Organic Chemistry

Section 5.14 Separating Enantiomers 211

In 1951, the Dutch chemists J. M. Bijvoet, A. F. Peerdeman, and A. J. van Bommel,
using X-ray crystallography and a new technique known as anomalous dispersion,
determined that the sodium rubidium salt of -tartaric acid had the R,Rconfigu-
ration. Because -tartaric acid could be synthesized from -glyceraldehyde,
-glyceraldehyde had to be the Senantiomer. The assumption, therefore, that
-glyceraldehyde had the Rconfiguration was correct!

The work of these chemists immediately provided absolute configurations for all
those compounds whose relative configurations had been determined by relating
them to -glyceraldehyde. Thus, -lactic acid has the configuration shown
above. If -glyceraldehyde had been the Senantiomer, -lactic acid would have
had the opposite configuration.

PROBLEM 36

What is the absolute configuration of the following?

a. acid c.

b. d. acid

PROBLEM 37

Which of the following is true?

a. If two compounds have the same relative configuration, they will have the same ab-

solute configuration.

b. If two compounds have the same relative configuration and you know the absolute

configuration of either one of them, you can determine the absolute configuration of the

other.

c. If two compounds have the same relative configuration, you can determine the absolute

configuration of only one of them.

d. An Rreactant always forms an Sproduct.

5.14 Separating Enantiomers

Enantiomers cannot be separated by the usual separation techniques such as fractional
distillation or crystallization because their identical boiling points and solubilities
cause them to distill or crystallize simultaneously. Louis Pasteur was the first to sepa-
rate a pair of enantiomers successfully. While working with crystals of sodium ammo-
nium tartrate, he noted that the crystals were not identical—some of the crystals were
“right-handed”and some were “left-handed.”He painstakingly separated the two
kinds of crystals with a pair of tweezers. He found that a solution of the right-handed
crystals rotated the plane of polarized light clockwise, whereas a solution of the left-
handed crystals rotated the plane of polarized light counterclockwise.

sodium ammonium

tartrate

right-handed crystals

COO− Na+

COO− NH 4

OH

HO H

H

+

sodium ammonium

tartrate

left-handed crystals

COO− Na+

COO− NH 4

H

H OH

HO

+

(+)-isoserine (+)-lactic

(-)-glyceric (-)-glyceraldehyde

(+) (-)

(+) (-)

(−)-glyceraldehyde

several steps

(+)-tartaric acid

HO H

HC O

CH 2 OH

HO H

COOH

COOH

H OH

R

S R

(+)

(-)

(+) (-)

(+)