para-Xylene. Molecular model of the aromatic hydrocarbon para-xylene (C8.H10). The structure is that of benzene with two methyl substituents. The substituents are opposite each other on the phenyl (benzene) ring, hence the term 'para'. This chemical is uysed on a large scale for the manufacture of terephtalic acid for polyester. Atoms are represented as spheres and are colour-coded: carbon (grey) and hydrogen (white).