Chapter C: C C N C

/ \ / \ / \ / \ / \
C / \ C C / \ C C / \ N C / \ C C / \ C
| | | | | | | | | |
| | | | | | | | | |
C \ / C C \ / N C \ / C C \ / C N \ / N
\ / \ / \ / \ / \ /
O O O O O

Pyrone Ortho-oxazine Meta-oxazine Paroxazine Azoxazine
or pentoxazoline

Penthiophene gives, by a similar introduction of nitrogen atoms, penthiazoline, corresponding to meta-oxazine, and para-thiazine, corresponding to paroxazine (para-oxazine). Pyridine gives origin to: pyridazine or ortho-diazine, pyrimidine or meta-diazine, pyrazine or para-diazine, osotriazine, _unsymmetrical_ triazine, _symmetrical_ triazine, osotetrazone and tetrazine. The skeletons of these types are (the carbon atoms are omitted for brevity):

N
/ \ / \ / \ / \
/ \ / \ / \ N / \
| | | | | | | |
| | | | | | | |
\ / \ / N \ / \ /
\ / \ / \ / \ /
N N N N

Pyridine Pyridazine Pyrimidine Pyrazine

/ \ / \ / \
/ \ N / \ N / \ N
| | | | | |
| | | | | |
N \ / N \ / N \ /
\ / \ / \ /
N N N
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Triazines

N N
/ \ / \
/ \ N N / \
| | | |
| | | |
\ / N \ / N
\ / \ /
N N

Osotetrazone Tetrazone

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.