We have previously referred to the condensation of heterocyclic ring
systems containing two vicinal carbon atoms with benzene, naphthalene
and other nuclei. 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-, naphtho- are prefixed to the name of the heterocyclic
ring. One or two benzene nuclei may suffer condensation with the
furfurane, thiophene and pyrrol rings, the common carbon atoms being
vicinal to the hetero-atom. The mono-benzo-derivatives are coumarone,
benzothiophene and indole; the dibenzo-derivatives are diphenylene
oxide, dibenzothiophene or diphenylene sulphide, and carbazole. Typical
formulae are (R denoting O, S or NH):
/\ /\ /\
/ \ ____ / \ ____ / \
| | | | | | |
| | | | | | |
\ / \ / \ / \ / \ /
\/ \/ , \/ \/ \/
R R
Isomers are possible, for the condensation may be effected on the two
carbon atoms symmetrically placed to the hetero-atom; these isomers,
however, are more of the nature of internal anhydrides. Benz-oxazoles
and -thiazoles have been prepared, benz-isoxazoles are known as
indoxazenes; benzo-pyrazoles occur in two structural forms, named
indazoles and isindazoles. Derivatives of osotriazol also exist in two
forms--azimides and pseudo-azimides.
Proceeding to the six-membered hetero-atomic rings, the benzo-,
dibenzo-and naphtho-derivatives are frequently of great commercial and
scientific importance, [alpha]-pyrone condenses with the benzene ring to
form coumarin and isocoumarin; benzo-[gamma]-pyrone constitutes the
nucleus of several vegetable colouring matters (chrysin, fisetin,
quercetin, &c., which are derivatives of flavone or phenyl
benzo-[gamma]-pyrone); dibenzo-[gamma]-pyrone 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 [alpha]- and
[beta]-naphthoquinolines and the anthrapyridines; anthracene gives
anthraquinoline; while two pyridine nuclei connected by an intermediate
benzene nucleus give the phenanthrolines. Naphthyridines and
naphthinolines result from the condensation of two pryridine and two
quinoline nuclei respectively; and quino-quinolines are unsymmetrical
naphthyridine nuclei condensed with a benzene nucleus.
Benzo-orthoxazines, -metoxazines and -paroxazines are known:
dibenzoparoxazine or phenoxazine is the parent of a valuable series of
dyestuffs; dibenzoparathiazine or thiodiphenylamine is important from the
same aspect. Benzo-ortho-diazines exist in two structural forms, cinnolin
and phthalazine; benzo-meta-diazines are known as quinazolines;
benzo-para-diazines are termed quinoxalines; the dibenzo-compounds are
named phenazines, this last group including many valuable
dyestuffs--indulines, safranines, &c. In addition to the types of
compounds enumerated above we may also notice purin, tropine and the
terpenes.