p-xylene

p-xylene is 1,4-dimethylbenzene, ${\mathrm{C}}_6{\mathrm{H}}_4(\mathrm{C}{\mathrm{H}}_3{)}_2$. Its two methyl substituents occupy opposite positions on the benzene ring.

It has the same molecular formula as the ortho and meta isomers but a different molecular symmetry and packing behavior. Comparing these isomers illustrates why a formula is insufficient to predict every physical property of an aromatic compound.