ving the formula C_{6}H_{12}O_{6}, namely, glucose, fructose,
mannose, and galactose, occur very commonly and widely distributed in
plants. In addition to these, thirteen others having the same percentage
composition have been artificially prepared, while seven additional forms
are theoretically possible. In other words, twenty-four different
compounds, all having the same empirical formula and similar sugar-like
properties are theoretically possible. In order to arrive at a conception
of this multiplicity of isomeric forms, it is necessary to understand the
two types of isomerism which are involved. One of these is _structural_
isomerism, and the other is _space_- or _stereo_-isomerism.

=Structural Isomerism.=--This refers to an actual difference in the
characteristic groups which are present in the molecule. As has been said,
all carbohydrates, from the standpoint of the characteristic groups which
they contain, are aldehyde-alcohols. The hexoses all contain five alcoholic
groups and one primary aldehyde, or one secondary aldehyde (ketone), group.
If the aldehyde oxygen is attached to the carbon atom which is at the end
of the six-membered chain, the structural arrangement is
|
that of an aldehyde, C=O and the sugar is of the type known
|
H
as "aldoses"; whereas, if the oxygen is attached to any other
|
carbon in the chain, the ketone arrangement, C=O results and
|
the sugar is a "ketose." This difference is illustrated in the Fischer
open-chain formulas for glucose (an aldose) and fructose (a ketose) as
follows:

Glucose Fructose

CH_{2}OH CH_{2}OH
| |
CHOH CHOH
| |
CHOH CHOH
| |
CHOH CHOH
| |
CHOH C=O
| |
CHO CH_{2}OH

=Stereo-isomerism=, or space isomerism, as its name indicates, depends upon
the different arrangement of the atoms or groups in the molecule in space,
and not upon any difference in the character of the constituent groups.
This possibility depends upon the existence