depend on the nature of the monomer, the chain length, and the degree of branching. For example, low-density poly(ethene) (LDPE) has extensive branching and is flexible, while high-density poly(ethene) (HDPE) has little branching and is rigid.

聚合物的性质取决于单体的性质、链长和支化度。例如，低密度聚乙烯 (LDPE) 支链多，柔韧性好；而高密度聚乙烯 (HDPE) 支链很少，刚性强。

8. Alkynes: Structure and Reactions | 炔烃：结构与反应

Alkynes contain a carbon–carbon triple bond and have the general formula CₙH₂ₙ₋₂ for non-cyclic structures. Each carbon of the triple bond is sp hybridised, giving a linear geometry with a bond angle of 180°. The triple bond consists of one σ bond and two perpendicular π bonds. The simplest alkyne is ethyne (acetylene, C₂H₂). Alkynes undergo electrophilic addition similarly to alkenes, but the reaction can occur in two stages because the triple bond can accept two molecules of reagent. For instance, the addition of bromine to ethyne first forms 1,2-dibromoethene and then 1,1,2,2-tetrabromoethane. Alkynes also exhibit acidic behaviour: terminal alkynes (R–C≡C–H) have a relatively acidic hydrogen (pKₐ ≈ 25) due to the high electronegativity of the sp-hybridised carbon, and they can form metal acetylides with bases like sodium amide (NaNH₂) or ammoniacal silver nitrate, which serves as a distinguishing test.

炔烃含有一个碳碳三键，非环状结构的通式为 CₙH₂ₙ₋₂。三键的每个碳均为 sp 杂化，呈直线形，键角 180°。三键由一个 σ 键和两个相互垂直的 π 键组成。最简单的炔烃是乙炔（电石气，C₂H₂）。炔烃与烯烃相似地发生亲电加成，但反应可分两步进行，因为三键可接受两分子试剂。例如，乙炔与溴加成先生成 1,2-二溴乙烯，再生成 1,1,2,2-四溴乙烷。炔烃还表现出酸性：末端炔烃 (R–C≡C–H) 的氢具有一定酸性 (pKₐ ≈ 25)，因 sp 杂化碳的电负性较高，它们可与氨基钠 (NaNH₂) 或氨性硝酸银生成金属炔化物，可用于鉴别。

9. Aromatic Hydrocarbons: Benzene and Its Stability | 芳香烃：苯及其稳定性

Benzene, C₆H₆, is the archetypal aromatic hydrocarbon. Its structure is described as a planar hexagonal ring with six delocalised π electrons spread equally over all six carbon atoms, giving a bond order of 1.5. This delocalisation imparts exceptional thermodynamic stability (resonance energy ~150 kJ mol⁻¹), which explains why benzene undergoes electrophilic substitution rather than addition: substituting a hydrogen preserves the aromatic sextet. Key electrophilic substitution reactions include nitration (using conc. HNO₃ and conc. H₂SO₄ to form nitrobenzene), halogenation (Cl₂ or Br₂ with a halogen carrier such as FeBr₃ or AlCl₃), Friedel–Crafts alkylation and acylation. The general mechanism involves generation of the electrophile, attack on the benzene ring to form a σ-complex (arenium ion), and loss of a proton to restore aromaticity.

苯，C₆H₆，是典型的芳香烃。其结构被描述为一个平面正六元环，六个离域 π 电子均匀分布在所有六个碳原子上，键级为 1.5。这种离域作用赋予了苯卓越的热力学稳定性（共振能约 150 kJ mol⁻¹），解释了为何苯发生亲电取代而非加成：取代氢原子保留了芳香六偶体。重要的亲电取代反应包括硝化（使用浓硝酸和浓硫酸生成硝基苯）、卤代（Cl₂ 或 Br₂ 在卤素载体如 FeBr₃ 或 AlCl₃ 存在下）、Friedel–Crafts 烷基化和酰基化。通用机理包括亲电试剂的生成、进攻苯环形成 σ 络合物（芳正离子）、然后失去质子恢复芳香性。

10. Combustion and Environmental Impact | 燃烧与环境影响

The combustion of hydrocarbons is the primary source of energy in internal combustion engines and power plants. However, the release of CO₂, a greenhouse gas, contributes to global warming. Incomplete combustion produces carbon monoxide, a toxic gas that binds to haemoglobin, and unburnt hydrocarbons, which are components of photochemical smog. Additionally, impurities in fossil fuels, especially sulfur compounds, lead to sulfur dioxide emissions, causing acid rain. Catalytic converters in vehicles use platinum, palladium, and rhodium to convert CO, NOₓ, and unburnt hydrocarbons into CO₂, N₂, and H₂O. In the context of A-Level chemistry, students should be able to write balanced equations for both complete and incomplete combustion and discuss the associated environmental issues.

碳氢化合物的燃烧是内燃机和发电站的主要能源。但释放的 CO₂ 是一种温室气体，加剧全球变暖。不完全燃烧会产生一氧化碳（一种与血红蛋白结合的有毒气体）和未燃烧的碳氢化合物，它们是光化学烟雾的成分。此外，化石燃料中的杂质，尤其是含硫化合物，导致二氧化硫排放，引发酸雨。汽车催化转化器使用铂、钯和铑将 CO、NOₓ 和未燃烧烃转化为 CO₂、N₂ 和 H₂O。在 A-Level 化学中，学生应能书写完全和不完全燃烧的配平方程式，并讨论相关的环境问题。

11. Summary and Key Concepts | 总结与核心概念

Hydrocarbons, though composed of only carbon and hydrogen, encompass a vast array of structures and reactions that are central to organic chemistry. The saturated alkanes serve as fuels and are feedstocks for halogenated compounds via free-radical substitution. Unsaturated alkenes and alkynes, with their π bonds, engage in electrophilic addition and polymerisation, providing routes to alcohols, halogenoalkanes, and synthetic materials. Aromatic hydrocarbons, typified by benzene, undergo electrophilic substitution while preserving their stable delocalised system. Mastery of nomenclature, isomerism, reaction mechanisms, and the influence of molecular structure on physical properties equips students to tackle more complex organic transformations and appreciate the role of hydrocarbons in both industrial and environmental contexts.

碳氢化合物虽仅由碳和氢组成，却涵盖了众多对有机化学至关重要的结构和反应。饱和烷烃既是燃料，又通过自由基取代作为卤代化合物的原料。不饱和的烯烃和炔烃凭借 π 键进行亲电加成和聚合，提供了合成醇、卤代烷烃以及高分子材料的路径。以苯为代表的芳香烃通过亲电取代保持其稳定的离域体系。掌握命名法、异构现象、反应机理以及分子结构对物理性质的影响，使学生有能力应对更复杂的有机转化，并理解碳氢化合物在工业与环境中的角色。

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Hydrocarbons | 碳氢化合物