Question

2. Sucrose is a disaccharide composed of glucose and fructose linked together. Hydrolysis of sucrose results in an equimolar solution of glucose and fructose. The specific rotation of sucrose is +66.5. If a solution of sucrose is completely hydrolyzed by strong acid how would you expect the optical rotation of the solution to change? Justify your answer.

3. If a mixture was made using 15 g of D-glucose and 15 g of L-glucose in 100 mL of water, what would be the expected optical rotation? Why?

Answer 1

2.

Sucrose on hydrolysis produces equimolar solution of D-glucose and D-fructose.

Specific rotation of sucrose = + 66.5^o

Specific rotation of D-glucose = + 52^o

Specific rotation of D-fructose = - 92^o

The specific rotation of equimolar mixture of glucose and fructose = [+ 52^o + (- 92^o)]/2

                                                                                                         = - 20^o

The optical rotation changed from dextrorotatory (+ 66.5^o) to levorotatory (- 20^o) on hydrolysis of sucrose. This is known as inversion of sucrose.

3.

Specific rotation of D-glucose = + 52^o

Specific rotation of L-glucose = - 52^o

If we made 100 mL solution using 15 g D-glucose and 15 g L-glucose, then concentration of both D- and L-glucose would be the same. The solution would be a racemic mixture of D-and L-glucose.

Therefore the optical rotation would cancel out. Expected optical rotation = 0^o.