In both cases the medial ring is most readily attacked; and various
formulae have been devised which are claimed by their authors to
represent this and other facts. According to Armstrong, anthracene
behaves unsymmetrically towards substituents, and hence one lateral ring
differs from the other; he represents the molecule as consisting of one
centric ring, the remaining medial and lateral ring being ethenoid.
Bamberger, on the other hand, extends his views on benzene and
naphthalene and assumes the molecule to be (1). For general purposes,
however, the symbol (2), in which the lateral rings are benzenoid and
the medial ring fatty, represents quite adequately the syntheses,
decompositions, and behaviour of anthracene.
/|\ /|\ /|\ / \ /|\ / \
/ \ / \ / \ / \ / | \ / \
|\ X X /| | | | | |
| / \ / \ | | | | | |
| | | | | | |
| \ / \ / | | | | | |
|/ X X \| | | | | |
\ / \ / \ / \ / \ | / \ /
\|/ \|/ \|/ \ / \|/ \ /
(1) (2)
Phenanthrene is regarded by Armstrong as represented by (3), the lateral
rings being benzenoid, and the medial ring fatty; Bamberger, however,
regards it as (4), the molecule being entirely aromatic. An interesting
observation by Baeyer, viz. that stilbene, C6H5.CH:CH.C6H5, is very
readily oxidized, while phenanthrene is not, supports, in some measure,
the views of Bamberger.
____ ____
/----\ /\ /\
____/ \____ ____/_ _\____
/ \ / \ /\ / \ /\
/ \____/ \ / __/__\__ \
\ / \ / \ / \ /
\____/ \____/ \/__\/ \/__\/
(3) (4)
_Heterocyclic Compounds._
During recent years an immense number of ringed or cyclic compounds have
been discovered, which exhibit individual characters more closely
resembling benzene, naphthalene, &c. than purely aliphatic substances,
inasmuch as in general they contain double linkages, yet withstand
oxidation, and behave as nuclei, forming derivatives in much the same
way as benzene. By reduction, the double linkages become saturated, and
compounds result which stand in much about the same relation to the
original nucleus as hexamethylene does to benzene. In general,
therefore, it may be considered that the double linkages are not of
exactly the same nature as the double linkage present in ethylene and
ethylenoid compounds, but that they are analogous to the potential
valencies of benzene. The centric hypothesis has been applied to these
rings by Bamberger and others; but as in the previous rings considered,
the ordinary representation with double and single linkages generally
represents the syntheses, decompositions, &c.; exceptions, however, are
known where it is necessary to assume an oscillation of the double
linkage. Five- and six-membered rings are the most stable and important,
the last-named group resulting from the polymerization of many
substances; three- and four-membered rings are formed with difficulty,
and are easily ruptured; rings containing seven or more members are
generally unstable, and are relatively little known. The elements which
go to form heterocyclic rings, in addition to carbon, are oxygen,
sulphur, selenium and nitrogen. It is remarkable that sulphur can
replace two methine or CH groups with the production of compounds
greatly resembling, the original one. Thus benzene, (CH)6, gives
thiophene, (CH)4S, from which it is difficultly distinguished; pyridine,
(CH)5N, gives thiazole, (CH)3.N.S, which is a very similar substance;
naphthalene gives thionaphthen, C8H6S, with which it shows great
analogies, especially in the derivatives. Similarly a CH group may be
replaced by a nitrogen atom with the production of compounds of similar
stability; thus benzene gives pyridine, naphthalene gives quinoline and
isoquinoline; anthracene gives acridine and [alpha] and [beta]
anthrapyridines. Similarly, two or more methine groups may be replaced
by the same number of nitrogen atoms with the formation of rings of
considerable stability.
Most of the simple ring systems which contain two adjacent carbon
atoms may suffer fusion with any other ring (also containing two
adjacent carbon atoms) with the production of nuclei of greater
complexity. Such _condensed nuclei_ are, in many cases, more readily
obtained than the parent nucleus. The more important types are derived
from aromatic nuclei, benzene, naphthalene, &c.; the
ortho-di-derivatives of the first named, lending themselves
particularly to the formation of condensed nuclei. Thus
ortho-phenylene diamine yields the following products:--
/\ N /\ N /\ N
/ \ /\\ / \ /\\ / \ /|\
| | \\ | | \\ | | | \
| | CH | | N | | | S
| | / | | / | | | /
\ / \ / \ / \ / \ / \|/
\/ NH , \/ NH , \/ N ,