Isomers of butylene

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but-1-ene

cis-but-2-ene

trans-but-2-ene

2-methylpropene

There are four isomers of alkenes which have the chemical formula C₄H₈. All four of these hydrocarbons have four carbon atoms and one double bond in their molecules, but have different chemical structures. The IUPAC and common names, respectively, of these chemical compounds are:

IUPAC name	common name
but-1-ene	α-butylene
cis-but-2-ene	cis-β-butylene
trans-but-2-ene	trans-β-butylene
2-methylpropene	isobutylene

The chemical structures are shown at right. The small blue numbers in the structure images are the numbering of the atoms in the main backbone chain of the molecules. Other organic compounds have the formula C₄H₈, namely cyclobutane and methylcyclopropane, but are not alkenes and are not discussed here. There are also four-carbon cyclic alkenes such as cyclobutene and methylcyclopropene, but they do not have the formula C₄H₈ and are not discussed here.

All four of these isomers are gases at room temperature and pressure, but can be liquified by lowering the temperature or raising the pressure on them, in a manner similar to pressurised butane. These gases are colourless, but do have distinct odours, and are highly flammable. Although not naturally present in petroleum in high percentages, they can be produced from petrochemicals or by catalytic cracking of petroleum. Although they are stable compounds, the carbon-carbon double bonds make them more reactive than similar alkanes, which are more inert compounds in various ways.

Because of the double bonds, these 4-carbon alkenes can act as monomers in the formation of polymers, as well as having other uses as petrochemical intermediates. They are used in the production of synthetic rubber. But-1-ene is a linear or normal alpha-olefin and isobutylene is a branched alpha-olefin. In a rather low percentage, but-1-ene is used as one of the comonomers, along with other alpha-olefins, in the production of high density polyethylene and linear low density polyethylene. Butyl rubber is made by cationic polymerisation of isobutylene with about 2 - 7% isoprene. Isobutylene is also used for the production of methyl tert-butyl ether (MTBE) and isooctane, both of which improve the combustion of gasoline.