11.8 Mutarotation

Normally D-(+)-glucose has a melting point of 146°C. However, when D-(+)-glucose is crystallized by evaporating an aqueous solution kept above 98°C, a second form of D-(+)-glucose with a melting point of 150°C can be obtained. When the optical rotations of these two forms are measured, they are found to be significantly different, but when an aqueous solution of either form is allowed to stand, its rotation changes. The specific rotation of one form decreases and the other increases, until both solutions show the same value. For example, a solution of α -D-(+)-glucose (mp 146°C) specific rotation gradually decreases from an initial value of + 112.2° to + 52.7°, while The β -D-(+)-glucose (mp 150°C) specific rotation gradually increases from an initial value of + 18.7° to + 52.7°. The three forms of glucose reach equilibrium concentrations with the specific rotation of +52.7. This change ("mutation") in the specific rotation toward equilibrium is called mutarotation (Scheme 6).

Scheme 6

11.9 Reducing and Non-reducing Sugars

The carbohydrates may also be classified as either reducing or non-reducing sugars. Cyclic acetals or ketals are not in equilibrium with their open chain carbonyl group containing forms in neutral or basic aqueous solutions. They cannot be oxidized by reagents such as Tollen's reagent (Ag⁺, NH₃, OH^-) or Br₂. So, these are referred as non-reducing sugars. Whereas hemiacetals or hemiketals are in equilibrium with the open-chain sugars in aqueous solution. These compounds can reduce an oxidizing agent (eg. Br₂), thus, they are classified as a reducing sugar.