Isobutylene
Names Preferred IUPAC name Other names Identifiers ChEBI ChemSpider ECHA InfoCard 100.003.697 EC Number RTECS number UNII UN number 1055In Liquefied petroleum gas: 1075 Properties[1] 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) −44.4·10−6 cm3/mol Structure C2v Hazards[2] GHS labelling: Danger H220 P210, P377, P381, P403 NFPA 704 (fire diamond) Flash point flammable gas 465 °C (869 °F; 738 K) Explosive limits 1.8-9.6% Related compounds 1-Butenecis-2-Butenetrans-2-Butene Isobutane
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]
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
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.
Isobutylene is a highly flammable gas.
- Butyl rubber
- Polythene
- Polybutene
- Perfluoroisobutene
- International Chemical Safety Card 1027
- SIDS Initial Assessment Report for Isobutylene from the Organisation for Economic Co-operation and Development (OECD)
