Question

Choose the correct answer from the alternatives given:
Invert sugar is an equimolar mixture of D-glucose and:
A. D-galactose
B. D-mannose
C. D-fructose
D. D-ribose

Answer 1

Hint: Sucrose on hydrolysis in aqueous solution is known as invert sugar because it shows a change in its optical rotation from dextrorotatory to laevorotatory due to the varying optical rotations of its constituent units. The whole phenomenon is known as inversion of sugar. Optical rotation is a property of sugars in which they can rotate the plane polarized light to right or left.

Complete solution:
The very thing that we need to know is that Sucrose is a disaccharide of D-glucose and D-fructose so on hydrolysis it will yield a mixture of D-fructose and D-glucose. Now, let’s see which option matches the facts about Sucrose and Invert sugar.
Option A: D-galactose is not the constituent of Sucrose so it can also not form Invert sugar.
Thus, option A is incorrect.
Option B: D-mannose is also not the constituent of Sucrose so it is not present in Invert sugar as well.
Thus, option B is not correct.
Option C: D-fructose forms a glycosidic bond with D-glucose and forms Sucrose. Sucrose on hydrolysis thus yields an equimolar mixture of D-glucose and D-fructose leading to the formation of Invert sugar.
Thus, option C is the correct answer.
Option D: D-ribose is a pentose sugar while Sucrose is a disaccharide formed of two hexose sugars thus this option is eliminated.
So, option D is not correct.

Therefore, Option D) D-fructose is the correct answer.

Note:
Sucrose has a dextrorotatory specific rotation of \[ + 66.5\% \], D-glucose has a dextrorotatory specific rotation of \[ + 53\] degrees, whereas D-fructose has a huge negative rotation of \[ - 92\] degrees. The resultant combination is laevorotatory because D-fructose has a higher specific rotation than D-glucose. Because of this, sucrose hydrolysis is referred to as sucrose inversion and the mixture thus obtained is called Invert sugar.