/\ NH /\ NH /\ N
/ \ / \ / \ /\\ / \ /\\
| | \ | | \\ | | CH
| | >CO or | | C.OH | | |
| | / | | / | | CH
\ / \ / \ / \ / \ / \//
\/ NH \/ NH , \/ N
\_______________________________/
Benzimidazolone Quinoxaline
In some cases oxidation of condensed benzenoid-heterocyclic nuclei
results in the rupture of the heterocyclic ring with the formation of
a benzene dicarboxylic acid; but if the aromatic nucleus be weakened
by the introduction of an amino group, then it is the benzenoid
nucleus which is destroyed and a dicarboxylic acid of the heterocyclic
ring system obtained.
Heterocyclic rings may be systematically surveyed from two aspects: (1)
by arranging the rings with similar hetero-atoms according to the
increasing number of carbon atoms, the so-called "homologous series"; or
(2) by first dividing the ring systems according to the number of
members constituting the ring, and then classifying these groups
according to the nature of the hetero-atoms, the so-called "isologous
series." The second method possesses greater advantages, for rings of
approximate stability come in one group, and, consequently, their
derivatives may be expected to exhibit considerable analogies.
As a useful preliminary it is convenient to divide heterocyclic ring
systems into two leading groups: (1) systems resulting from simple
internal dehydration (or similar condensations) of saturated aliphatic
compounds--such compounds are: the internal anhydrides or cyclic ethers
of the glycols and thioglycols (ethylene oxide, &c.); the cyclic
alkyleneimides resulting from the splitting off of ammonia between the
amino groups of diamino-paraffins (pyrrolidine, piperazine, &c.); the
cyclic esters of oxycarboxylic acids (lactones, lactides); the internal
anhydrides of aminocarboxylic acids (lactams, betaines); cyclic
derivatives of dicarboxylic acids (anhydrides, imides, alkylen-esters,
alkylen-amides, &c.). These compounds retain their aliphatic nature, and
are best classified with open-chain compounds, into which, in general,
they are readily converted. (2) Systems which are generally unsaturated
compounds, often of considerable stability, and behave as nuclei; these
compounds constitute a well-individualized class exhibiting closer
affinities to benzenoid substances than to the open-chain series.
The transition between the two classes as differentiated above may be
illustrated by the following cyclic compounds, each of which contains
a ring composed of four carbon atoms and one oxygen atom:
CH2.CH2\ CH2.CO \ CH2.CO\ CH.CO\ CH=CH\
| O | O | O || O | O
CH2.CH2/ CH2.CH2/ CH2.CO/ CH.CO/ CH=CH/
Tetramethylene Butyrolactone. Succinic Maleic Furfurane.
oxide. anhydride. anhydride.
The first four substances are readily formed from, and converted into,
the corresponding dihydroxy open-chain compound; these substances are
truly aliphatic in character. The fifth compound, on the other hand,
does not behave as an unsaturated aliphatic compound, but its
deportment is that of a nucleus, many substitution derivatives being
capable of synthesis. Reduction, however, converts it into an
aliphatic compound. This is comparable with the reduction of the
benzene nucleus into hexamethylene, a substance of an aliphatic
character.
True ring systems, which possess the characters of organic nuclei, do
not come into existence in three-and four-membered rings, their first
appearance being in penta-atomic rings. The three primary members are
furfurane, thiophene and pyrrol, each of which contains four methine or
CH groups, and an oxygen, sulphur and imido (NH) member respectively; a
series of compounds containing selenium is also known. The formulae of
these substances are:
CH=CH\ CH=CH\ CH=CH\ CH=CH\
| O | S | Se | NH
CH=CH/ CH=CH/ CH=CH/ CH=CH/
Furfurane. Thiophene. Selenophene. Pyrrol.
By substituting one or more CH groups in these compounds by nitrogen
atoms, ring-systems, collectively known as _azoles_, result. Obviously,
isomeric ring-systems are possible, since the carbon atoms in the
original rings are not all of equal value. Thus furfurane yields the
following rings by the introduction of one and two nitrogen atoms:
CH==N\ N==CH\ N===N\
| O | O | O
CH=CH/ CH=CH/ CH=CH/
Isoxazole. Oxazole. Diazo-oxides.
CH=N\ N=CH\ N=CH\
| O | O | O
CH=N/ CH=N/ N=CH/
Furazane. Azoximes. Oxybiazole.
Thiophene yields a similar series: isothiazole (only known as the
condensed ring, isobenzothiazole), thiazole, diazosulphides,
piazthioles, azosulphimes and thiobiazole (the formulae are easily
derived from the preceding series by replacing oxygen by sulphur).
Thiophene also gives rise to triazsulphole, three nitrogen atoms being
introduced. Selenophene gives the series: selenazole, diazoselenide and
piaselenole, corresponding to oxazole, diazo-oxides and furazane. Pyrrol
yields an analogous series: pyrazole, imidazole or glyoxaline, azimide
or osotriazole, triazole and tetrazole:
CH==N\ N==CH\ N===N\
| NH | NH | NH
CH=CH/ CH=CH/ CH=CH/
Pyrazole. Imidazole. Azimide.
N=CH\ N==N\
| NH | NH
N=CH/ N=CH/
Triazole. Tetrazole.
Six-membered ring systems can be referred back, in a manner similar to
the above, to pyrone, penthiophene and pyridine, the substances
containing a ring of five carbon atoms, and an oxygen, sulphur and
nitrogen atom respectively. As before, only _true_ ring nuclei, and not
internal anhydrides of aliphatic compounds, will be mentioned. From the
pyrone ring the following series of compounds are derived (for brevity,
the hydrogen atoms are not printed):
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