Nomenclature of Aromatic Compounds

Aromatic compounds are conventionally described in two parts: the nucleus (the benzene ring itself, drawn as a hexagon with alternating double bonds, a circle inside a hexagon, or explicitly as six CH units) and the side chain (any alkyl or other aliphatic group hanging off the ring in place of a ring hydrogen). Replacing one, two or three ring hydrogens with some other group gives, respectively, a monosubstituted, disubstituted or trisubstituted derivative. For a disubstituted ring specifically, the two substituents' relative positions have their own shorthand names: ortho (adjacent, the 1,2 relationship, 60 degrees apart around the ring, abbreviated o-), meta (one carbon further apart, the 1,3 relationship, 120 degrees apart, abbreviated m-), and para (directly across the ring, the 1,4 relationship, abbreviated p-); once three or more ring hydrogens are replaced, plain numerals (1,2,3...) are used instead of these letters.

Aromatic compounds themselves split into two naming families depending on WHERE their functional group sits. Nuclear-substituted aromatic compounds have their functional group attached directly to the ring, and are named as derivatives of benzene itself -- toluene (methylbenzene), the three xylene isomers (ortho-, meta- and para-dimethylbenzene), mesitylene (1,3,5-trimethylbenzene) and the various chloro- and dichlorobenzenes are all built this way. Side-chain-substituted aromatic compounds instead carry their functional group out on the alkyl chain hanging off the ring, and are named as phenyl derivatives of the corresponding aliphatic compound -- benzyl chloride (phenylchloromethane), benzaldichloride (phenyldichloromethane) and benzotrichloride (phenyltrichloromethane) illustrate the pattern. A standard set of aryl-group names recurs throughout this naming system: phenyl (C6H5-, the benzene ring itself as a substituent), benzyl (C6H5-CH2-), benzal (C6H5-CH=), benzo (C6H5-C(tr …