[Footnote 1: Ueber die raumliche Anordnung der Atome in
organischen Molekulen, and ihre Bestimmung in
geometrisch-isomeren ungesattigten Verbindungen. Von Johannes
Wislicenus.--Abhandlungen der mathemalisch-physischen Klasse der
Konigl. Sachsischen Gesellschaft der Wissenechaften. Band XIV.,
No. 1.]
The expression "chemical structure," as commonly used by chemists, has,
as is well known, nothing to do with the arrangement of atoms in space.
The structural formula does not profess to represent spatial relations,
but simply the connections which, after a careful study of the
transformations and modes of formation of the compound represented, are
believed to exist between the atoms. Nevertheless, although we do not
commonly consider the question of space relations, it is clear that
atoms must have some definite positions in space in the molecules, and
the only reason why we do not represent these positions is because we
know practically nothing about them. The most definite suggestion
concerning space relations of atoms which has been made is that of Le
Bel and Van't Hoff. The well known hypothesis of these authors was put
forward to account for a certain kind of so-called physical isomerism
which shows itself in the action of substances upon polarized light.
Since this hypothesis was proposed, the number of cases of "abnormal
isomerism," that is to say, of cases of isomerism which cannot be
accounted for by the commonly accepted method of explaining structure,
has increased to a considerable extent, and the necessity for some new
hypothesis, or for some modification of the old ones, has come to be
pretty generally recognized. Among the cases of isomerism which it is at
least difficult to explain by the aid of the prevailing views are those
of maleic and fumaric acids; citraconic and mesaconic acids; certain
halogen derivatives of crotonic acid and of cinnamic acid; and coumaric
and coumarinic acids.
More than one hypothesis has been proposed to account for these cases of
isomerism, but no one has shown itself to be entirely satisfactory.
Quite recently Johannes Wislicenus, Professor of Chemistry in the
University of Liepsic, has made what has the appearance of being an
important contribution toward the solution of the problem referred to.
The author shows that many of the facts known in regard to the relations
between maleic and fumaric acids, and the other substances which
furnish examples of "abnormal isomerism," may be explained by the aid of
an extension of the Le Bel-Van't Hoff hypothesis. It is difficult
without the aid of models to give a clear idea concerning the hypothesis
of Wislicenus, but some idea of it may be gained from the following. If
we suppose a carbon atom to exert its affinities in the directions of
the solid angles of a tetrahedron, as is done in the Le Bel-Van't Hoff
hypothesis, then, when two carbon atoms unite, as in ethane, the union
will be between two solid angles of two tetrahedrons. If the two carbon
atoms unite by the ethylene kind of union, the union will be along a
line corresponding to one of the edges of each tetrahedron. In the
former case, in which single union exists, the two parts of the molecule
represented by the two tetrahedrons can be supposed to be capable of
revolving around an axis either in the same direction or in opposite
directions. This axis corresponds to the straight line joining the two
carbon atoms. In the case in which double union exists no such
revolution is possible. Again, if, by addition to an unsaturated
compound like ethylene, a saturated compound is formed, the kind of
union between the carbon atoms is changed, and the possibility of
revolution of the two parts of the compound is given. Whether such
revolution take place or not will be determined largely by the structure
of the compound. The tendency will be for those parts of the molecule
which have the greatest specific affinity for one another to take those
positions in which they are nearest to one another. Thus, suppose that
chlorine is added to ethylene. By following the change on the model, it
is seen that in the resulting figure the two chlorine atoms in ethylene
chloride are situated at angles of the two tetrahedrons which are
nearest each other. But chlorine has a stronger affinity for hydrogen
than it has for chlorine, and therefore each chlorine atom would tend to
get as near a hydrogen atom as possible. This involves a partial
revolution of the two tetrahedrons in opposite directions around their
common axis. So also hydrogen would tend to take a position as near as
possible to hydroxyl and to carboxyl, while hydroxyl would avoid
hydroxyl, and carboxyl would avoid carboxyl. These views are suggested
as a result of a careful application of the original Le Bel-Van't Hoff
hypothesis, and are, of course, of little value unless they can be shown
to be in accordance with the facts.
The chief merit of the work of Wislicenus consists in the fact that he
has shown that a large number of phenomena which have been observed in
the study of such cases of isomerism as were mentioned above find a
ready explanation in terms of the new hypothesis, whereas for most of
these phenomena no explanation whatever has thus far been presented. The
most marked case presented is that of maleic and fumaric acids. One by
one, the author discusses the transformations of these acids and their
substitution products, and becomes to this conclusion: "There is not to
my knowledge a single fact known in regard to the relations between
fumaric and maleic acids which is not explained by the aid of the above
geometrical considerations, not one which does not clearly support the
new hypothesis." Among the facts which he discusses in the light of the
hypothesis are these: The formation of fumaric and maleic acids from
malic acid; the quantitative transformation of maleic into fumaric acid
by contact with strong acids; the transformation of the ethereal salts
of maleic acid into those of fumaric acid by the action of a minute
quantity of free iodine; the formation of brommaleic acid and
hydrobromic acid from the dibromsuccinic acid formed by the addition of
two bromine atoms to fumaric acid; the formation of dibromsuccinic acid
from brommaleic acid and of isodibromsuccinic acid from bromfumaric acid
by the action of fuming hydrobromic acid; the conversion of brommaleic
acid into fumaric and then into succinic acid by the action of sodium
amalgam; the formation of one and the same tribromsuccinic acid by the
action of bromine on brommaleic and on bromfumaric acid; and finally,
the conversion of maleic into inactive tartaric acid, and of fumaric
into racemic acid by potassium permanganate. All these facts are shown
to find a ready explanation by the aid of the new hypothesis. Further,
it is shown that the decompositions of the salts of certain halogen
derivatives of organic acids, which give up halogen salt and carbon
dioxide, as well as the formation of lactones and of anhydrides of
dibasic acids, are in perfect harmony with the hypothesis. But the only
way to get a clear conception in regard to the mass of material which
the author has brought together and which he has shown to support his
hypothesis is by a careful study of the original paper, and the object
of this notice is mainly to call the attention of American chemists to
it.
As to the question what value to attach to the speculations which
Wislicenus has brought to our notice, it is difficult to give any but a
general answer. No one can well have a greater fear of mere speculation,
which is indulged in independently of the facts, than the writer of this
notice. Great harm has been done chemistry, and probably every other
branch of knowledge, by unwarranted speculation, and every one who has
looked into the matter knows how extremely difficult it is to emancipate
one's self from the influence of a plausible hypothesis, even when it
can be shown that it is not in accordance with the facts. It behooves
every one, therefore, before accepting a new hypothesis, no matter how
fascinating it may appear at first sight, to look carefully into the
facts, and to endeavor to determine independently whether it is well
founded or not. On the other hand, there is some danger to be
apprehended from a tendency, sometimes observed, to denounce everything
speculative, no matter how broad the basis of facts upon which it rests
may be. Without legitimate speculation, it is clear that there could be
no great progress in any subject. As far as the hypothesis under
consideration is concerned, the writer is firmly of the opinion that it
is likely to prove of great value in dealing with a large number of
chemical facts, and that, as it suggests many lines of research, it will
undoubtedly in the course of a few years exert a profound influence on
chemistry. Whether the evidence which will be accumulated will or will
not confirm the view that the tetrahedron form is characteristic of the
simplest molecules of carbon compounds is not the most important
question to be asked under the circumstances. We should rather ask
whether the testing of the hypothesis is or is not likely to bring us
nearer to the truth. It is a proposition that admits of no denial that a
hypothesis which can be tested by experiment, and which suggests lines
of work and stimulates workers to follow them, is a gain to science, no
matter what the ultimate fate of the hypothesis may be.--_Amer. Chem.
Jour._
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