Question

Cholesterol, when isolated from natural sources, is obtained as a single enantiomer. The observed rotation a of a 0.3 g sample of cholesterol in 15 mL of chloroform solution contained in a 10 cm polarimeter tube is –0.78º. Calculate the specific rotation of cholesterol. A sample of synthetic cholesterol was prepared consisting entirely of (+) - cholesterol. This synthetic (+)- cholesterol was mixed with some natural (–)-cholesterol. The mixture had a specific rotation of –13º. What fraction of the mixture was (+)- cholesterol ?

Answer 1

Specific rotation, 

Enantiomeric excess = 

Therefore (+) - cholesterol is of 33.3 % and (–) -cholesterol is of 66.6 % in the mixture.