Question
Sucrose is dextrorotatory but the mixture obtained after hydrolysis is laevorotatory. Explain.
✓
Answer
Sucrose is dextrorotatory having. $[\alpha]_\text{D}=+66.5^\circ$ on hydrolysis with dilute acids or enzymes, it gives equimolar D-(+)-glucose and D-(-)- fructose.
$\text{C}_{12}\text{H}_{22}\text{O}_{11}+\text{H}_{2}\text{O}\ \xrightarrow{\ \ \ \ \ \ \ \ \ }\text{C}_{6}\text{H}_{12}\text{O}_{6}+\text{C}_{6}\text{H}_{12}\text{O}_{6}$
Since D-(-)-fructose has larger specific rotation than D-(-)-fructose has largre specific retation than D-(+)-glucose, the resulting mixture has specific. Therefore, the mixture is laevorotatory.
$\text{C}_{12}\text{H}_{22}\text{O}_{11}+\text{H}_{2}\text{O}\ \xrightarrow{\ \ \ \ \ \ \ \ \ }\text{C}_{6}\text{H}_{12}\text{O}_{6}+\text{C}_{6}\text{H}_{12}\text{O}_{6}$
Since D-(-)-fructose has larger specific rotation than D-(-)-fructose has largre specific retation than D-(+)-glucose, the resulting mixture has specific. Therefore, the mixture is laevorotatory.
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