text: Aromatic compounds play key roles in the biochemistry of all living things.  The four aromatic amino acids histidine, phenylalanine, tryptophan, and tyrosine each serve as one of the 20 basic building-blocks of proteins.  Further, all 5 nucleotides (adenine, thymine, cytosine, guanine, and uracil) that make up the sequence of the genetic code in DNA and RNA are aromatic purines or pyrimidines.  The molecule heme contains an aromatic system with 22 π electrons. Chlorophyll also has a similar aromatic system.
  Aromatic compounds are important in industry.  Key aromatic hydrocarbons of commercial interest are benzene, toluene, ortho-xylene and para-xylene.  About 35 million tonnes are produced worldwide every year.  They are extracted from complex mixtures obtained by the refining of oil or by distillation of coal tar, and are used to produce a range of important chemicals and polymers, including styrene, phenol, aniline, polyester and nylon.