Xylene

In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are substituted determines which of three structural isomers results. It is a colorless, flammable, slightly greasy liquid of great industrial value. The mixture is referred to as both xylene and, more precisely, xylenes. Mixed xylenes refers to a mixture of the xylenes plus ethylbenzene. The four compounds have identical empirical formulas . Typically the four compounds are produced together by various catalytic reforming and pyrolysis methods. Xylenes are an important petrochemical produced by catalytic reforming and also by coal carbonisation in the manufacture of coke fuel. They also occur in crude oil in concentrations of about 0.5–1%, depending on the source. Small quantities occur in gasoline and aircraft fuels. Xylenes are produced mainly as part of the BTX aromatics (benzene, toluene, and xylenes) extracted from the product of catalytic reforming known as reformate. Several million tons are produced annually. In 2011, a global consortium began construction of one of the world's largest xylene plants in Singapore. Xylene was first isolated and named in 1850 by the French chemist Auguste Cahours (1813–1891), having been discovered as a constituent of wood tar. Xylenes are produced by the methylation of toluene and benzene. Commercial or laboratory-grade xylene produced usually contains about 40–65% of m-xylene and up to 20% each of o-xylene, p-xylene and ethylbenzene. The ratio of isomers can be shifted to favor the highly valued p-xylene via the patented UOP-Isomar process or by transalkylation of xylene with itself or trimethylbenzene. These conversions are catalyzed by zeolites. ZSM-5 is used to facilitate some isomerization reactions leading to mass production of modern plastics. The physical properties of the isomers of xylene differ slightly.