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.
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