📚 Alkanes: Structure, Reactions and Fuels | 烷烃：结构、反应与燃料
Alkanes are saturated hydrocarbons containing only carbon and hydrogen, with single covalent bonds between carbon atoms. They form the backbone of the fuels industry and are the starting point for understanding organic reaction mechanisms. In AQA A-level Chemistry topic 3.2, you need to link structure, bonding and physical properties to the industrial processes of fractional distillation and cracking, as well as to combustion and free-radical substitution.
烷烃是仅由碳和氢组成的饱和烃，碳原子之间仅存在单共价键。它们构成了燃料工业的基础，也是理解有机反应机理的起点。在 AQA A-level 化学 3.2 主题中，你需要将结构、成键和物理性质与分馏、裂化的工业过程，以及燃烧和自由基取代反应联系起来。
1. General Formula and Homologous Series | 通式与同系列
Alkanes are a homologous series with the general formula CₙH₂ₙ₊₂. Each member differs from the next by a CH₂ unit. The first four straight-chain members are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).
烷烃是一个同系列，通式为 CₙH₂ₙ₊₂。相邻成员之间相差一个 CH₂ 单元。前四个直链成员分别是甲烷 (CH₄)、乙烷 (C₂H₆)、丙烷 (C₃H₈) 和丁烷 (C₄H₁₀)。
Homologous series show a gradual trend in physical properties, such as boiling point, while their chemical properties remain similar because they share the same functional group. For alkanes, the functional group is effectively the saturated C–C and C–H single-bond framework.
同系列的物理性质（如沸点）呈现逐渐变化的趋势，而化学性质保持相似，因为它们具有相同的官能团。对烷烃而言，官能团实际上就是饱和的 C–C 和 C–H 单键骨架。
2. Bonding and Shape | 成键与形状
Each carbon atom in an alkane uses four electron pairs in single covalent bonds. The electron-pair geometry around each carbon is tetrahedral, with a bond angle of approximately 109.5°.
烷烃中的每个碳原子使用四对电子形成单共价键。每个碳周围的电子对几何构型为四面体形，键角约为 109.5°。
Carbon-carbon single bonds are sigma bonds formed by end-on overlap of atomic orbitals. Sigma bonds allow free rotation of groups around the bond, so alkanes are not rigid molecules and can adopt many conformations.
碳-碳单键是通过原子轨道端对端重叠形成的 σ 键。σ 键允许基团围绕键自由旋转，因此烷烃不是刚性分子，可以采取多种构象。
Because carbon and hydrogen have very similar electronegativity, C–C and C–H bonds are almost non-polar. This has a direct effect on the physical properties of alkanes, particularly their low solubility in water and their dependence on weak intermolecular forces.
由于碳和氢的电负性非常相近，C–C 和 C–H 键几乎是非极性的。这直接影响烷烃的物理性质，尤其是它们在水中的低溶解度以及对弱分子间作用力的依赖。
3. Physical Properties and Boiling Points | 物理性质与沸点
Alkanes are non-polar or almost non-polar molecules because carbon and hydrogen have similar electronegativity. The only intermolecular forces between alkane molecules are London dispersion forces, also called van der Waals forces.
烷烃是非极性或几乎非极性的分子，因为碳和氢的电负性相近。烷烃分子之间唯一的分子间作用力是伦敦色散力，也称为范德华力。
As chain length increases, the number of electrons increases and the surface contact area becomes larger, so London forces become stronger. This explains why boiling points increase from methane to long-chain alkanes.
随着链长增加，电子数增多，分子接触面积变大，伦敦力变强。这解释了为什么从甲烷到长链烷烃沸点逐渐升高。
Branching lowers boiling point. Branched isomers have smaller surface areas and cannot pack as closely as straight-chain isomers, so their London forces are weaker and their boiling points are lower.
支链会降低沸点。支链异构体的表面积较小，不能像直链异构体那样紧密堆积，因此伦敦力较弱，沸点较低。
4. Fractional Distillation of Crude Oil | 原油分馏
Crude oil is a mixture of many hydrocarbons, mainly alkanes. Fractional distillation separates the mixture into fractions with similar boiling points. The column is hot at the bottom and cooler at the top.
原油是多种烃类（主要是烷烃）的混合物。分馏将混合物分离为沸点相近的馏分。分馏塔底部温度高，顶部温度低。
Smaller alkanes with low boiling points rise to the top of the column and are collected as gases or petrol. Larger alkanes with high boiling points condense lower down, giving fractions such as kerosene, diesel and fuel oil.
沸点低的小分子烷烃上升到塔顶，以气体或汽油形式收集。沸点高的大分子烷烃在较低处冷凝，得到煤油、柴油和燃料油等馏分。
The process is physical rather than chemical because no covalent bonds are broken; the components are only separated by evaporation and condensation.
该过程是物理过程而非化学过程，因为没有共价键断裂；各组分只是通过蒸发和冷凝得到分离。
5. Cracking: Thermal and Catalytic | 裂化：热裂化与催化裂化
Cracking converts long-chain, less useful alkanes into shorter-chain alkanes and alkenes, which are in higher demand as fuels and as chemical feedstock. There are two main types: thermal cracking and catalytic cracking.
裂化将长链、用途较少的烷烃转化为需求量更大的短链烷烃和烯烃，用作燃料和化学原料。主要有两种类型：热裂化和催化裂化。
Thermal cracking uses high temperature, around 700–1200 K, and high pressure, up to 7000 kPa. It produces a high proportion of alkenes and is used to make compounds such as ethene.
热裂化使用高温（约 700–1200 K）和高压（最高约 7000 kPa）。它产生高比例的烯烃，用于制造乙烯等化合物。
Catalytic cracking uses a zeolite catalyst, lower temperature, around 720 K, and lower pressure. It produces branched alkanes, cycloalkanes and aromatic compounds, which burn more smoothly in petrol engines and improve fuel quality.
催化裂化使用沸石催化剂、较低温度（约 720 K）和较低压力。它产生支链烷烃、环烷烃和芳香族化合物，这些成分在汽油发动机中燃烧更平稳，可提高燃料质量。
C₁₀H₂₂ → C₆H₁₄ + C₄H₈
The example shows a long-chain alkane cracking into a shorter alkane and an alkene. The exact products depend on the conditions and the catalyst used.
该示例显示长链烷烃裂化为较短烷烃和烯烃。具体产物取决于反应条件和所用催化剂。
6. Combustion of Alkanes | 烷烃的燃烧
Alkanes are important fuels because they burn in oxygen to release large amounts of energy. Complete combustion occurs when there is an excess of oxygen, producing carbon dioxide and water.
烷烃是重要的燃料，因为它们在氧气中燃烧释放大量能量。当氧气过量时发生完全燃烧，生成二氧化碳和水。
