Xylene

Xylene (from Greek ξύλο, xylo, 'wood'), xylol or dimethylbenzene is any one of three isomers of dimethylbenzene, or a combination thereof. With the formula (CH3)2C6H4, each of the three compounds has a central benzene ring with two methyl groups attached at substituents. They are all colorless, flammable liquids, some of which are of great industrial value. The mixture is referred to as both xylene and, more precisely, xylenes. Xylene (from Greek ξύλο, xylo, 'wood'), xylol or dimethylbenzene is any one of three isomers of dimethylbenzene, or a combination thereof. With the formula (CH3)2C6H4, each of the three compounds has a central benzene ring with two methyl groups attached at substituents. They are all colorless, flammable liquids, some of which are of great industrial value. The mixture is referred to as both xylene and, more precisely, xylenes. 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. The xylene mixture is a slightly greasy, colorless liquid commonly encountered as a solvent. 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. Xylene exists in three isomeric forms. The isomers can be distinguished by the designations ortho- (o-), meta- (m-) and para- (p-), which specify to which carbon atoms (of the benzene ring) the two methyl groups are attached. By counting the carbon atoms around the ring starting from one of the ring carbons bonded to a methyl group, and counting towards the second methyl group, the o-isomer has the IUPAC name of 1,2-dimethylbenzene, the m-isomer is 1,3-dimethylbenzene and the p-isomer is 1,4-dimethylbenzene. Of the three isomers, the p-isomer is the most industrially sought after since it can be oxidized to terephthalic acid. 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 chemical and physical properties of xylene differ according to the respective isomers. The melting point ranges from −47.87 °C (−54.17 °F) (m-xylene) to 13.26 °C (55.87 °F) (p-xylene)—as usual, the para isomer's melting point is much higher because it packs more readily in the crystal structure. The boiling point for each isomer is around 140 °C (284 °F). The density of each isomer is around 0.87 g/mL (7.26 lb/U.S. gallon or 8.72 lb/imp gallon) and thus is less dense than water. Xylene in air can be smelled at concentrations as low as 0.08 to 3.7 ppm (parts of xylene per million parts of air) and can be tasted in water at 0.53 to 1.8 ppm.