JEE Main 2020 · Chemistry · Biomolecules and Polymers · hard

The specific rotations of sucrose, D-glucose and D-fructose are +66.5^°, +52.5^° and -92.0^° respectively. Complete hydrolysis of sucrose gives equal…

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The specific rotations of sucrose, D-glucose and D-fructose are

+66.5^\circ

+52.5^\circ

and

-92.0^\circ

respectively. Complete hydrolysis of sucrose gives equal masses of glucose and fructose. Taking the specific rotation of the resulting mixture as the average of the two sugars, its value is

A. $-19.75^\circ$
B. $+19.75^\circ$
C. $-39.5^\circ$
D. $+59.5^\circ$

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Answer: (A) $-19.75^\circ$

Sucrose (

\mathrm{C_{12}H_{22}O_{11}}

342

)

+\mathrm{H_2O}\to

glucose (

180

) + fructose (

180

); each gives

180/342

g per g sucrose, so equal masses. Mixture rotation

=\dfrac{52.5+(-92.0)}{2}=-19.75^\circ

. The sign changes from dextro to laevo, which is why the product is called invert sugar.

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