烯烃Alkene

双键碳原子为 sp² 杂化，与相连的原子共平面，键角约 120°。C=C 由一个 σ 键和一个 π 键组成，π 键由两个平行 p 轨道侧面重叠形成，电子云分布在分子平面上下，较易受亲电试剂进攻。π 键使双键不能自由旋转，因此双键两端各连两个不同基团时存在顺反（E/Z）异构，如顺-2-丁烯和反-2-丁烯。

The double-bond carbons are sp² hybridised and lie in a plane with their attached atoms, at about 120°. C=C is one σ bond plus one π bond; the π bond forms by sideways overlap of parallel p orbitals, with electron density above and below the molecular plane, which is open to attack by electrophiles. The π bond prevents rotation, so when each end carries two different groups there are cis–trans (E/Z) isomers, such as cis- and trans-but-2-ene.

亲电加成：与溴水加成使其褪色，可用来检验 C=C；与 HBr 加成生成卤代烷；在磷酸催化下与水蒸气加成生成醇；在镍催化下与 H₂ 加成生成烷烃。氧化：冷的稀酸性高锰酸钾把烯烃氧化为二醇，热的浓高锰酸钾使双键断裂。加聚：许多烯烃分子相互加成生成聚乙烯、聚丙烯、聚氯乙烯等高分子。

Electrophilic addition: bromine water is decolourised, a test for C=C; HBr gives halogenoalkanes; steam with a phosphoric acid catalyst gives alcohols; H₂ over nickel gives alkanes. Oxidation: cold dilute acidified permanganate gives diols, and hot concentrated permanganate cleaves the double bond. Addition polymerisation: many alkene molecules join to give poly(ethene), poly(propene), PVC and other polymers.

工业上主要由石油馏分的裂化（尤其是蒸汽裂化）得到乙烯和丙烯，它们是产量最大的有机化工原料，用于生产塑料、乙醇、环氧乙烷等。乙烯也是植物激素，能促进果实成熟。

Industrially, ethene and propene come mainly from cracking (especially steam cracking) of petroleum fractions; they are the largest-volume organic feedstocks, used to make plastics, ethanol, epoxyethane and more. Ethene is also a plant hormone that speeds fruit ripening.