Question

Sucrose is dextrorotatory but the mixture obtained after hydrolysis is laevorotatory. Why?

Hint:

Fructose dominates the optical rotation due to its higher magnitude of negative rotation. The phenomenon is called \textit{inversion} and the product is called \textit{invert sugar}.

Answer 1

Step 1: Sucrose rotates plane-polarized light to the right (dextrorotatory).
Step 2: On hydrolysis, sucrose produces one molecule each of glucose and fructose: \[ \text{Sucrose} + \text{H}_2\text{O} \rightarrow \text{Glucose} (+52.7^\circ) + \text{Fructose} (-92.4^\circ) \] Step 3: The overall rotation becomes negative because fructose has a greater negative specific rotation.
Conclusion: The resulting mixture becomes laevorotatory, a phenomenon known as inversion of sugar.