to higher sugars, Emil Fischer and his students having carried the process
as far as the production of glucodecose (C_{10}H_{20}O_{10}). It usually
happens, however, that two stereo-isomers result from the "step-up" by way
of the nitrile reaction; thus, arabinose yields a mixture of glucose and
mannose, glucose yields glucoheptose and mannoheptose, etc.

The reverse process, or the so-called "degradation" of a sugar into another
containing fewer carbon atoms, may be readily accomplished in either one or
two ways. In Wohl's process, the aldehyde group of the sugar is first
converted into an _oxime_, by treatment with hydroxylamine; the oxime, on
being heated with concentrated sodium hydroxide solution, splits off water
and becomes the corresponding _nitrile_; this, on further heating, splits
off HCN and yields an aldose having one less carbon atom than the original
sugar. This process is the exact reverse of the nitrile synthesis,
described above. The second method of degradation, suggested by Ruff, makes
use of Fenton's method of oxidizing aldehyde sugars to the corresponding
monobasic acid, using hydrogen peroxide and ferrous sulfate as the
oxidizing mixture; the _aldonic acid_ thus formed is then converted into
its calcium salt, which, when further oxidized, splits off its carboxyl
group and one of the hydrogens of the adjacent alcoholic group, leaving an
aldose having one less carbon atom than the original aldose sugar.

=Enolic Forms.=--A final avenue for the interconversion of glucose,
mannose, and fructose into one another, is through the spontaneous
transformations which these undergo when dissolved in water containing
sodium hydroxide or potassium hydroxide. This change is due to the
conversion of the sugar, in the alkaline solution, into an _enol_, which is
identical for all three sugars, and which may subsequently be reconverted
into any one of the three isomeric hexoses. The relationships involved are
illustrated in the following formulas:

CHO CHO CH_{2}OH CHOH
| | | |
H-C-OH HO-C-H C=O C-OH
| | | |
HO-C-H HO-C-H HO-C-H HO-C-H
| | | |
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
H-C-OH