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xylene
Xylene (from Greek ξύλο, ''xylo'', "wood"), xylol or dimethylbenzene is an aromatic hydrocarbon mixture consisting of a benzene ring with two methyl groups at various substituted positions. The three isomers of xylene have the molecular formula C8H10, also represented by the semi-structural formula C6H4(CH3)2. Xylene is a major petrochemical produced by catalytic reforming and also by coal carbonisation in the manufacture of coke fuel. It represents about 0.5–1% of crude oil (depending on the source), and is found in small quantities in gasoline and aircraft fuels. Xylenes are mainly produced as part of the BTX aromatics (benzene, toluene and xylenes) extracted from the product of catalytic reforming known as "reformate". The mixture is a slightly greasy, colorless liquid commonly encountered as a solvent. 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.〔Cahours, Auguste (1850) ("Recherches sur les huiles légéres obtenues dans la distillation du bois" ) (Investigations of light oils obtained by the distillation of wood), ''Compte rendus'', 30 : 319-323 ; see especially p. 321. From p. 321: ''"Je le désignerai sous le nomme ''xylène''."'' (I will designate it by the name of ''xylene''.) Note: Cahours' empirical formula for xylene is incorrect because chemists at that time used the wrong atomic mass for carbon (8 instead of 16).〕 Several million tons are produced annually.〔Fabri, Jörg; Graeser, Ulrich, and Simo, Thomas A., (Xylenes ), Ullmann's Encyclopedia of Industrial Chemistry, 2000, Wiley-VCH, Weinheim. Accessed 2015-2-8 .〕 In 2011, a global consortium began construction of one of the world’s largest xylene plants in Singapore.〔Tremblay, Jean-François, (Making Aromatics In Singapore ), ''Chem. Eng. News'', 89 (38), 2011-9-19, pp 18–19. Accessed 2012-4-28.〕
==Isomers==
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.〔
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