Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

SOME KBMAKKS ON CAKBOHYDKATBS 395
needles. Melting point 220°-222°. Specific rotation + 42° (chloroform).
The yield can be increased by working up the mother liquor.
Cellobiose. 1 —Dissolve 10 g. of the acetyl compound in 20 c.c. of
chloroform and cool strongly in a freezing mixture. With continued
cooling and vigorous shaking add to the solution 0'5 g. of sodium
dissolved in 25 c.c. of absolute methyl alcohol and previously strongly
cooled also. At first a clear solution is formed but very soon a
gelatinous addition compound with sodium ethoxide separates.
After five minutes decompose this compound by dropping in icewater.
Now transfer the reaction mixture to a small separating
funnel and rapidly evaporate 1-2 c.c. of the chloroform layer on a
large watch glass. A small residue only should remain—a sign that
the hydrolysis has been successful. Run off the chloroform, neutralise
the aqueous solution with acetic acid and evaporate in a vacuum
until syrupy. Part of the disaccharide crystallises at this stage.
Mix the syrup with 25 c.c. of absolute alcohol, so precipitating the
cellobiose in colourless crystalline form. Leave over night, filter at
the pump; wash with a little absolute alcohol and dry in a vacuum
desiccator. Yield 3-4 g.
SOME REMARKS ON CARBOHYDRATES (WITH REFERENCE
TO SECTIONS 2-7)
Two disaccharides have been hydrolysed and in each case the more
readily crystallisable sugar has been isolated—d-glucose from cane sugar,
i-galactose from lactose. The hydrolysis of bioses, by cleavage at the
oxygen bridge, is biologically accelerated by the catalytic action of
specific enzymes (invertase, lactase). The more universal hydrolysis
by acids is a unimolecular reaction and its velocity is proportional to
the hydrogen ion concentration.
The highly polymerised compound cellulose is broken down into
its octa-acetyl disaccharide cellobiose by " acetolysis " (Franchimont,
Skraup, Ost). The bioses belong to the important group of glucosides.
The isomerism of a- and j8-glucose is to be attributed to the spatially
different arrangement of the H and OH-groups attached to the asymmetric
carbon atom 1. This atom is asymmetric in the cyclic " lactol "
formula (Tollens). The mutarotation of the sugars, i.e. the gradual
change to the final stationary value of the optical rotation, is to be
explained by an equilibrium occurring in solution between the various
1 G. Zemplen, Ber., 1926, 59, 1258.