But-2-ene

But-2-ene (/ˈbjuːt.tu.in/) 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-but-2-ene ((Z)-but-2-ene) and trans-but-2-ene ((E)-but-2-ene).

But-2-ene
cis
trans
cis
trans
Names
Preferred IUPAC name
But-2-ene
Other names
β-Butylene
Identifiers
3D model (JSmol)
1718755 1361341
ChEBI
ChemSpider
ECHA InfoCard 100.003.140
EC Number
  • 203-452-9
25196 1140 1141
RTECS number
  • EM2932000 1718756
UNII
  • InChI=1S/C4H8/c1-3-4-2/h3-4H,1-2H3 checkY
    Key: IAQRGUVFOMOMEM-UHFFFAOYSA-N checkY
  • InChI=1/C4H8/c1-3-4-2/h3-4H,1-2H3
    Key: IAQRGUVFOMOMEM-UHFFFAOYAW
  • (cis): InChI=1S/C4H8/c1-3-4-2/h3-4H,1-2H3/b4-3-
    Key: IAQRGUVFOMOMEM-ARJAWSKDSA-N
  • (trans): InChI=1S/C4H8/c1-3-4-2/h3-4H,1-2H3/b4-3+
    Key: IAQRGUVFOMOMEM-ONEGZZNKSA-N
  • C(=CC)C
  • (cis): C/C=C\C
  • (trans): C/C=C/C
Properties
C4H8
Molar mass 56.106 g/mol
Density 0.641 g/ml (cis, 3.7 °C)[1]
0.626 g/ml (trans, 0.9 °C)[2]
Melting point −138.9 °C (−218.0 °F; 134.2 K) (cis)[1]
-105.5 °C (trans)[2]
Boiling point 0.8 to 3.7 °C (33.4 to 38.7 °F; 273.9 to 276.8 K) (Z = 3.7 °C)[1]
(E = 0.8 °C)[2]
  • -42.6·10−6 cm3/mol (cis)
  • -43.3·10−6 cm3/mol (trans)
Hazards[3]
GHS labelling:
Flam. Gas 1Press. Gas
Danger
H220
P210, P377, P381, P403
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
1
4
0
Flash point −72 °C (−98 °F; 201 K)[1][2]
325 °C (617 °F; 598 K)[1][2]
Related compounds
Related butenes
 1-Butene
cis-2-Butene
trans-2-Butene
Isobutene
Related compounds
 Butane
Butyne
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)
Infobox references

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 gasoline (petrol) and butadiene,[4] although some but-2-ene is also used to produce the solvent butanone via hydration 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[5]). However, separation is unnecessary in most industrial settings, as both isomers behave similarly in most of the desired reactions. A typical industrial but-2-ene mixture is 70% (Z)-but-2-ene (cis-isomer) and 30% (E)-but-2-ene (trans-isomer). Butane and but-1-ene are common impurities, present at 1% or more in industrial mixtures, which also contain smaller amounts of isobutene, butadiene and butyne.[4]

References
  1. Record in the GESTIS Substance Database of the Institute for Occupational Safety and Health
  2. Record in the GESTIS Substance Database of the Institute for Occupational Safety and Health
  3. cis-2-Butene, International Chemical Safety Card 0397, Geneva: International Programme on Chemical Safety, March 1996. trans-2-Butene, International Chemical Safety Card 0398, Geneva: International Programme on Chemical Safety, March 1996.
  4. 2-Butene (PDF), SIDS Initial Assessment Report, Geneva: United Nations Environment Programme, February 1995.
  5. Chemical Safety Information from Intergovernmental Organizations Archived December 9, 2009, at the Wayback Machine
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