Page 12 - A TEXTBOOK OF ORGANIC CHEMISTRY

6 INTRODUCTION

Polymerism and Isomerism
The determination of molecular weight is of the first im-
portance, because different substances very frequently have the
same percentage composition and therefore the same empirical
formula, and yet are totally distinct from one another. This
difference is often due to differences in the complexities of the
molecules. Thus formic aldehyde, CH,O; acetic acid, C,1,0,
lactic acid, CH,O; and grape-sugar, 0,1,O% have all the
same percentage composition; as have also ethylene, CIH;
propylene, (Hg; and butylene, CH,. Compounds standing
in such relation to each other are termed polymers. Very
frequently, however, substances which are totally distinct
from each other possess both the same percentage composition
and the same molecular weight; that is to say, these com-
pounds are made up not only of the same elements, but also
of an equal number of atoms of these elements; such sub-
stances are termed isomers or metamers. (See Ethers.)
Thus, for instance, common alcohol and methyl ether, the
latter of which is obtained by heating methyl alcohol with sul-
phuric acid, have one and the same molecular formula, CH,O.
The striking phenomenon of isomerism is most readily ex-
plicable on the assumption that for the molecule of each com-
pound there is a definite arrangement of the atoms, and that
this arrangement or grouping is different in the molecules of
the two isomerides. This difference in grouping may be con-
sidered as being due to a difference in the linking powers of
the atoms, as is indicated by the dissimilar chemical behaviour
of isomers, and explained by the theory of valency.

Chemical Theories; the Theory of Valency
After the fall of the Electro-Chemical theory, unitary
i
formula n contradistinction to the earlier dualistic formulse
-were much used; thus alcohol had the formula CH,O,
(using the old equivalent weights). The necessity for com-
paring substances of complicated composition with simpler
ones, taken as " Types ", had already repeatedly led to the
propounding of new theories for representing the constitution
of organic compounds, e.g. the older Type theory (Dumas), and
the Nucleus theory (Laurent).

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