The simple carbohydrates and the glucosides / by E. Frankland Armstrong.

  • Armstrong, E. Frankland (Edward Frankland), 1878-1945.
Date:
1912
Catalogue details

Licence: In copyright

Credit: The simple carbohydrates and the glucosides / by E. Frankland Armstrong. Source: Wellcome Collection.

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    The More Important Derivatives of Glucose. The experimental work of the last ten years has shown that most of the derivatives of glucose likewise exist in two forms differing in physical properties, more particularly crystalline form, optical rotatory power and melting-point. The chemical behaviour of all these sub- stances is such that it must be assumed that the aldehydic function has disappeared giving rise to the closed-ring structure already formulated. Glucose Pentacetates.—Under proper experimental conditions, all five hydroxyl groups in glucose become acetylated, the a- or /3-pent- acetate predominating in the product according to the method adopted. As these compounds form the starting-point for a number of syntheses, it is important to understand fully the methods of preparing them. They have the following formulae:— AcO—CH HC [Ac = CsHsO] OAc :h, . oac a-Glucose pentacetate. HC—OAc CHa . OAc 5-Glucose pentacetate. To obtain the a-pentacetate it is necessary to acetylate glucose instantly before isomeric change can take place, since the presence of acid greatly accelerates the isomeric change from a- to /3-glucose. This is done by adding anhydrous a-glucose to boiling acetic anhydride con- taining a small quantity of zinc chloride as catalyst. A violent action ensues, and the sugar passes into solution. The product is poured into water, which is changed from time to time to remove the acetic acid ; finally the a-glucose pentacetate solidifies. The crude product contains both isomerides: it is purified by crystallisation from alcohol. The a- pentacetate predominates also when glucose is acetylated in pyridine solution at o°. To obtain the /9-pentacetate, glucose is mixed with acetic anhydride and sodium acetate, and heated for some time at the temperature of the water bath. As the change from a- to /3-glucose in this case pre- cedes acetylation, /3-glucose pentacetate predominates in the final product, and may be separated by fractional crystallisation.