Ladenburg's prism admits of one mono-substitution derivative and three
di-derivatives. Furthermore, it is in accordance with certain simple
syntheses of benzene derivatives (e.g. from acetylene and acetone);
but according to Baeyer (_Ber._, 1886, 19, p. 1797) it fails to
explain the formation of dioxyterephthalic ester from succinosuccinic
ester, unless we make the assumption that the transformation of these
substances is attended by a migration of the substituent groups. For
succinosuccinic ester, formed by the action of sodium on two molecules
of succinic ester, has either of the formulae (I) or (II); oxidation
of the free acid gives dioxyterephthalic acid in which the
para-positions must remain substituted as in (I) and (II). By
projecting Ladenburg's prism on a plane and numbering the atoms so as
to correspond with Kekule's form, viz. that 1.2 and 1.6 should be
ortho-positions, 1.3 and 1.5 meta-, and 1.4 para-, and following out
the transformation on the Ladenburg formula, then an
ortho-dioxyterephthalic acid (IV) should result, a fact denied by
experience, and inexplicable unless we assume a wandering of atoms.
Kekule's formula (III), on the other hand, is in full agreement
(Baeyer). This explanation has been challenged by Ladenburg (_Ber._,
1886, 19, p. 971; _Ber._, 1887, 20, p. 62) and by A.K. Miller (_J.C.S.
Trans._, 1887, p. 208). The transformation is not one of the oxidation
of a hexamethylene compound to a benzenoid compound, for only two
hydrogen atoms are removed. Succinosuccinic ester behaves both as a
ketone and as a phenol, thereby exhibiting desmotropy; assuming the
ketone formula as indicating the constitution, then in Baeyer's
equation we have a migration of a hydrogen atom, whereas to bring
Ladenburg's formula into line, an oxygen atom must migrate.
The relative merits of the formulae of Kekule, Claus and Dewar were
next investigated by means of the reduction products of benzene, it
being Baeyer's intention to detect whether double linkages were or
were not present in the benzene complex.
To follow Baeyer's results we must explain his nomenclature of the
reduced benzene derivatives. He numbers the carbon atoms placed at the
corners of a hexagon from 1 to 6, and each side in the same order, so
that the carbon atoms 1 and 2 are connected by the side 1, atoms 2 and
3 by the side 2, and so on. A doubly linked pair of atoms is denoted
by the sign [DELTA] with the index corresponding to the side; if there
are two pairs of double links, then indices corresponding to both
sides are employed. Thus [DELTA]^1 denotes a tetrahydro derivative in
which the double link occupies the side 1; [DELTA]^{1.3}, a dihydro
derivative, the double links being along the sides 1 and 3. Another
form of isomerism is occasioned by spatial arrangements, many of the
_reduced_ terephthalic acids existing in two stereo-isomeric forms.
Baeyer explains this by analogy with fumaric and maleic acids: he
assumes the reduced benzene ring to lie in a plane; when both carboxyl
groups are on the same side of this plane, the acids, in general,
resemble maleic acids, these forms he denotes by [GAMMA]_cis-cis_, or
shortly _cis_-; when the carboxyl groups are on opposite sides, the
acids correspond to fumaric acid, these forms are denoted by
[GAMMA]_cis-trans_, or shortly _trans_-.
By reducing terephthalic acid with sodium amalgam, care being taken to
neutralize the caustic soda simultaneously formed by passing in carbon
dioxide, [DELTA]^{2.5} dihydroterephthalic acid is obtained; this
results from the splitting of a _para_-linkage. By boiling with water
the [DELTA]^{2.5} acid is converted into the [DELTA]^{1.5}
dihydroterephthalic acid. This acid is converted into the
[DELTA]^{1.4} acid by soda, and into the [DELTA]^2 tetrahydro acid by
reduction. From this acid the [DELTA]^{1.3} dihydro and the [DELTA]^1
tetrahydro acids may be obtained, from both of which the hexahydro
acid may be prepared. From these results Baeyer concluded that Claus'
formula with three para-linkings cannot possibly be correct, for the
[DELTA]^{2.5} dihydroterephthalic acid undoubtedly has two ethylene
linkages, since it readily takes up two or four atoms of bromine, and
is oxidized in warm aqueous solution by alkaline potassium
permanganate. But the formation of the [DELTA]^{2.5} acid as the first
reduction product is not fully consistent with Kekule's symbol, for we
should then expect the [DELTA]^{1.3} or the [DELTA]^{1.5} acid to be
first formed (see also POLYMETHYLENES).
The stronger argument against the ethylenoid linkages demanded by
Kekule's formula is provided by the remarkable stability towards
oxidizing and reducing agents which characterizes all benzenoid
compounds. From the fact that reduction products containing either one
or two double linkages behave exactly as unsaturated aliphatic
compounds, being readily reduced or oxidized, and combining with the
halogen elements and haloid acids, it seems probable that in benzenoid
compounds the fourth valencies are symmetrically distributed in such a
manner as to induce a peculiar stability in the molecule. Such a
configuration was proposed in 1887 by H.E. Armstrong (_J.C.S. Trans._,
1887, p. 258), and shortly afterwards by Baeyer (_Ann._, 1888, 245, p.
103). In this formula, the so-called "centric formula," the assumption
made is that the fourth valencies are simply _directed_ towards the
centre of the ring; nothing further is said about the fourth valencies
except that they exert a pressure towards the centre. Claus maintained
that Baeyer's view was identical with his own, for as in Baeyer's
formula, the fourth valencies have a different function from the
peripheral valencies, being united at the centre in a form of potential
union.
It is difficult to determine which configuration most accurately
explains the observed facts; Kekule's formula undoubtedly explains the
synthetical production of benzenoid compounds most satisfactorily, and
W. Marckwald (_Ann._, 1893, 274, p. 331; 1894, 279, p. 14) has supported
this formula from considerations based on the syntheses of the quinoline
ring. Further researches by Baeyer, and upon various nitrogenous ring
systems by E. Bamberger (a strong supporter of the centric formula),
have shown that the nature of the substituent groups influences the
distribution of the fourth valencies; therefore it may be concluded that
in compounds the benzene nucleus appears to be capable of existence in
two tautomeric forms, in the sense that each particular derivative
possesses a definite constitution. The benzene nucleus presents a
remarkable case, which must be considered in the formulation of any
complete theory of valency. From a study of the reduction of compounds
containing two ethylenic bonds united by a single bond, termed a
"conjugated system," E. Thiele suggested a doctrine of "partial
valencies," which assumes that in addition to the ordinary valencies,
each doubly linked atom has a partial valency, by which the atom first
interacts. When applied to benzene, a twofold conjugated system is
suggested in which the partial valencies of adjacent atoms neutralize,
with the formation of a potential double link. The stability of benzene
is ascribed to this conjugation.[14]
Physico-chemical methods.
Physico-chemical properties have also been drawn upon to decide whether
double unions are present in the benzene complex; but here the
predilections of the observers apparently influence the nature of the
conclusions to be drawn from such data. It is well known that singly,
doubly and trebly linked carbon atoms affect the physical properties of
substances, such as the refractive index, specific volume, and the heat
of combustion; and by determining these constants for many substances,
fairly definite values can be assigned to these groupings. The general
question of the relation of the refractive index to constitution has
been especially studied by J.W. Bruhl, who concluded that benzene
contained 3 double linkages; whereas, in 1901, Pellini (_Gazetta_, 31,
i. p. 1) calculated that 9 single linkages were present. A similar
contradiction apparently exists with regard to the specific volume, for
while benzene has a specific volume corresponding to Claus' formula,
toluene, or methylbenzene, rather points to Kekule's. The heat of
combustion, as first determined by Julius Thomsen, agreed rather better
with the presence of nine single unions. His work was repeated on a
finer scale by M.P.E. Berthelot of Paris, and F.C.A. Stohmann of
Leipzig; and the new data and the conclusions to be drawn from them
formed the subject of much discussion, Bruhl endeavouring to show how
they supported Kekule's formula, while Thomsen maintained that they
demanded the benzene union to have a different heat of combustion from
the acetylene union. Thomsen then investigated heats of combustion of
various benzenoid hydrocarbons--benzene, naphthalene, anthracene,
phenanthrene, &c.--in the crystallized state. It was found that the
results were capable of expression by the empirical relation C_{a}H_{2b}
= 104.3b + 49.09m + 105.47n, where C_{a}H_{2b} denotes the formula of
the hydrocarbon, m the number of single carbon linkings and n the number
of double linkings, m and n being calculated on the Kekule formulae.
But, at the same time, the constants in the above relation are not
identical with those in the corresponding relation empirically deduced
from observations on fatty hydrocarbons; and we are therefore led to
conclude that a benzene union is considerably more stable than an
ethylene union.
Mention may be made of the absorption spectrum of benzene. According to
W.N. Hartley (_J.C.S._, 1905, 87, p. 1822), there are six bands in the
ultra-violet, while E.C.C. Baly and J.N. Collie (_J.C.S._, 1905, 87, p.
1332; 1906, 89, p. 524) record seven. These bands are due to molecular
oscillations; Hartley suggests the carbon atoms to be rotating and
forming alternately single and double linkages, the formation of three
double links giving three bands, and of three single links another
three; Baly and Collie, on the other hand, suggest the making and
breaking of links between adjacent atoms, pointing out that there are
seven combinations of one, two and three pairs of carbon atoms in the
benzene molecule.
_Stereo-chemical Configurations._--Simultaneously with the discussions
of Kekule, Ladenburg, Claus, Baeyer and others as to the merits of
various plane formulae of the benzene complex, there were published many
suggestions with regard to the arrangement of the atoms in space, all of
which attempted to explain the number of isomers and the equivalence of
the hydrogen atoms. The development of stereo-isomerism at the hands of
J. Wislicenus, Le Bel and van 't Hoff has resulted in the introduction
of another condition which formulae for the benzene complex must
satisfy, viz. that the hydrogen atoms must all lie in one plane. The
proof of this statement rests on the fact that if the hydrogen atoms
were not co-planar, then substitution derivatives (the substituting
groups not containing asymmetric carbon atoms) should exist in
enantiomorphic forms, differing in crystal form and in their action on
polarized light; such optical antipodes have, however, not yet been
separated. Ladenburg's prism formula would give two enantiomorphic
ortho-di-substitution derivatives; while forms in which the hydrogen
atoms are placed at the corners of a regular octahedron would yield
enantiomorphic tri-substitution derivatives.
The octahedral formula discussed by Julius Thomsen (_Ber._, 1886, 19,
p. 2944) consists of the six carbon atoms placed at the corners of a
regular octahedron, and connected together by the full lines as shown
in (I); a plane projection gives a hexagon with diagonals (II).
Reduction to hexamethylene compounds necessitates the disruption of
three of the edges of the octahedron, the diagonal linkings remaining
intact, or, in the plane projection, three peripheral linkages, the
hexamethylene ring assuming the form (III):
[Illustration: I II III]
In 1888 J.E. Marsh published a paper (_Phil. Mag._ [V.], 26, p. 426)
in which he discussed various stereo-chemical representations of the
benzene nucleus. (The stereo-chemistry of carbon compounds has led to
the spatial representation of a carbon atom as being situated at the
centre of a tetrahedron, the four valencies being directed towards the
apices; see above, and ISOMERISM.) A form based on Kekule's formula
consists in taking three pairs of tetrahedra, each pair having a side
in common, and joining them up along the sides of a regular hexagon by
means of their apices. This form, afterwards supported by Carl Graebe
(_Ber._, 1902, 35, p. 526; see also Marsh's reply, _Journ. Chem. Soc.
Trans._, 1902, p. 961) shows the proximity of the ortho-positions, but
fails to explain the identity of 1.2 and 1.6 compounds. Arrangements
connected with Claus' formula are obtained by placing six tetrahedra
on the six triangles formed by the diagonals of a plane hexagon. The
form in which the tetrahedra are all on one side, afterwards discussed
by J. Loschmidt (_Monats._, 1890, II, p. 28), would not give
stereo-isomers; and the arrangement of placing the tetrahedra on
alternate sides, a form afterwards developed by W. Vaubel (_Journ. Pr.
Chem._, 1894[2], 49, p. 308), has the advantage of bringing the
meta-positions on one side, and the ortho- and para- on opposite
sides, thus exhibiting the similarity actually observed between these
series of compounds. Marsh also devised a form closely resembling that
of Thomsen, inasmuch as the carbon atoms occupied the angles of a
regular octahedron, and the diagonal linkages differed in nature from
the peripheral, but differing from Thomsen's since rupture of the
diagonal and not peripheral bonds accompanied the reduction to
hexamethylene.
We may also notice the model devised by H. Sachse (_Ber._, 1888, 21,
2530; _Zeit. fur phys. Chem._, II, p. 214; 23, p. 2062). Two parallel
triangular faces are removed from a cardboard model of a regular
octahedron, and on the remaining six faces tetrahedra are then placed;
the hydrogen atoms are at the free angles. This configuration is,
according to Sachse, more stable than any other form; no oscillation
is possible, the molecule being only able to move as a whole. In 1897,
J.N. Collie (_Journ. Chem. Soc. Trans._, p. 1013) considered in detail
an octahedral form, and showed how by means of certain simple
rotations of his system the formulae of Kekule and Claus could be
obtained as projections. An entirely new device, suggested by B. Konig
(_Chem. Zeit._, 1905, 29, p. 30), assumed the six carbon atoms to
occupy six of the corners of a cube, each carbon atom being linked to
a hydrogen atom and by single bonds to two neighbouring carbon atoms,
the remaining valencies being directed to the unoccupied corners of
the cube, three to each, where they are supposed to satisfy each
other.
_Condensed Nuclei._
Restricting ourselves to compounds resulting from the fusion of benzene
rings, we have first to consider naphthalene, C10H8, which consists of
two benzene rings having a pair of carbon atoms in common. The next
members are the isomers anthracene and phenanthrene, C14H10, formed from
three benzene nuclei. Here we shall only discuss the structure of these
compounds in the light of the modern benzene theories; reference should
be made to the articles NAPHTHALENE, ANTHRACENE and PHENANTHRENE for
syntheses, decompositions, &c.
_Naphthalene._--Of the earlier suggestions for the constitution of
naphthalene we notice the formulae of Wreden (1) and (2), Berthelot and
Balls (3), R.A.C.E. Erlenmeyer (4) and Adolf Claus (5).
//\ //\ /|\ /|\ /|\ //\ /\\
// \ // \ / | \ / | \ / | \ // \ / \\
// \ __CH2 // \/ | \ / | \/ | \ // \/ \\
| || | | || | || || | || | || | || |
| || | /||CH | || | || || | || | || | || |
| || |/ || | || | || || | || | || | || |
\\ /---C || \\ /\ | / \ | /\ | / \\ /\ //
\\ / \ || \\ / \ | / \ | / \ | / \\ / \ //
\\/ \||CH \\/ \|/ \|/ \|/ \\/ \//
(1) (2) (3) (4)
/|\ /|\
/ | \ / | \
|\ | /| | ||
| \|/ | | ||
| | | | ||
| /|\ | | ||
|/ | \| | ||
\ | / \ | /
\|/ \|/
(5)
The first suggestion is quite out of the question. C. Graebe in 1866
(_Ann._ 149, p. 20) established the symmetry of the naphthalene nucleus,
and showed that whichever half of the molecule be oxidized the same
phthalic acid results. Therefore formula (2), being unsymmetrical, is
impossible. The third formula is based on Dewar's benzene formula, which
we have seen to be incorrect. Formula (4) is symmetrical and based on
Kekule's formula: it is in full accord with the syntheses and
decompositions of the naphthalene nucleus and the number of isomers
found. In 1882 Claus suggested a combination of his own and Dewar's
benzene formulae. This is obviously unsymmetrical, consisting of an
aliphatic and an aromatic nucleus; Claus explained the formation of the
same phthalic acid from the oxidation of either nucleus by supposing
that if the aromatic group be oxidized, the aliphatic residue assumes
the character of a benzene nucleus. Bamberger opposed Claus' formula on
the following grounds:--The molecule of naphthalene is symmetrical,
since 2.7 dioxynaphthalene is readily esterified by methyl iodide and
sulphuric acid to a dimethyl ether; and no more than two
mono-substitution derivatives are known. The molecule is aromatic but
not benzenoid; however, by the reduction of one half of the molecule,
the other assumes a benzenoid character.
If [beta]-naphthylamine and [beta]-naphthol be reduced, tetrahydro
products are obtained in which the amino- or oxy-bearing half of the
molecule becomes aliphatic in character. The compounds so obtained,
alicyclic-[beta]-tetrahydronaphthylamine and
alicyclic-[beta]-tetrahydronaphthol, closely resemble
[beta]-aminodiethylbenzene, C6H4(C2H5).C2H4NH2, and
[beta]-oxydiethylbenzene, C6H4(C2H5).C2H4OH. If [alpha]-naphthylamine
and [alpha]-naphthol be reduced, the hydrogen atoms attach themselves
to the non-substituted half of the molecule, and the compounds so
obtained resemble aminodiethylbenzene, C6H3.NH2(C2H5)2 and
oxydiethylbenzene, C6H3.OH(C2H5)2. Bamberger's observations on reduced
quinoline derivatives point to the same conclusion, that condensed
nuclei are not benzenoid, but possess an individual character, which
breaks down, however, when the molecule is reduced.
It remains, therefore, to consider Erlenmeyer's formula and those
derived from the centric hypothesis. The former, based on Kekule's
symbol for benzene, explains the decompositions and syntheses of the
ring, but the character of naphthalene is not in keeping with the
presence of five double linkages, although it is more readily acted upon
than benzene is. On the centric hypothesis two formulae are possible:
(i) due to H.E. Armstrong, and (2) due to E. Bamberger.
/|\ /|\ /|\ /|\
/ \ / \ / \ / \
|\ -|- /| |\ X /|
| | | | / \ |
| | | | |
| | | | \ / |
|/ -|- \| |/ X \|
\ / \ / \ / \ /
\|/ \|/ \|/ \|/
(1) (2)
In the first symbol it is assumed that one of the affinities of each of
the two central carbon atoms common to the two rings _acts into_ both
rings, an assumption involving a somewhat wide departure from all
ordinary views as to the manner in which affinity acts. This symbol
harmonizes with the fact that the two rings are in complete sympathy, the
one responding to every change made in the other. Then, on account of the
relatively slight--because divided--influence which would be exercised
upon the two rings by the two affinities common to both, the remaining
four centric affinities of each ring would presumably be less attracted
into the ring than in the case of benzene; consequently they would be
more active outwards, and combination would set in more readily. When, as
in the formation of naphthalene tetrachloride, for example, the one ring
becomes saturated, the other might be expected to assume the normal
centric form and become relatively inactive. This is absolutely the
case. On the other hand, if substitution be effected in the one ring, and
the affinities in that ring become attracted inwards, as apparently
happens in the case of benzene, the adjoining ring should become
relatively more active because the common affinities would act less into
it. Hence, unless the radical introduced be one which exercises a special
attractive influence, substitution should take place in preference in the
previously unsubstituted ring. In practice this usually occurs; for
example, on further bromination, [alpha]-bromonaphthalene yields a
mixture of the (1.4) and (1.5) dibromonaphthalenes; and when
nitronaphthalene is either brominated, or nitrated or sulphonated, the
action is practically confined to the second ring. The centric formula
proposed by Bamberger represents naphthalene as formed by the fusion of
two benzene rings, this indicates that it is a monocyclic composed of ten
atoms of carbon. The formula has the advantage that it may be constructed
from tetrahedral models of the carbon atom; but it involves the
assumption that the molecule has within it a mechanism, equivalent in a
measure to a system of railway points, which can readily close up and
pass into that characteristic of benzene.
_Anthracene and Phenanthrene._--These isomeric hydrocarbons, of the
formula C14H10, are to be regarded as formed by the fusion of three
benzenoid rings as represented by the symbols:--
____
/\ /\ /\ / \
/ \ / \ / \ ____/ \____
| | | | / \ / \
| | | | / \____/ \
\ / \ / \ / \ / \ /
\/ \/ \/ \____/ \____/