The Homologous Group of Alkanes | 烷烃同系物

📚 The Homologous Group of Alkanes | 烷烃同系物

Alkanes form one of the most fundamental homologous series in organic chemistry. Understanding their structure, naming, physical trends and key reactions is essential for the Cambridge A-Level Chemistry syllabus.

烷烃是有机化学中最基础的同系物之一。理解烷烃的结构、命名、物理性质递变规律和主要反应，是剑桥 A-Level 化学课程的核心要求。

1. What Is a Homologous Series? | 什么是同系物？

A homologous series is a family of organic compounds that share the same general formula, the same functional group and similar chemical properties. Each successive member differs from the previous one by a CH₂ unit, called a methylene group.

同系物是指具有相同通式、相同官能团和相似化学性质的一类有机化合物。相邻成员之间相差一个 CH₂（亚甲基）单元。

Because members of a homologous series have a repeating structural unit, their physical properties such as boiling point and viscosity change in a regular way as molar mass increases.

由于同系物具有重复的结构单元，它们的沸点、黏度等物理性质会随着摩尔质量增加而呈现规律性变化。

Alkanes are the simplest homologous series in organic chemistry, and they provide a clear model for understanding how molecular size and shape influence properties.

烷烃是有机化学中最简单的同系物，为理解分子大小和形状如何影响性质提供了一个清晰的模型。

2. General Formula of Alkanes | 烷烃的通式

Alkanes are saturated hydrocarbons that contain only single C–C and C–H bonds. The general formula for the straight-chain and branched alkane homologous series is:

烷烃是只含 C–C 单键和 C–H 单键的饱和烃。直链和支链烷烃同系物的通式为：

CₙH₂ₙ₊₂

For example, when n = 1 the formula is CH₄ (methane); when n = 2 it is C₂H₆ (ethane); when n = 3 it is C₃H₈ (propane).

例如，当 n = 1 时分子式为 CH₄（甲烷）；n = 2 时为 C₂H₆（乙烷）；n = 3 时为 C₃H₈（丙烷）。

The term ‘saturated’ means the molecule contains the maximum possible number of hydrogen atoms per carbon atom, because there are no double or triple bonds.

“饱和”表示分子中每个碳原子连接的氢原子数已达到最大，因为没有双键或三键。

The general formula CₙH₂ₙ₊₂ applies to open-chain alkanes. Cycloalkanes have the general formula CₙH₂ₙ and are not part of this particular homologous series.

通式 CₙH₂ₙ₊₂ 适用于开链烷烃。环烷烃的通式为 CₙH₂ₙ，不属于这一同系物。

3. Structure and Bonding in Alkanes | 烷烃的结构与成键

Each carbon atom in an alkane is sp³ hybridised and forms four sigma (σ) bonds arranged in a tetrahedral shape. The H–C–H bond angle in methane is approximately 109.5°.

烷烃中每个碳原子均为 sp³ 杂化，形成四个 σ 键，空间构型为四面体。甲烷中 H–C–H 键角约为 109.5°。

C–C and C–H bonds are both strong and almost non-polar. Carbon and hydrogen have similar electronegativity values, so alkanes do not carry significant partial charges.

C–C 键和 C–H 键都很强，而且几乎非极性。碳和氢的电负性相近，因此烷烃中不存在明显的部分电荷。

Because alkane molecules are non-polar, the only intermolecular forces between them are weak London dispersion forces, also called instantaneous dipole–induced dipole forces.

由于烷烃分子为非极性分子，分子间仅存在微弱的伦敦色散力，也称为瞬时偶极–诱导偶极力。

Free rotation is possible around C–C single bonds at room temperature, so alkane chains are flexible rather than rigid. This affects their shape and packing.

常温下 C–C 单键可以自由旋转，因此烷烃碳链是柔性的，而非刚性结构。这会影响分子的形状与堆积方式。

4. Naming Alkanes Using IUPAC Rules | 烷烃的 IUPAC 命名

The first ten straight-chain alkanes have the stem names meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non- and dec-, each with the suffix -ane.

前十个直链烷烃的词头分别为 meth-、eth-、prop-、but-、pent-、hex-、hept-、oct-、non- 和 dec-，后缀均为 -ane。