Crystallization of D-glucose

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

Experiment.—Heat a solution of 2 g. of phenylhydrazine in dilute acetic acid (1-5 c.c. of acid, 15 c.c. of water) with 1 g. of d-glucose dissolved in 5 c.c. of water on the water bath at 80°. After about twenty minutes the osazone begins to separate in fine small yellow needles. After the lapse of an hour collect the crystals at the pump, wash with water, and dry in the air. Melting point 205°. 

One of the best-known systems involving epimerization is that of d-glucose. In aqueous solution the epimeric a- and 0-forms are in equilibrium, but each can be obtained by crystallization ethanol readily affords the pure a-form, whereas pure 0 may be obtained from pyridine. 

Nitrosorbite (Sorbitol hexanitrate) d-Sorbite occurs in the berries of the mountain ash, but is more readily procured by the electrolytic reduction of d-glucose. It crystallizes with one molecule of water in small crystals which lose their water when heated and melt at about 110°. Nitrosorbite, isomeric with nitromannite, exists as a viscous liquid and has never been obtained in the crystalline state. It is used in non-freezing dynamites. 

Dextrose monohydrate is purified, crystalline D-glucose containing one molecule of water of crystallization per molecule of D-glucose. 

Branched cydodextrins are also used to increase the solubility of complexes. Two methods are used to make branched cydodextrins, an enzymic method and a pyrolytic method. In the enzymic method, a starch debranching enzyme, such as pul-lulanase, is added to a solution of cyclodextrin and a large excess of D-glucose or maltose to force the reaction to proceed in the reverse direction, i.e. to add rather than remove a branch.69 Since the equilibrium favors the debranching reaction, yields are low and the product typically contains —15% branched cyclodextrin and —85% glucose or maltose. Purification is difficult because of the high solubility of both the glucose or maltose and the branched cyclodextrin, but much of the unreacted cyclodextrin can be removed by crystallization. 

An aqueous solution of D-glucose contains an equilibrium mixture of a-D-glucopyranose, /3-D-glucopyranose, and the intermediate open-chain form. Crystallization below 98 °C gives the a anomer, and crystallization above 98 °C gives the fi anomer. 

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