乙烯Ethylene

两个碳原子均为 sp² 杂化，形成一个 σ 键和一个由 p 轨道侧面重叠的 π 键，C=C 键长 133.9 pm，H–C–H 键角约 117°。π 键使分子不能绕 C=C 自由旋转，这是顺反异构的根源。π 电子暴露在分子平面两侧，容易受到亲电试剂进攻。

Both carbons are sp² hybridised and joined by a σ bond and a π bond formed by sideways overlap of p orbitals; the C=C bond is 133.9 pm and the H–C–H angle about 117°. The π bond prevents free rotation about C=C, which is the origin of cis–trans isomerism. The π electrons lie above and below the molecular plane and are open to attack by electrophiles.

1669 年贝歇尔加热乙醇与硫酸得到乙烯。1795 年四位荷兰化学家发现它与氯气生成油状液体（1,2-二氯乙烷），因此乙烯曾被称为“成油气”（olefiant gas），这也是 olefin（烯烃）一词的来源。现代工业通过石脑油或乙烷的蒸汽裂解生产乙烯，全球年产能超过 2 亿吨。

Becher obtained ethylene in 1669 by heating ethanol with sulfuric acid. In 1795 four Dutch chemists found it reacts with chlorine to give an oily liquid, 1,2-dichloroethane, so it was called olefiant gas, the source of the word olefin. Today it is made by steam cracking of naphtha or ethane, with world capacity above 200 million tonnes a year.

约 60% 的乙烯用于生产聚乙烯，其余制环氧乙烷（乙二醇）、氯乙烯（PVC）、苯乙烯和乙醇。乙烯也是植物催熟激素，果实成熟时会自身释放，商业上用于香蕉等水果的催熟。

About 60% of ethylene becomes polyethylene; the rest goes to ethylene oxide (and ethylene glycol), vinyl chloride (PVC), styrene and ethanol. It is also a plant hormone that fruit releases as it ripens, and it is used commercially to ripen bananas and other fruit.