Question

(a) Write the Haworth structure of the sucrose.
(b) Sucrose is a non-reducing sugar. Why?

Answer 1

Hint: Carbohydrates are one of the class biomolecules composed of carbon, hydrogen, and oxygen elements. The general formula of the carbohydrates is \[{{\text{C}}_{\text{x}}}{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)_{\text{y}}}\]. Starch, sugars, and fiber are some examples of carbohydrates. Based on the forming units or units present carbohydrates are classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Complete step-by-step answer:
(a)Here, in the question, sucrose is given which is a disaccharide composed of the two monosaccharides that are glucose and fructose.In the case of sucrose, glucose and fructose are linked by \[{\text{\alpha - 1,\beta - 2 - glycosidic linkage}}\].
The Haworth structure of the sucrose is as follows:

Here, in the structure, a six membered ring compound is glucose and five membered ring compounds is fructose. Glucose and fructose both are the monosaccharides. Here, glucose and fructose are linked together by the glycosidic linkage as shown in the above structure.

(b)In the above structure, we can see that there is no free aldehyde or ketone group adjacent to \[{\text{ - CHOH}}\] group. Sucrose is the non-reducing sugar because the reducing groups of glucose and fructose are involved in the formation of the \[{\text{\alpha - 1,\beta - 2 - glycosidic linkage}}\] formation. Hence, it is not possible to reduce the sucrose molecule, and hence, acts as a non-reducing sugar.

Note: Based on the groups present in the sugars they are classified as reducing sugars and non-reducing sugars.Reducing sugars contain free aldehyde or ketone groups while non-reducing sugars do not contain free aldehyde or ketone groups. Reducing sugars gives Fehling’s test positive. All monosaccharides and some disaccharides are reducing sugars while some disaccharides and all polysaccharides are non-reducing sugars. Non-reducing sugars give negative Fehling’s test