The more important nuclei of this type have received special and non-systematic names; when this is not the case, such terms as phen-, benzo-, naphthoare prefixed to the name of the heterocyclic ring.
The mono-benzo-derivatives are coumarone, benzothiophene and indole; the dibenzo-derivatives are diphenylene oxide, dibenzothiophene or diphenylene sulphide, and carbazole.
Benz-oxazoles and -thiazoles have been prepared, benz-isoxazoles are known as indoxazenes; benzo-pyrazoles occur in two structural forms, named indazoles and isindazoles.
Proceeding to the six-membered hetero-atomic rings, the benzo-, dibenzoand naphtho-derivatives are frequently of great commercial and scientific importance.
A-pyrone condenses with the benzene ring to form coumarin and isocoumarin; benzo-'y-pyrone constitutes the nucleus of several vegetable colouring matters (chrysin, fisetin, quercetin, &c., which are derivatives of flavone or phenyl benzo-y-pyrone); dibenzo--ypyrone is known as xanthone; related to this substance are fluorane (and fluorescein), fluorone, fluorime, pyronine, &c. The pyridine ring condenses with the benzene ring to form quinoline and isoquinoline; acridine and phenanthridine are dibenzo-pyridines; naphthalene gives rise to a-and /3-naphthoquinolines and the anthrapyridines; anthracene gives anthraquinoline; while two pyridine nuclei connected by an intermediate benzene nucleus give the phenanthrolines.