Organic Chemistry

216 CHAPTER 5 Stereochemistry

The carbon to which the enantiotopic hydrogens are attached is called a prochiral

carbonbecause it will become a chirality center (an asymmetric carbon) if one of the

hydrogens is replaced by a deuterium (or any group other than or OH). If the

hydrogen is replaced by a deuterium, the asymmetric carbon will have the Rconfigu-

ration. Thus, the hydrogen is called the pro-R-hydrogen. The hydrogen is

called the pro-S-hydrogenbecause if it is replaced by a deuterium, the asymmetric

carbon will have the Sconfiguration. The molecule containing the prochiral carbon is

called a prochiral molecule because it would become a chiral molecule if one of the

enantiotopic hydrogens is replaced.

The pro-R- and pro-S-hydrogens are chemically equivalent, so they have the same

chemical reactivity and cannot be distinguished by achiral reagents. For example,

when ethanol is oxidized by pyridinium chlorochromate (PCC) to acetaldehyde, one

of the enantiotopic hydrogens is removed. (PCC is discussed in Section 20.2.) Because

the two hydrogens are chemically equivalent, half the product results from removing

the hydrogen and the other half results from removing the hydrogen.

Enantiotopic hydrogens, however, are not chemically equivalent toward chiral

reagents. An enzyme can distinguish between them because an enzyme is chiral

(Section 5.15). For example, when the oxidation of ethanol to acetaldehyde is cat-

alyzed by the enzyme alcohol dehydrogenase, only one of the enantiotopic hydrogens

is removed.

If a carbon is bonded to two hydrogens and replacing each of them in turn with deu-

terium (or another group) creates a pair of diastereomers, the hydrogens are called

diastereotopic hydrogens.

Ha and Hb are

diastereotopic hydrogens

CH 3

CH 3

H Br

CH 3

Hb

H Br

D

CH 3

CH 3

CH 3

D

H Br

Ha Hb

Ha

replace Ha with a D

diastereomers

diastereotopic

hydrogens

replace Hb with a D

CH 3

C

O

100%

CH 3 COH

Hb

Hb

Ha alcohol

dehydrogenase

(Ha)

H Hb Ha

b

Ha

CH 3 COH

CH 3

C

+

O

CH 3

C

O

PCC

pyridine

50% 50%

Ha Hb

CH 3 OH

Hb

Ha pro-R-hydrogen

prochiral

carbon

pro-S-hydrogen

Ha Hb

CH 3 Ha