Question

An incompletely resolved mixture of (–)--methylbenzylamine and (+)-- methylbenzylamine contains 75% of the (–)-enantiomer. What is the optical purity of the sample?

Answer 1

Enantiomer ratio :  75% of the (–)-enantiomer :  25% of the (+)-enantiomer

Optical purity = 75 -25 = 50 %

If the mixture has a 50% enantiomeric excess, 50% of the mixture is excess - enantiomer and 50 % is a racemic mixture. Half of the racemic mixture plus the amount of excess - enantiomer equals the amount of the - enantiomer present in the mixture. Thus, 50% of the mixture is the - enantiomer (1/2 X 50 + 50) and 25% is the + enantiomer.