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Magazine Chapter 23: Q20. (page 1225)
Which of the following sugars are reducing sugars? Comment on the common name sucrose for table sugar.
(a) (b) (an aldohexose)
(c) (d)
(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 鈥�-ose鈥� is a reducing sugar while the sugar whose name ends with suffix 鈥�-oside鈥� is a non-reducing sugar.
Identification of the given sugars as reducing or non-reducing.
(a) The name for the sugar ends with the suffix鈥�-oside鈥�. Hence, it is a non-reducing sugar (acetal).
(b) The name for the sugar ends with the suffix鈥�-ose鈥�. Hence, it is a reducing sugar (hemiacetal).
(c) The name for the sugar ends with the suffix鈥�-ose鈥�. Hence, it is a reducing sugar (hemiacetal).
(d) The name for the sugar h ends with the suffix鈥�-oside鈥�. 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
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