p-xylene

'''p-Xylene (para-xylene') is an aromatic hydrocarbon. It is one of the three isomers of dimethylbenzene known collectively as xylenes. The p- stands for para-, indicating that the two methyl groups in p-xylene occupy the diametrically opposite substituent positions 1 and 4. It is in the positions of the two methyl groups, their arene substitution pattern, that it differs from the other isomers, o-xylene and m-xylene. All have the same chemical formula C6H4(CH3)2. All xylene isomers are colorless and highly flammable. The odor threshold of p-xylene is 0.62 parts per million (ppm).

==Production== The production of p-xylene is industrially significant, with annual demand estimated at 37 million tons in 2014, and still on the increase. p-Xylene is produced by catalytic reforming of petroleum naphtha as part of the BTX aromatics (benzene, toluene and the xylene isomers) extracted from the catalytic reformate. The p-xylene is then separated out in a series of distillation, adsorption or crystallization and reaction processes from the m-xylene, o-xylene, and ethylbenzene. Its melting point is the highest among this series of isomers, but simple crystallization does not allow easy purification due to the formation of eutectic mixtures.