📚 Alkenes | 烯烃
industrial uses. Understanding the electron-rich C=C bond is key to explaining reactivity.
烯烃是含有至少一个碳碳双键的不饱和烃。本部分涵盖其成键、异构现象、特征加成反应及工业用途。理解富电子的 C=C 双键是解释反应性的关键。
1. Structure and Bonding in Alkenes | 烯烃的结构与成键
Alkenes follow the general formula CₙH₂ₙ for acyclic alkenes with one double bond. The simplest alkene is ethene, C₂H₄, in which each carbon atom uses sp² hybrid orbitals to form three σ bonds, giving a trigonal planar geometry with bond angles of approximately 120°.
烯烃的通式为 CₙH₂ₙ（含一个双键的无环烯烃）。最简单的烯烃是乙烯 C₂H₄，每个碳原子使用 sp² 杂化轨道形成三个 σ 键，呈平面三角形，键角约为 120°。
d formed by end-on overlap of sp² orbitals and a π bond formed by sideways overlap of adjacent p orbitals. The π bond restricts rotation around the double bond and is the reactive centre because it is more exposed and weaker than the σ bond.
C=C 双键由一个 sp² 轨道的头对头重叠形成的 σ 键，以及相邻 p 轨道侧面重叠形成的 π 键组成。π 键限制双键旋转，并且由于比 σ 键更暴露、更弱，因此成为反应中心。
2. Nomenclature and E/Z Isomerism | 命名与 E/Z 异构
ble bond the lowest possible locant. For example, CH₃CH=CHCH₃ is but-2-ene, not but-1-ene.
烯烃命名时选择含有 C=C 双键的最长碳链，将 -ane 结尾改为 -ene，并使双键位次尽可能小。例如 CH₃CH=CHCH₃ 是丁-2-烯，而不是丁-1-烯。
nd gives rise to stereoisomerism. If each carbon of the double bond carries two different groups, cis-trans or E/Z isomers can exist. E/Z notation uses Cahn-Ingold-Prelog priority: the E isomer has the higher priority groups on opposite sides of the double bond, while the Z isomer has them on the same side.
由于 C=C 双键旋转受限，可产生立体异构。如果双键每个碳上连有两个不同基团，就可能存在 cis-trans 或 E/Z 异构体。E/Z 标记法使用 Cahn-Ingold-Prelog 优先规则：E 异构体中高优先基团位于双键两侧，而 Z 异构体中高优先基团位于同侧。
methyl groups can be on the same side (Z) or opposite sides (E). cis-trans terminology is only used when the two substituents on each carbon are identical, such as the two H atoms and two CH₃ groups in but-2-ene.
对丁-2-烯 CH₃CH=CHCH₃ 而言，两个甲基可以在同侧（Z）或两侧（E）。cis-trans 术语仅用于每个碳上两个取代基相同的场合，如丁-2-烯中两个 H 和两个 CH₃。
3. Physical Properties and Reactivity | 物理性质与反应性
Alkenes are essentially non-polar molecules, so their intermolecular forces are weak van der Waals forces only. They are insoluble in water but soluble in non-polar solvents such as hexane. Boiling points increase with increasing chain length because larger molecules have stronger van der Waals forces.
烯烃基本上是非极性分子，因此分子间作用力仅为微弱的范德华力。它们不溶于水，但溶于己烷等非极性溶剂。沸点随碳链增长而升高，因为分子越大，范德华力越强。
The high electron density of the π bond makes alkenes attractive to electrophiles. Since the π bond is weaker than a σ bond, it breaks readily to allow addition reactions, turning the unsaturated alkene into a saturated compound.
π 键的高电子密度使烯烃对亲电试剂有吸引力。由于 π 键比 σ 键弱，它容易断裂并发生加成反应，使不饱和烯烃转变为饱和化合物。
4. Electrophilic Addition Mechanism | 亲电加成机理
lectrophilic addition, the C=C double bond attacks the electrophile, and the π bond breaks
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