HC+-----+CH CH CH
|\ /| /|\ /|\
| \ / | / | \ HC--/-+-\--CH
| |CH| HC+--+--+CH \/ \|/ \/
HC+--|--+CH HC+--+--+CH /\ /|\ /\
\ | / \ | / HC--\-+-/--CH
\|/ \|/ \|/
CH CH CH
IV V VI
\______________________ ________________________/
v
Ladenburg
Objections to Kekule's formula.
One of the earliest and strongest objections urged against Kekule's
formula was that it demanded two isomeric ortho-di-substitution
derivatives; for if we number the carbon atoms in cyclical order from
1 to 6, then the derivatives 1.2 and 1.6 should be different.[13]
Ladenburg submitted that if the 1.2 and 1.6 compounds were identical,
then we should expect the two well-known crotonic acids,
CH3.CH:CH.COOH and CH2:CH.CH2.COOH, to be identical. This view was
opposed by Victor Meyer and Kekule. The former pointed out that the
supposed isomerism was not due to an arrangement of atoms, but to the
disposition of a valency, and therefore it was doubtful whether such a
subtle condition could exert any influence on the properties of the
substance. Kekule answered Ladenburg by formulating a dynamic
interpretation of valency. He assumed that if we have one atom
connected by single bonds to (say) four other atoms, then in a
certain unit of time it will collide with each of these atoms in turn.
Now suppose two of the attached atoms are replaced by one atom, then
this atom must have two valencies directed to the central atom; and
consequently, in the same unit of time, the central atom will collide
once with each of the two monovalent atoms and twice with the
divalent. Applying this notion to benzene, let us consider the impacts
made by the carbon atom (1) which we will assume to be doubly linked
to the carbon atom (2) and singly linked to (6), h standing for the
hydrogen atom. In the first unit of time, the impacts are 2, 6, h, 2;
and in the second 6, 2, h, 6. If we represent graphically the impacts
in the second unit of time, we perceive that they point to a
configuration in which the double linkage is between the carbon atoms
1 and 6, and the single linkage between 1 and 2. Therefore, according
to Kekule, the double linkages are in a state of continual
oscillation, and if his dynamical notion of valency, or a similar
hypothesis, be correct, then the difference between the 1.2 and 1.6
di-derivatives rests on the insufficiency of his formula, which
represents the configuration during one set of oscillations only. The
difference is only apparent, not real. An analogous oscillation
prevails in the pyrazol nucleus, for L. Knorr (_Ann._, 1894, 279, p.
188) has shown that 3- and 5-methylpyrazols are identical.
Ladenburg's formula.
The explanation thus attempted by Kekule was adversely criticized,
more especially by A. Ladenburg, who devoted much attention to the
study of the substitution products of benzene, and to the support of
his own formula. His views are presented in his Pamphlet: _Theorie der
aromatischen Verbindungen_, 1876. The prism formula also received
support from the following data: protocatechuic acid when oxidized by
nitrous acid gives carboxytartronic acid, which, on account of its
ready decomposition into carbon dioxide and tartronic acid, was
considered to be HO.C(COOH)3. This implied that in the benzene complex
there was at least one carbon atom linked to three others, thus
rendering Kekule's formula impossible and Ladenburg's and Claus'
possible. Kekule (_Ann._, 1883, 221, p. 230), however, reinvestigated
this acid; he showed that it was dibasic and not tribasic; that it
gave tartaric acid on reduction; and, finally, that it was
dioxytartaric acid, HOOC.C(OH)2.C(OH)2.COOH. The formation of this
substance readily follows from Kekule's formula, while considerable
difficulties are met with when one attempts an explanation based on
Ladenburg's representation. Kekule also urged that the formation of
trichlorphenomalic acid, shown by him and O. Strecker to be
trichloracetoacrylic acid, was more favourably explained by his
formula than by Ladenburg's.
Baeyer's researches.
Other objections to Ladenburg's formula resulted from A. von Baeyer's
researches (commenced in 1886) on the reduced phthalic acids. Baeyer
pointed out that although benzene derivatives were obtainable from
hexamethylene compounds, yet it by no means follows that only
hexamethylene compounds need result when benzene compounds are
reduced. He admitted the possibility of the formulae of Kekule, Claus,
Dewar and Ladenburg, although as to the last di-trimethylene
derivatives should be possible reduction products, being formed by
severing two of the prism edges; and he attempted to solve the problem
by a systematic investigation of the reduced phthalic acids.
CO C.OH C.OH (1)OH
/ \ // \ // \ / | \
H2C/ \CH.CO2Et HC// \CH.CO2Et HC// \C.CO2Et / | \
| | | | | || EtO2C.(6)|---+---|(5)H
| | or | | --> | || | | |
| | | | | || EtO2C.(3)|---+---|(2)OH
EtO2C.HC\ /CH2 EtO2C.C\\ /CH2 EtO2C.C\\ /CH \ | /
\ / \\ / \\ / \ | /
CO C.OH C.OH (4)H