📚 Classification of Hydrocarbons | 碳氢化合物的分类
Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They form the backbone of organic chemistry and serve as the primary constituents of fossil fuels, plastics, and many synthetic materials. Based on their structure - specifically the types of bonds between carbon atoms and whether they form chains or rings - hydrocarbons are classified into several families, including alkanes, alkenes, alkynes, cycloalkanes, cycloalkenes, and aromatic compounds. Understanding this classification is fundamental to predicting physical properties, chemical reactivity, and environmental impact.
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，是化石燃料、塑料及许多合成材料的主要成分。根据碳原子之间的成键方式以及是否形成链或环，碳氢化合物可分为烷烃、烯烃、炔烃、环烷烃、环烯烃和芳香族化合物等多个类别。掌握这种分类对于预测物理性质、化学反应性及环境影响而言至关重要。
1. Definition and Scope | 定义与范围
A hydrocarbon is any organic molecule that contains only carbon and hydrogen. The carbon atoms may be joined in straight chains, branched chains, or rings, and the bonds between carbon atoms may be single, double, or triple. Hydrogen atoms fill the remaining valencies of carbon. Because hydrocarbons contain only C-C and C-H bonds, they are generally non-polar, insoluble in water, and soluble in non-polar organic solvents such as hexane or benzene.
碳氢化合物是只含碳和氢的有机分子。碳原子可以连成直链、支链或环，碳原子之间的键可以是单键、双键或三键。氢原子补足碳剩余价键。由于碳氢化合物只含 C-C 键和 C-H 键，它们通常是非极性的，不溶于水，可溶于己烷、苯等非极性有机溶剂。
The scope of hydrocarbon chemistry is very broad. Fossil fuels such as natural gas, petroleum, and coal contain mixtures of hydrocarbons. Plastics and many synthetic materials are derived from hydrocarbon feedstocks, especially ethene, propene, and benzene. Therefore, the classification of hydrocarbons is not only an academic exercise but also the basis for understanding fuels, polymers, and environmental pollution.
碳氢化合物化学的范围非常广。天然气、石油和煤等化石燃料含有多种碳氢化合物的混合物。塑料和许多合成材料来自碳氢化合物原料，尤其是乙烯、丙烯和苯。因此，碳氢化合物的分类不仅是学术练习，也是理解燃料、聚合物和环境污染的基础。
2. Basis of Classification | 分类依据
Hydrocarbons are classified according to three main structural features. The first is the type of carbon-carbon bond: single bonds give saturated hydrocarbons, double bonds give alkenes, and triple bonds give alkynes. The second is the carbon skeleton: the chain may be acyclic, open-chain, or cyclic, meaning it forms a ring. The third is the presence or absence of an aromatic ring system, especially a benzene ring with delocalised π electrons.
碳氢化合物主要依据三个结构特征分类。第一是碳碳键型：单键形成饱和烃，双键形成烯烃，三键形成炔烃。第二是碳骨架：链可以是无环的开链，也可以是环状。第三是是否存在芳香环体系，尤其是具有离域 π 电子的苯环。
These features are not independent. For example, a cyclic hydrocarbon may still be saturated if it contains only single C-C bonds, which is why cycloalkanes are placed with saturated hydrocarbons. A cyclic compound with a double bond is unsaturated and shows addition reactions. Aromatic compounds contain rings, but their bonding and reactivity are very different from ordinary cycloalkenes because of delocalisation.
这些特征并非彼此独立。例如，环状烃如果只含 C-C 单键，仍然是饱和烃，因此环烷烃被归入饱和烃。含双键的环状化合物是不饱和烃，能发生加成反应。芳香族化合物也含环，但由于离域作用，其成键和反应性与普通环烯烃差别很大。
Aliphatic → acyclic + cyclic; Aromatic → benzene-like rings
3. Classification Overview | 分类总览
Hydrocarbons are broadly divided into aliphatic and aromatic compounds. Aliphatic hydrocarbons can be acyclic, meaning open-chain, or cyclic. Acyclic aliphatic hydrocarbons are further split into alkanes, alkenes, and alkynes depending on whether they contain single, double, or triple carbon-carbon bonds. Cyclic aliphatic hydrocarbons include cycloalkanes and cycloalkenes. Aromatic hydrocarbons contain a benzene ring or a related delocalised ring system.
碳氢化合物大体上分为脂肪族化合物和芳香族化合物。脂肪族碳氢化合物可以是非环的开链结构，也可以是环状结构。非环脂肪烃根据含单键、双键或三键，进一步分为烷烃、烯烃和炔烃。环状脂肪烃包括环烷烃和环烯烃。芳香族碳氢化合物含有苯环或相关的离域环体系。
| Family | 类别 | Bonding | 成键 | General formula | 通式 | Example | 实例 |
|---|---|---|---|
| Alkane | 烷烃 | C-C single bonds | CₙH₂ₙ₊₂ | CH₄, C₂H₆ |
| Alkene | 烯烃 | One C=C double bond | CₙH₂ₙ | C₂H₄, C₃H₆ |
| Alkyne | 炔烃 | One C≡C triple bond | CₙH₂ₙ₋₂ | C₂H₂, C₃H₄ |
| Cycloalkane | 环烷烃 | Ring of C-C single bonds | CₙH₂ₙ | C₃H₆, C₆H₁₂ |
| Cycloalkene | 环烯烃 | Ring with one C=C bond | CₙH₂ₙ₋₂ | C₅H₈, C₆H₁₀ |
| Aromatic | 芳香烃 | Delocalised π system | Often CₙH₂ₙ₋₆ for arenes | C₆H₆, C₇H₈ |
4. Saturated and Unsaturated Hydrocarbons | 饱和烃与不饱和烃
Saturated hydrocarbons contain only single covalent bonds between carbon atoms. Alkanes and cycloalkanes are saturated. They have the maximum possible number of hydrogen atoms for a given carbon skeleton, so they cannot undergo addition reactions. Their main reactions are combustion and, under suitable conditions, substitution with halogens.
饱和烃的碳原子之间只含单共价键。烷烃和环烷烃是饱和烃。对于给定的碳骨架，它们含有最大可能数目的氢原子，因此不能发生加成反应。它们的主要反应是燃烧，以及在适当条件下与卤素发生取代反应。
Unsaturated hydrocarbons contain at least one carbon-carbon double or triple bond. Alkenes, alkynes, and cycloalkenes are unsaturated. They have fewer hydrogen atoms than the corresponding saturated compounds and can undergo addition reactions. A common test is bromine water: unsaturated hydrocarbons decolourise orange-brown bromine water quickly, while alkanes do not in the dark.
不饱和烃至少含一个碳碳双键或三键。烯烃、炔烃和环烯烃是不饱和烃。它们比相应的饱和化合物含氢更少，能发生加成反应。常用检验方法是溴水：不饱和烃会迅速使橙棕色溴水褪色，而烷烃在暗处不会。
C₂H₄ + Br₂ → C₂H₄Br₂ (rapid addition, bromine water decolourised)
5. Alkanes | 烷烃
Alkanes have the general formula CₙH₂ₙ₊₂ and contain only single C-C and C-H bonds. Each carbon atom is sp³ hybridised and tetrahedral. The C-C bond angle is about 109.5°. Alkanes are called paraffins because of their relatively low reactivity. They are non-polar, insoluble in water, and often used as fuels. Examples include methane CH₄, ethane C₂H₆, propane C₃H₈, and butane C₄H₁₀.
烷烃的通式为 CₙH₂ₙ₊₂，只含 C-C 单键和 C-H 单键。每个碳原子为 sp³ 杂化，呈四面体构型。C-C 键角约为 109.5°。烷烃因反应性较低而被称为石蜡烃。它们非极性，不溶于水，常被用作燃料。例子包括甲烷 CH₄、乙烷 C₂H₆、丙烷 C₃H₈ 和丁烷 C₄H₁₀。
Alkanes burn in excess oxygen to form carbon dioxide and water. This is a highly exothermic reaction and explains their use as fuels. In limited oxygen, incomplete combustion produces carbon monoxide and soot, which are harmful. Alkanes also react with halogens in the presence of ultraviolet light through free-radical substitution.
烷烃在过量氧气中燃烧生成二氧化碳和水。该反应高度放热，解释了其作为燃料的用途。在氧气不足时，不完全燃烧会产生一氧化碳和炭黑，它们是有害的。烷烃还在紫外光存在下与卤素发生自由基取代反应。
CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O
6. Alkenes | 烯烃
. The double bond consists of one σ bond and one π bond. The carbon atoms of the double bond are sp² hybridised and trigonal planar, with bond angles of about 120°. Ethene C₂H₄ is the simplest alkene.
烯烃至少含一个碳碳双键 C=C。对于只含一个双键的无环烯烃，通式为 CₙH₂ₙ，其中 n ≥ 2。双键由一个 σ 键和一个 π 键组成。双键碳原子为 sp² 杂化，呈平面三角形，键角约为 120°。乙烯 C₂H₄ 是最简单的烯烃。
The π bond is weaker and more exposed than the σ bond, so alkenes are much more reactive than alkanes. They undergo electrophilic addition with bromine, hydrogen, hydrogen halides, and water. They also undergo addition polymerisation. For example, ethene polymerises to poly(ethene), a common plastic.
π 键比 σ 键更弱、更暴露，因此烯烃比烷烃活泼得多。它们能与溴、氢气、卤化氢和水发生亲电加成反应。它们也能发生加聚反应。例如，乙烯聚合成聚乙烯，这是一种常见塑料。
C₂H₄ + H₂ → C₂H₆ (nickel catalyst, heat)
n C₂H₄ → [-CH₂-CH₂-]ₙ (poly(ethene))
7. Alkynes | 炔烃
Alkynes contain at least one carbon-carbon triple bond, C≡ sists of one σ bond and two π bonds. The carbon atoms are sp hybridised and linear, with a bond angle of 180°. Ethyne C₂H₂, also called acetylene, is the simplest alkyne.
炔烃至少含一个碳碳三键 C≡C。对于只含一个三键的无环炔烃，通式为 CₙH₂ₙ₋₂，其中 n ≥ 2。三键由一个 σ 键和两个 π 键组成。碳原子为 sp 杂化，呈直线形，键角为 180°。乙炔 C₂H₂ 是最简单的炔烃。
Alkynes are unsaturated and undergo addition reactions, similar to alkenes but often less readily. Ethyne is used in oxyacetylene welding because it burns with a very hot flame. A terminal alkyne has a hydrogen atom attached to a triple-bonded carbon, which is weakly acidic and can be removed by a strong base.
炔烃是不饱和烃，能发生加成反应，与烯烃类似，但通常较不活泼。乙炔用于氧乙炔焊接，因为它燃烧时产生非常高温的火焰。末端炔烃的三键碳上连有氢原子，该氢有弱酸性，可被强碱脱去。
C₂H₂ + 2 Br₂ → C₂H₂Br₄ (addition can occur twice)
8. Cycloalkanes and Cycloalkenes | 环烷烃与环烯烃
contain only single C-C bonds, so their chemical behaviour is similar to that of alkanes. They undergo combustion and free-radical substitution. Small rings such as cyclopropane and cyclobutane have significant ring strain, which makes them more reactive than larger cycloalkanes.
环烷烃是饱和的环状碳氢化合物。含一个由单键构成的碳环时，通式为 CₙH₂ₙ，其中 n ≥ 3。它们只含 C-C 单键，因此化学行为与烷烃相似，能发生燃烧和自由基取代。环丙烷、环丁烷等小环具有明显的环张力，因此比大环烷烃更活泼。
rated and undergo addition reactions across the C=C bond. Cyclohexene C₆H₁₀ is a common example and is often used to demonstrate the addition of bromine.
环烯烃的环内含有碳碳双键。含一个双键时，通式为 CₙH₂ₙ₋₂，其中 n ≥ 3。它们是不饱和烃，能在 C=C 键上发生加成反应。环己烯 C₆H₁₀ 是常见例子，常用于演示溴的加成反应。
C₆H₁₀ + Br₂ → C₆H₁₀Br₂ (bromine water decolourised)
9. Aromatic Hydrocarbons | 芳香烃
Aromatic hydrocarbons contain a benzene ring or a related delocalised π system. Benzene, C₆H₆, is the simplest and most important aromatic hydrocarbon. It is a planar, hexagonal molecule in which six p orbitals overlap to form a delocalised π system above and below the ring. All six C-C bonds are equal in length, intermediate between single and double bonds.
芳香烃含有苯环或相关的离域 π 体系。苯 C₆H₆ 是最简单、最重要的芳香烃。它是一个平面六边形分子，六个 p 轨道重叠，在环上下方形成离域 π 体系。六个 C-C 键长度相等，介于单键和双键之间。
The delocalisation makes benzene unusually stable, so it does not undergo addition reactions as readily as alkenes. Instead, aromatic compounds typically undergo electrophilic substitution, such as nitration, halogenation, and Friedel-Crafts alkylation or acylation. Substituted benzenes include toluene, C₆H₅CH₃, and phenol, C₆H₅OH, although phenol also contains an -OH group and is not a pure hydrocarbon.
离域作用使苯异常稳定，因此它不像烯烃那样容易发生加成反应。相反，芳香族化合物通常发生亲电取代，如硝化、卤代以及 Friedel-Crafts 烷基化或酰基化。取代苯包括甲苯 C₆H₅CH₃ 和苯酚 C₆H₅OH，不过苯酚还含 -OH 基团，不是纯碳氢化合物。
C₆H₆ + Br₂ → C₆H₅Br + HBr (FeBr₃ catalyst, electrophilic substitution)
10. Homologous Series and Isomerism | 同系物与同分异构
A homologous series is a family of organic compounds with the same general formula and similar chemical properties, in which successive members differ by a CH₂ unit. Alkanes, alkenes, alkynes, cycloalkanes, and cycloalkenes each form homologous series. As the number of carbon atoms increases, the physical properties change gradually, while the characteristic chemical reactions remain similar.
同系物是具有相同通式和相似化学性质的一族有机化合物，相邻成员相差一个 CH₂ 单元。烷烃、烯烃、炔烃、环烷烃和环烯烃各自形成同系物系列。随着碳原子数增加，物理性质逐渐变化，而特征化学反应保持相似。
Isomerism is also important in hydrocarbon classification. Structural isomers have the same molecular formula but different arrangements of atoms. Chain isomers differ in the branching of the carbon skeleton. Position isomers differ in the position of the double or triple bond or of a substituent. Functional group isomers belong to different families, for example ethanol and dimethyl ether both have the formula C₂H₆O, but only one is a hydrocarbon-related alcohol.
同分异构在碳氢化合物分类中也很重要。结构异构体分子式相同但原子排列不同。链异构体的碳骨架支链不同。位置异构体的双键、三键或取代基位置不同。官能团异构体属于不同类别，例如乙醇和二甲醚的分子式都是 C₂H₆O，但只有前者是醇类，并非纯碳
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