Question

Which of the following sugars are reducing sugars? Comment on the common name sucrose for table sugar.

(a) $\mathbf{methyl}\mathbf{}\alpha \mathbf{‐}\mathbf{D}\mathbf{‐}\mathbf{galactopyranoside}$ (b) $\beta \mathbf{‐}\mathbf{L}\mathbf{‐}\mathbf{idopyranose}$ (an aldohexose)

(c) $\alpha \mathbf{‐}\mathbf{D}\mathbf{‐}\mathbf{allopyranose}$ (d) $\beta \mathbf{‐}\mathbf{L}\mathbf{‐}\mathbf{ribofuranoside}$

(e) (f)

Short answer

(a) non-reducing sugar

(b) reducing sugar

(c) reducing sugar

(d) non-reducing sugar

(e) reducing sugar

(f) non-reducing sugar

Step-by-step solution

Reducing and non-reducing sugars

Carbohydrates are further divided into two categories: reducing sugars and non-reducing sugars. Reducing sugars are carbohydrates that can reduce Fehling's solution and Tollens reagent, but non-reducing sugars cannot reduce Fehling's solution or Tollens reagent. Except for sucrose, all monosaccharides and disaccharides are categorised as reducing sugars. Furthermore, hemiacetals are reducing sugars, whereas acetals are non-reducing sugars.

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.

(a) The name for the sugar $\mathrm{methyl}\alpha ‐\mathrm{D}‐\mathrm{galactopyranoside}$ends with the suffix“-oside”. Hence, it is a non-reducing sugar (acetal).

(b) The name for the sugar $\beta ‐\mathrm{L}‐\mathrm{idopyranose}$ends with the suffix“-ose”. Hence, it is a reducing sugar (hemiacetal).

(c) The name for the sugar ends with the suffix“-ose”. Hence, it is a reducing sugar (hemiacetal).

(d) The name for the sugar h ends with the suffix“-oside”. Hence, it is a non-reducing sugar (acetal).

(e) The structure of the sugar is shown below

Sugar

(f) Sucrose is a non-reducing sugar since their glycosidic bond is between their respective hemiacetal carbon atom. It is a disaccharide and often called table sugar or cane sugar.

Sucrose