Organic Chemistry

196 CHAPTER 5 Stereochemistry

For example, if a sample of 2-bromobutane has an observed specific rotation of

its optical purity is 0.40. In other words, it is 40% optically pure—40% of the

mixture consists of an excess of a single enantiomer.

Because the observed specific rotation is positive, we know that the solution con-

tains excess (S)- -2-bromobutane. The enantiomeric excess (ee)tells us how much

excess (S)- -2-bromobutane is in the mixture. As long as the compound is chemi-

cally pure, enantiomeric excess and optical purity will be the same.

If the mixture has a 40% enantiomeric excess, 40% of the mixture is excess Senan-

tiomer and 60% is a racemic mixture. Half of the racemic mixture plus the amount of

excess Senantiomer equals the amount of the Senantiomer present in the mixture.

Thus, 70% of the mixture is the Senantiomer and 30% is the R

enantiomer.

PROBLEM 15

acid has a specific rotation of What would be the observed specific

rotation of each of the following mixtures?

a. 25% acid and 75% acid

b. 50% acid and 50% acid

c. 75% acid and 25% acid

PROBLEM 16

Naproxen, a nonsteroidal anti-inflammatory drug, is the active ingredient in Aleve.

Naproxen has a specific rotation of in chloroform. One commercial preparation re-

sults in a mixture that is 97% optically pure.

a. Does naproxen have the Ror the Sconfigurtion?

b. What percent of each enantiomer is obtained from the commercial preparation?

PROBLEM 17 SOLVED

A solution prepared by mixing 10 mL of a 0.10 M solution of the Renantiomer and 30 mL

of a 0.10 M solution of the Senantiomer was found to have an observed specific rotation of

What is the specific rotation of each of the enantiomers?

SOLUTION One millimole (mmol) of the Renantiomer is mixed

with 3 mmol of the Senantiomer; 1 mmol of the Renantiomer plus

1 mmol of the Senantiomer will form 2 mmol of a racemic mixture. There will be 2 mmol

of Senantiomer left over. Therefore, 2 mmol out of 4 mmol is excess Senantiomer

The solution is 50% optically pure.

The Senantiomer has a specific rotation of the Renantiomer has a specific rotation

of -9.6°.

+9.6°;

x=+9.6°

0.50=

+4.8°

x

optical purity=0.50=

observed specific rotation

specific rotation of the pure enantiomer

12 > 4 =0.50 2.

(30.0 mL*0.10 M)

(10.0 mL*0.10 M)

+4.8°.

+ 66 °

(-)-mandelic (+)-mandelic

(-)-mandelic (+)-mandelic

(-)-mandelic (+)-mandelic

(+)-Mandelic + 158 °.

11 > 2 * 60 + 402

enantiomeric excess excess of a single enantiomer

entire mixture

=× 100 %

40 % 40 %

100 %

==

(+)

(+)

optical purity=

+9.2°

+23.1°

=0.40 or 40%

+9.2°,