Q.Sucrose is dextrorotatory, but the mixture obtained after hydrolysis is Leavorotatory. Explain.

Sucrose is dextrorotatory ($+66.5^\circ$), but on hydrolysis it gives glucose ($+52^\circ$) and fructose ($-92^\circ$); the strong negative rotation of fructose exceeds the positive rotation of glucose, so the product mixture is net laevorotatory. This sign reversal is called inversion, and the equimolar product is invert sugar.

Concept. Optical rotation of a mixture is the algebraic sum of the rotations of its components (weighted by amount). Sucrose on acid or enzymatic (invertase) hydrolysis breaks into equal moles of D-glucose and D-fructose:

$\text{Sucrose (}+66.5^\circ\text{)} + \text{H}_2\text{O} \xrightarrow{\text{H}^+/\text{invertase}} \text{D-glucose (}+52^\circ\text{)} + \text{D-fructose (}-92^\circ\text{)}$

Why the mixture is laevorotatory.

• Net specific rotation of the equimolar mixture $\approx \dfrac{(+52) + (-92)}{2} = -20^\circ$ (negative).
• Because fructose is more strongly laevorotatory than glucose is dextrorotatory, the overall rotation changes sign from $(+)$ to $(-)$. …