2-Butene

0.626 g/ml (trans, 0.9 °C) −105.5 °C (trans) (E = 0.8 °C)

• −42.6·10−6 cm3/mol (cis)
• −43.3·10−6 cm3/mol (trans) |NFPA-H = 1 |NFPA-F = 4 |NFPA-R = 0 cis-2-Butene trans-2-Butene Isobutene Butyne

2-Butene is an acyclic alkene with four carbon atoms. It is the simplest alkene exhibiting cis/trans-isomerism (also known as (E/Z)-isomerism); that is, it exists as two geometric isomers cis-2-butene ((Z)-but-2-ene) and *trans-*2-butene ((E)-but-2-ene).

It is a petrochemical, produced by the catalytic cracking of crude oil or the dimerization of ethylene. Its main uses are in the production of high-octane gasoline (petrol) on alkylation units and butadiene, although some 2-butene is also used to produce the solvent butanone via hydration reaction to 2-butanol followed by oxidation.

The two isomers are extremely difficult to separate by distillation because of the proximity of their boiling points (~4 °C for cis and ~1 °C for trans). However, separation is unnecessary in most industrial settings, as both isomers behave similarly in most of the desired reactions. A typical industrial 2-butene mixture is 70% (Z)-but-2-ene (cis-isomer) and 30% (E)-but-2-ene (trans-isomer). Butane and 1-butene are common impurities, present at 1% or more in industrial mixtures, which also contain smaller amounts of isobutene, butadiene and butyne.