Question

An aqueous solution containing

10 g

of an optically pure compound was diluted to

500 mL

with water and was found to have a specific rotation of −111

°

.

If this

solution were mixed with

500 mL

of a solution containing

7 g

of a racemic mixture of the compound, what would the specific rotation of the resulting mixture of the compound?

What would

be its optical purity?

[

α

]_D =

Optical purity = %

Answer 1

Amount of enantiomer with –ve rotation = 10 + 7/2 = 13.5 g

Total amount of compound = 10 + 7 = 17 g

% of enantiomer with –ve rotation = (13.5/17) x 100 = 79.41 %

% of enantiomer with +ve rotation = 100 – 79.41 = 20.59 %

Optical purity = 79.41 – 20.59 = 58.82%

Optical purity = observed rotation of mixture/Specific rotation of pure enantiomer

Observed rotation of mixture = Optical purity x Specific rotation of pure enantiomer

                                                     = 0. 5882 x (-111)

                                                     = - 65.29