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198 THE CARBOHYDRATES.
Gluconic acid = CH2 (OH).[CH(OH)]4 .COOH;
Saccharic acid = COOH.[CH(OH)]4 .COOH.
The monocarboxylic acids are easily transformed into their anhydrides
(lactones), and these latter are of special interest because, as shown by
Fischer, they can be changed into the corresponding aldehyde, i.e.,
the corresponding aldose, by nascent hydrogen.
The monosaccharides are converted into the corresponding poly-
hydric alcohol by nascent hydrogen. Thus arabinose, which is a
pentose, C5H10O5, is transformed into the pentatomic alcohol, arabite,
C5H12O5. The three hexoses, glucose, mannose, and galactose,
C6H12O6, are transformed into the corresponding three hexites, sorbite,
mannite, and dulcite, CeH^Oe- The ketoses, on the contrary, due
to their constitution, yield a mixture of two alcohols on the same treat-
ment. From d-fructose, for example, we obtain a mixture of d-sorbite
and Z-mannite. On careful oxidation of the polyhydric alcohols the cor-
responding sugar can be prepared.
Numerous isomers occur among the monosaccharides, and especially
in the hexose group. In certain cases, as, for instance, in glucose and
fructose, we are dealing with a different constitution (aldoses and ketoses),
but in most cases we have stereoisomerism due to the presence of asym-
metric carbon atoms.
As the monosaccharides from the trioses upward contain asymmetric
carbon atoms they occur as optically active bodies in an 1-, dr and racemic
form, r or d-l form, which is a mixture or a combination of the I- and
d-form in equal parts. As the number of asymmetric carbon atoms
increases so does the number of possible stereoisomeric forms enlarge.
As the number according to van’t Hoff is 2n
, where n represents the num-
ber of asymmetric carbon atoms, then for the aldo-hexose, which con-
tains 4 asymmetric carbon atoms, 24 = 16 stereo-chemically different forms
can exist. In fact, of these, 12 have been prepared and their geometric
structure has been explained and for which Fischer has given configura-
tion formulae.
As these relations are readily conceived we will, for example, give only the
configuration formula? for the most important pentoses and hexoses occurring
in the animal body.
COH
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