Benzene, toluene, and xylene. These aromatic molecules are very important components of gasoline and petrochemicals. Benzene, the simplest aromatic, is carcinogenic and its level in gasoline is severely restricted. Toluene and xylene have benzene rings to which are attached one or two methyl (CH3) groups, respectively; xylene has three isomers, with the methyls adjacent on the ring ( ortho), separated by one carbon ( meta), or separated by two carbons ( para); see figure.