The Alkenes | 烯烃

📚 The Alkenes | 烯烃

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond (C=C). The double bond is the functional group responsible for the characteristic addition reactions of alkenes, making them important starting materials in organic synthesis and the petrochemical industry.

烯烃是一类含有至少一个碳碳双键 (C=C) 的不饱和烃。双键是决定烯烃特征加成反应的官能团，使烯烃成为有机合成和石化工业中重要的起始原料。

1. Structure and Bonding | 结构与成键

In ethene (CH₂=CH₂), each carbon atom uses three sp² hybrid orbitals to form three sigma (σ) bonds: one to the other carbon and two to hydrogen atoms. The remaining unhybridised 2p orbital on each carbon overlaps sideways to form a pi (π) bond above and below the plane of the molecule.

在乙烯 (CH₂=CH₂) 中，每个碳原子使用三个 sp² 杂化轨道形成三个 σ 键：一个与另一个碳原子成键，两个与氢原子成键。每个碳上剩余的一个未杂化的 2p 轨道在分子平面的上下方侧向重叠，形成一个 π 键。

The double bond therefore consists of one σ bond and one π bond. The π bond is weaker than the σ bond, and the carbon-carbon double bond is shorter and stronger than a single C–C bond. The bond angle around each carbon is about 120°, giving a planar arrangement.

因此，双键由一个 σ 键和一个 π 键组成。π 键比 σ 键弱，碳碳双键比单键 C–C 更短、更强。每个碳周围的键角约为 120°，分子呈平面排列。

Rotation around the C=C double bond is restricted because the π bond would have to break. This restricted rotation gives rise to geometric isomerism.

由于 π 键必须断裂才能转动，围绕 C=C 双键的旋转受到限制。这种旋转受限导致了几何异构现象。

2. General Formula and Homologous Series | 通式与同系物

For acyclic alkenes containing one double bond, the general formula is CₙH₂ₙ. Each member of the homologous series differs from the next by a –CH₂– unit, and they show similar chemical properties because they contain the same C=C functional group.

对于含有一个双键的无环烯烃，其通式为 CₙH₂ₙ。同系物中相邻成员相差一个 –CH₂– 单元，由于都含有相同的 C=C 官能团，它们表现出相似的化学性质。

Examples include ethene (C₂H₄), propene (C₃H₆), but-1-ene and but-2-ene (C₄H₈), and pent-1-ene (C₅H₁₀). As the chain length increases, boiling points increase due to stronger van der Waals forces.

例子包括乙烯 (C₂H₄)、丙烯 (C₃H₆)、丁-1-烯和丁-2-烯 (C₄H

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