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Assertion : A solution of sucrose in water is dextrorotatory. But on hydrolysis in the presence of a little hydrochloric acid, it becomes laevorotatory.
Reason : Sucrose on hydrolysis gives unequal amounts of glucose and fructose. As a result of this, change in sign of rotation is observed.
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Reason : Sucrose on hydrolysis gives unequal amounts of glucose and fructose. As a result of this, change in sign of rotation is observed.
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If assertion is true but reason is false.
The hydrolysis of sucrose by boiling with a mineral acid $(\mathrm{HCl})$, produces a mixture of equal molecules of $D$-glucose and $D$-fructose. Sucrose solution is dextrorotatory, specific rotation $=+66.5^{\circ}$. But on hydrolysis, it becomes laevorotatory. The specific rotation of $D$-glucose is $+52^{\circ}$ and of $D$-fructose is $-92^{\circ}$. Therefore, the net specific rotation of an equimolar mixture of both is :
$\frac{+52^{\circ}-92^{\circ}}{2}=-20^{\circ}$
$\therefore \quad$ laevorotatory.
$\frac{+52^{\circ}-92^{\circ}}{2}=-20^{\circ}$
$\therefore \quad$ laevorotatory.
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