📚 Hydrocarbons: Alkanes, Alkenes and Reaction Pathways | 烃：烷烃、烯烃与反应路径
Hydrocarbons are organic compounds that contain only carbon and hydrogen. They are the simplest family in organic chemistry and the starting point for fuels, polymers, solvents and large-scale industrial synthesis. In the Cambridge A-Level syllabus the examinable core is the structure and nomenclature of alkanes and alkenes, their physical properties, combustion, free-radical substitution, electrophilic addition, and the reaction pathways that link them to haloalkanes, alcohols, aldehydes, ketones and carboxylic acids.
烃是仅由碳和氢两种元素组成的有机化合物，是有机化学中最基础的一族，也是燃料、聚合物、溶剂与大规模工业合成的起点。在剑桥 A-Level 考纲中，考试核心是烷烃与烯烃的结构和命名、物理性质、燃烧、自由基取代、亲电加成，以及把它们与卤代烃、醇、醛、酮、羧酸联系起来的反应路径。
1. Classification and General Formulas | 烃的分类与通式
saturated or unsaturated. Saturated hydrocarbons contain only C-C single bonds, while unsaturated hydrocarbons contain at least one C=C double bond.
烃可分为脂肪烃、脂环烃和芳香烃三大类。脂肪烃具有直链或支链，可以是饱和的，也可以是不饱和的。饱和烃只含 C-C 单键，而不饱和烃至少含一个 C=C 双键。
Alkanes have the general formula CₙH₂ₙ₊₂ and alkenes with one double bond have the general formula CₙH₂ₙ. Cycloalkanes are alicyclic saturated hydrocarbons with the formula CₙH₂ₙ, the same as alkenes but with a ring structure rather than a double bond.
烷烃的通式为 CₙH₂ₙ₊₂，含一个双键的烯烃通式为 CₙH₂ₙ。环烷烃是脂环饱和烃，通式为 CₙH₂ₙ，与烯烃相同，但具有环状结构而不是双键。
| Family | 类别 | General formula | 通式 | Example | 实例 | Feature | 结构特征 |
| Alkane 烷烃 | CₙH₂ₙ₊₂ | CH₄, C₂H₆, C₃H₈ | Saturated, only σ C-C bonds 饱和，仅 C-C 单键 |
| Alkene 烯烃 | CₙH₂ₙ | C₂H₄, C₃H₆ | One C=C double bond 含一个 C=C 双键 |
| Cycloalkane 环烷烃 | CₙH₂ₙ | C₃H₆ (cyclopropane) | Ring, saturated 成环，饱和 |
| Arene 芳香烃 | CₙH₂ₙ₋₆ (mono-ring) | C₆H₆ (benzene) | Delocalised π system 离域 π 体系 |
A key exam point is that CₙH₂ₙ does not automatically mean an alkene: the same molecular formula can represent a cycloalkane, so the molecular formula alone is not enough to identify a functional group.
一个关键考点是：CₙH₂ₙ 并不一定就是烯烃，同一分子式也可能代表环烷烃，因此仅凭分子式不足以确定官能团。
2. Nomenclature and Isomerism | 命名与同分异构
Systematic IUPAC names are built from the longest continuous carbon chain. Prefixes such as meth-, eth-, prop-, but-, pent- and hex- indicate the number of carbon atoms in that chain. The suffix -ane is used for alkanes and -ene for alkenes, with a number showing the position of the double bond.
系统命名以最长的连续碳链为基础。meth-、eth-、prop-、but-、pent-、hex- 等前缀表示主链碳原子数。烷烃用后缀 -ane，烯烃用后缀 -ene，并用数字标出双键的位置。
nd the double bond takes priority over alkyl side chains. For example, CH₃CH=CHCH₃ is but-2-ene, not but-3-ene, and the double bond position is written immediately before the suffix.
编号有两条规则。必须从使双键（或取代基）位次最小的那一端开始编号，且双键的优先级高于烷基侧链。例如 CH₃CH=CHCH₃ 是 but-2-ene（丁-2-烯），而不是 but-3-ene，双键位次紧写在后缀之前。
Structural isomerism arises when the same molecular formula gives different arrangements of atoms. Chain isomerism appears in C₄H₁₀ (butane and 2-methylpropane, two isomers) and C₅H₁₂ (three isomers). Position isomerism appears in but-1-ene and but-2-ene, while functional group isomerism links C₄H₈ as an alkene, as cyclobutane and as methylcyclopropane.
结构异构是指同一分子式对应不同的原子连接方式。链异构见于 C₄H₁₀（丁烷与 2-甲基丙烷，共 2 种）和 C₅H₁₂（共 3 种）；位置异构见于 but-1-ene 与 but-2-ene；官能团异构则把 C₄H₈ 联系为烯烃、环丁烷和甲基环丙烷等多种形式。
s restricted by the π bond, but-2-ene exists as cis and trans isomers (E/Z), which have different physical properties and different boiling points.
烯烃还具有立体异构。由于 π 键限制 C=C 键的自由旋转，but-2-ene 存在顺式（cis）与反式（trans）异构体（即 E/Z 构型），两者物理性质和沸点不同。
3. Structure and Bonding: sp³ and sp² | 结构与成键：sp³ 与 sp²
In an alkane each carbon is sp³ hybridised. The four σ bonds point to the corners of a tetrahedron with bond angles of about 109.5°, and rotation about the C-C single bond is free.
烷烃中每个碳都是 sp³ 杂化，四个 σ 键指向四面体的四个顶点，键角约为 109.5°，C-C 单键可以自由旋转。
ls overlap side-on to form a π bond above and below the plane. The C=C unit is therefore planar and rigid.
烯烃双键中的每个碳为 sp² 杂化，三个 σ 键位于同一平面，键角约 120°；剩余的 p 轨道侧面重叠，在平面上下形成 π 键。因此 C=C 单元是平面且刚性的。
| Feature | 参数 | C-C single | C-C 单键 | C=C double | C=C 双键 |
| Bond length 键长 | 154 pm | 134 pm |
| Bond enthalpy 键焓 | 347 kJ mol⁻¹ | 612 kJ mol⁻¹ |
| Hybridisation 杂化 | sp³ | sp² |
| Shape 形状 | Tetrahedral 四面体 | Trigonal planar 平面三角 |
ot twice as strong. Subtracting the σ contribution shows that the π bond alone is worth roughly 265 kJ mol⁻¹, which is why the π electrons are more exposed, more easily polarised and attacked by electrophiles.
C=C 键比 C-C 键强，但并非两倍强。减去 σ 键的贡献后可知，单独的 π 键约为 265 kJ mol⁻¹，这正是 π 电子更暴露、更易被极化并受到亲电试剂进攻的原因。
4. Physical Properties and Intermolecular Forces | 物理性质与分子间作用力
Hydrocarbons are non-polar molecules held together only by instantaneous dipole-induced dipole (London dispersion) forces. They are insoluble in water but dissolve readily in non-polar organic solvents such as hexane, and they are less dense than water.
烃是非极性分子，分子间只有瞬时偶极诱导偶极作用（伦敦色散力）。它们不溶于水，但易溶于己烷等非极性有机溶剂，密度小于水。
Boiling points increase with chain length because a longer chain has more electrons and a larger surface area, so the London forces between molecules become stronger. Increasing branching lowers the boiling point, because a more compact, spherical molecule has less surface contact with its neighbours.
沸点随碳链增长而升高，因为链越长电子数越多、表面积越大，分子间伦敦力越强。支链增多则使沸点降低，因为分子更紧凑、更接近球形，与相邻分子的接触面积减小。
ightly more polarisable, an alkene boils at a very similar, usually marginally higher, temperature than the corresponding alkane.
室温下，C₁-C₄ 烷烃为气体，C₅-C₁₇ 为液体，更长的链为蜡状固体。由于 C=C 使电子云更易极化，烯烃与其对应烷烃的沸点非常接近，通常略高一些。
5. Combustion, Cracking and Fuel Chemistry | 燃烧、裂解与燃料化学
Alkanes and alkenes burn exothermically in excess oxygen to give carbon dioxide and water. Complete combustion of an alkane follows the general equation below.
烷烃和烯烃在足量氧气中放热燃烧，生成二氧化碳和水。烷烃完全燃烧遵循下面的通用方程式。
CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O
ut 890 kJ mol⁻¹. In a limited supply of air, combustion is incomplete and produces carbon monoxide or soot, which is both less energy-efficient and more dangerous, since CO is toxic and binds strongly to haemoglobin.
对甲烷而言，方程式简化为 CH₄ + 2O₂ → CO₂ + 2H₂O，放热约 890 kJ mol⁻¹。空气不足时燃烧不完全，生成一氧化碳或炭黑，既降低了能量利用效率，也更加危险，因为 CO 有毒且与血红蛋白结合很强。
Cracking converts long-chain alkanes from crude oil into shorter, more useful molecules. Thermal cracking uses high temperature, while catalytic cracking uses a zeolite catalyst at around 500 °C, and both produce a mixture of short-chain alkanes and alkenes. Steam cracking of naphtha is the main industrial source of ethene and propene for polymer manufacture.
裂解把原油中的长链烷烃转化为更短、更有用的分子。热裂解使用高温，催化裂解使用沸石催化剂并在约 500 °C 下进行，两者都得到短链烷烃与烯烃的混合物。石脑油蒸汽裂解是工业上生产乙烯和丙烯、用于制造聚合物的主要来源。
6. Free-Radical Substitution of Alkanes | 烷烃的自由基取代反应
Alkanes are unreactive towards most reagents because the C-C and C-H bonds are strong and non-polar. Their characteristic reaction is free-radical substitution with a halogen, which requires ultraviolet light or a temperature of about 300 °C to break the halogen molecule homolytically.
由于 C-C 键和 C-H 键强度高且无极性，烷烃对大多数试剂不活泼。其特征反应是与卤素发生自由基取代，需要紫外光或约 300 °C 的高温使卤素分子均裂。
The mechanism has three stages: initiation, propagation and termination. Using methane and chlorine as the example, the initiation step splits chlorine into two radicals, a process called homolytic fission.
该机理分三个阶段：引发、增长和终止。以甲烷与氯气为例，引发阶段使氯分子均裂生成两个自由基，这一过程称为均裂。
Initiation: Cl₂ → 2 Cl•
Propagation: Cl• + CH₄ → CH₃• + HCl then CH₃• + Cl₂ → CH₃Cl + Cl•
Termination: Cl• + Cl• → Cl₂ CH₃• + Cl• → CH₃Cl CH₃• + CH₃• → C₂H₆
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