Organic Chemistry

Section 5.8 Optical Purity and Enantiomeric Excess 195

dextrorotatory or levorotatory is to put the compound in a polarimeter. For ex-
ample, (S)-lactic acid and (S)-sodium lactate have the same configuration, but
(S)-lactic acid is dextrorotatory whereas (S)-sodium lactate is levorotatory. When we
know the direction an optically active compound rotates the plane of polarization, we
can incorporate or into its name.

PROBLEM 13

a. Is (R)-lactic acid dextrorotatory or levorotatory?

b. Is (R)-sodium lactate dextrorotatory or levorotatory?

A mixture of equal amounts of two enantiomers—such as (R)- -lactic acid and
(S)- -lactic acid—is called a racemic mixtureor a racemate. Racemic mixtures
do not rotate the plane of polarized light. They are optically inactive because for
every molecule in a racemic mixture that rotates the plane of polarization in one di-
rection, there is a mirror-image molecule that rotates the plane in the opposite direc-
tion. As a result, the light emerges from a racemic mixture with its plane of
polarization unchanged. The symbol is used to specify a racemic mixture. Thus,
-2-bromobutane indicates a mixture of -2-bromobutane and an equal amount
of -2-bromobutane.

PROBLEM 14

glutamate (MSG) is a flavor enhancer used in many foods. Some

people have an allergic reaction to MSG (headache, chest pains, and an overall feeling of

weakness). “Fast food”often contains substantial amounts of MSG, and it is widely used

in Chinese food as well. MSG has a specific rotation of

a. What is the specific rotation of (R)- -monosodium glutamate?

b. What is the specific rotation of a racemic mixture of MSG?

5.8 Optical Purity and Enantiomeric Excess

Whether a particular sample consists of a single enantiomer or a mixture of enantiomers
can be determined by its observed specific rotation. For example, an enantiomerically
puresample—meaning only one enantiomer is present—of (S)- -2-bromobutane will
have an observed specific rotationof because the specific rotationof (S)- -
2-bromobutane is If, however, the sample of 2-bromobutane has an observed
specific rotation of 0°, we will know that the compound is a racemic mixture. If the ob-
served specific rotation is positive but less than we will know that we have a
mixture of enantiomers and the mixture contains more of the enantiomer with the Scon-
figuration than the enantiomer with the Rconfiguration. From the observed specific ro-
tation, we can calculate the optical purity (op)of the mixture.

optical purity=

observed specific rotation

specific rotation of the pure enantiomer

+23.1°,

+23.1°.

+23.1° (+)

(+)

(-)

C

−OOCCH

2 CH 2

COO−^ Na+

H

+NH 3

(S)-(+)-monosodium glutamate

+ 24 °.

(S)-(+)-Monosodium

(-)

(;) (+)

(;)

(+)

(-)

C

HO

CH 3

H

HO

H

COOH COO−Na+

C

CH 3

(S)-(+)-lactic acid (S)-(−)-sodium lactate

(+) (-)

(+) (-)