Question

Which of the sugars mentioned in Problem 23-53 and 23-54 are reducing sugars? Which ones undergo mutarotation?

Short answer

Reducing sugars can undergo mutarotation and they are as:

2,3,4,6 - tetra - O - ethyl - D - mannopyranose

1,3,6 - tri - O - ethyl - D - fructofuranose

4 - O- (α - D - glucopyranosyl) - D - galactopyranose

6 - O - (β -D - galactopyranosyl) - D - glucopyranose

Step-by-step solution

Reducing and non-reducing sugars

Carbohydrates are also classified on the basis of reducing or non-reducing sugars. Reducing sugars are defined as those carbohydrates which have the ability to reduce Fehling’s solution and Tollen’s reagent while non-reducing sugars does not have the ability to reduce Fehling’s solution and Tollen’s reagent. All monosaccharides and disaccharides except the sucrose are classified as reducing sugars. Moreover, reducing sugars are hemiacetals while non-reducing sugars are acetals.

General rule for identifying reducing and non-reducing sugars

A sugar whose name ends with the suffix (-ose) is a reducing sugar while the sugar whose name ends with suffix (oside) is a non-reducing sugar.

Identification of the given sugars as reducing or non-reducing.

The name for the sugar ethyl - β - D - glucopyranoside ends with the suffix (-oside) . Hence, it is a non-reducing sugar (acetal).

The name for the sugar 2,3,4,6 - tetra - O - ethyl - D - mannopyranose ends with the suffix (-ose). Hence, it is a reducing sugar (hemiacetal).

The name for the sugar 1,3,6 - tri - O - ethyl - D - fructofuranose ends with the suffix (-ose) . Hence, it is a reducing sugar (hemiacetal).

The name for the sugar ethyl - 2,3,4,6 - tetra - O - methyl - β - D - galactopyranoside ends with the suffix (-oside) . Hence, it is a non-reducing sugar (acetal).

The name for the sugar 4 - O - (α - D - glucopyranosyl) - D - galactopyranose ends with the suffix . Hence, it is a reducing sugar (hemiacetal).

The name for the sugar α - D - fructopyranosyl -β- D - mannopyranoside ends with the suffix (-oside) . Hence, it is a non-reducing sugar (acetal).

The name for the sugar 6 - O -β -D - galactopyranosyl) - D - glucopyranose ( ends with the suffix (-ose). Hence, it is a reducing sugar (hemiacetal).

Mutarotation

A spontaneous change in the specific rotation of a solution of an optically active compound is known as mutarotation. This implies that the two samples are different but in solution they form an equilibrium mixture. Sugars which are reducing can undergo mutarotation while non-reducing sugars cannot undergo mutarotation.