Isobutylene

Isobutylene (or 2-methylpropene) is a hydrocarbon with the chemical formula (CH₃)₂C=CH₂. It is a four-carbon branched alkene (olefin), one of the four isomers of butylene. It is a colorless flammable gas, and is of considerable industrial value.^[3]

Isobutylene

Skeletal formula of isobutylene
Ball-and-stick model of isobutylene Space-filling model of isobutylene
Names
Preferred IUPAC name 2-Methylprop-1-ene
Other names 2-Methylpropene Isobutene γ-Butylene 2-Methylpropylene Methylpropene
Identifiers
CAS Number	115-11-7 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:43907 N
ChemSpider	7957 Y
ECHA InfoCard	100.003.697
EC Number	204-066-3
PubChem CID	8255
RTECS number	UD0890000
UNII	QA2LMR467H Y
UN number	1055 In Liquefied petroleum gas: 1075
CompTox Dashboard (EPA)	DTXSID9020748
InChI InChI=1S/C4H8/c1-4(2)3/h1H2,2-3H3 Y Key: VQTUBCCKSQIDNK-UHFFFAOYSA-N Y InChI=1/C4H8/c1-4(2)3/h1H2,2-3H3
SMILES CC(=C)C
Properties[1]
Chemical formula	C4H8
Molar mass	56.106 g/mol
Appearance	Colorless gas
Density	0.5879 g/cm3, liquid
Melting point	−140.3 °C (−220.5 °F; 132.8 K)
Boiling point	−6.9 °C (19.6 °F; 266.2 K)
Magnetic susceptibility (χ)	−44.4·10−6 cm3/mol
Structure
Point group	C2v
Hazards[2]
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H220
Precautionary statements	P210, P377, P381, P403
NFPA 704 (fire diamond)	1 4 0
Flash point	flammable gas
Autoignition temperature	465 °C (869 °F; 738 K)
Explosive limits	1.8–9.6%
Related compounds
Related butenes	1-Butene cis-2-Butene trans-2-Butene
Related compounds	Isobutane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Production

Polymer and chemical grade isobutylene is typically obtained by dehydrating tertiary butyl alcohol (TBA) or catalytic dehydrogenation of isobutane.^[4] Gasoline additives methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE), respectively, are produced by reacting methanol or ethanol with isobutylene contained in butene streams from olefin steam crackers or refineries, or with isobutylene from dehydrated TBA. Isobutylene is not isolated from the olefin or refinery butene stream before the reaction, as separating the ethers from the remaining butenes is simpler. Isobutylene can also be produced in high purities by "back-cracking" MTBE or ETBE at high temperatures and then separating the isobutylene by distillation from methanol.

Isobutylene is a byproduct in the ethenolysis of diisobutene to prepare neohexene:^[5]

(CH₃)₃C-CH=C(CH₃)₂ + CH₂=CH₂ → (CH₃)₃C-CH=CH₂ + (CH₃)₂C=CH₂

Uses

Isobutylene is used in the production of a variety of products. It is alkylated with butane to produce isooctane or dimerized to diisobutylene (DIB) and then hydrogenated to make isooctane, a fuel additive. Isobutylene is also used in the production of methacrolein. Polymerization of isobutylene produces butyl rubber (polyisobutylene or PIB). Antioxidants such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) are produced by Friedel-Crafts alkylation of phenols with isobutylene.

tert-Butylamine is produced commercially by amination of isobutylene using zeolite catalysts:^[6]

NH₃ + CH₂=C(CH₃)₂ → H₂NC(CH₃)₃

Applications are found in the calibration of photoionization detectors.

Safety

Isobutylene is a highly flammable gas.

See also

• Butyl rubber
• Polythene
• Polybutene
• Perfluoroisobutene