Question

Q)a)An aqueous solution containing 10g of optically pure fructose was diluted to 500mL with water and placed in a polarimeter tube 20 cm long.The measured rotation was -5.20 degrees.Calculate the specific rotation of fructose.
b)If this solution were mixed with 500 mL of a solution containing 5 g of racemic fructose,what would be the specific rotation of the resulting fructose mixture?What would be its optical purity?

Answer 1

Specific rotatiof a compound is a chemical property of any chiral compound It is explained as the rotation caused in the path of a plain polarized light when it passes through a solution of known concentration. The instrument used to measure the specific rotation is called The expression for the specific rotation in a polarimeter is as follows Here, [a],'s the specific rotation of the compound, α įs the observed rotation of the compound, is the length of the path covered by the light in the solution (in dm), and c is the concentration of the solution of the compound (in g/ml) The mass of the optically active fructose is 10 g The volume of the solution is 500 ml The concentration of the solution is, 10g 500 mL 0.02g/ml The length of the polarimeter tube is 20 cm. 20 cm 2 dm The specific rotation of the compound is, TX C T 5.20° [a],' _ 2dm×0.02g/mL

Thus, the specific rotation of the compound is-130°/dm The amount of the racemic mixture added is 5 g The amount of pure fructose is 10 g The total amount of the fructose that will rotate the light in same direction after mixing is, = 12.5 g The amount of the sisomer remaining in the racemic mixture is 2.5 g [R] = 12.5 g/L [S] = 2.5 g/L The expression for enantiomeric excess is, x100% R+S 12.5-2.5 12.5+2.5 x100% = 66.67% The expression for optical purity is, specific rotation of the sample specific rotation pure enantatiomer optical purity = 100 specific rotation of the sample 130 66.67%-100×

specific rotation of the new mixture -86.67 The specific rotation of the compound is -130 dm- The optical purity of the compound is 66.67% The specific rotation of the new mixture is |-86.67" /dm mL 흐 mL