Evidence for Hemiacetal Formations as Exemplified with Glucose

3 EVIDENCE FOR HEMIACETAL FORMATION AS EXEMPLIFIED WITH GLUCOSE 

Glucose does not form a bisulfite adduct or give a magenta color with the Schiff reagent. 

Naturally occurring ( + )-glucose is obtained in two forms mp=146°C, [al =+112° and mp = 1.50°C, [o], = +19°. The specific rotation of each changes (mutarotates) in water, and both reach a constant value of +52.7°. 

Unlike typical aldehydes, 1 mol of glucose reacts with only 1 mol of ROH in dry HCI to give two isomeric acetals (alkyl glycosides). See Problem 15.35. 

To explain the above results, glucose is assumed to exist as a cyclic hemiacetal in equilibrium with a small amount of the open-chain aldehyde. In the hemiacetal, C is chiral and two diastereomers (anomers) are possible they are called a- and jS-glucosides. The C of C=0 is always the anomeric position. Cyclic hemiacetals form when an OH and a CHO group in the same molecule can form a five- or six-membered ring that is more stable than the open chain. 

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