First slide

Disaccharides

Question

Statement 1: A solution of sucrose in water is dextrorotatory. But on hydrolysis in the presence of a little hydrochloric acid, it becomes laevorotatory.
Statement 2: Sucrose on hydrolysis gives unequal amounts of glucose and fructose. As a result of this, change in sign of rotation is observed.

Moderate

A

Both statements 1 and 2 are correct
B

statements 1 is correct and statement 2 is incorrect
C

Statement 1 is incorrect and statement 2 is correct
D

Both Statements 1 and 2 are incorrect

Solution

The hydrolysis of sucrose by boiling with a mineral acid (HCl), produces a mixture of equal molecules of D-glucose and D-fructose. Sucrose solution is dextrorotatory
specific rotation - + 66.5⁰. But on hydrolysis, it becomes laevorotatory. The specific rotation of D-glucose is + 52^o and of D-fructose is 92^o. Therefore, the net specific rotation of an equimolar mixture of both is: $\frac{+ 52^{o} - 92^{o}}{2} = - 20^{o}$

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