Question

Sucrose is dextrorotatory but the mixture obtained after hydrolysis is laevorotatory. Explain.

Answer 1

Sucrose is dextrorotatory having [α]_D = + 66.5°). On hydrolysis with dilute acids or enzymes, it gives equimolar D – (+) – glucose and D – (–) – fructose.

\[\ce{\underset{Sucrose}{C12H22O11} + H2O ->[HCl] \underset{[α]_D = + 52.4°}{\underset{D - (+) - Glucose}{C6H12O6}} + \underset{[α]_D =- 92.4°}{\underset{D - (+) - Fructose}{C6H12O6}}}\]

Since D – (–) – fructose has larger specific rotation than D – (–) – fructose has larger specific rotation than D – (+) – glucose, the resulting mixture has specific rotation of – 39.9°. Therefore, the mixture is laevorotatory.