Dimethyl benzenes

Xylenes () are an important petrochemical produced by catalytic reforming and also by coal carbonization 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 gasoaircraft fuels.

Xylene (Dimethyl benzenes)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.

The most widely used dimethylbenzene is 1,4-dimethylbenzene (p-xylene). It is oxidized to benzene-1,4-dicarboxylic acid (terehalic acid) which reacts with ethane-1,2-diol (ethylene glycol) to form the monomer for the polyester, polyethylene terehalate (PET).1,2-Dimethylbenzene (o-xylene) is oxidized to benzene-1,2-dicarboxylic anhydride (halic anhydride) which is in turn converted into the esters of benzene-1,2-dicarboxylic acid, for example di-octylbenzene-1,2-dicarboxylate, which are used as plasticizers. It is also used to make alkyd resins.