Organic Chemistry

198 CHAPTER 5 Stereochemistry

groups on the same side of the carbon chain are called the erythro enantiomers

(Section 22.3). Those with similar groups on opposite sides are called the threo enan-

tiomers. Therefore, 1 and 2 are the erythro enantiomers of 3-chloro-2-butanol (the hy-

drogens are on the same side), whereas 3 and 4 are the threo enantiomers. In each of

the Fischer projections shown here, the horizontal bonds project out of the paper to-

ward the viewer and the vertical bonds extend behind the paper away from the viewer.

Groups can rotate freely about the carbon–carbon single bonds, but Fischer projec-

tions show the stereoisomers in their eclipsed conformations.

A Fischer projection does not show the three-dimensional structure of the molecule,

and it represents the molecule in a relatively unstable eclipsed conformation. Most

chemists, therefore, prefer to use perspective formulas because they show the mole-

cule’s three-dimensional structure in a stable, staggered conformation, so they provide

a more accurate representation of structure. When perspective formulas are drawn to

show the stereoisomers in their less stable eclipsed conformations, it can easily be

seen—as the eclipsed Fischer projections show—that the erythro isomers have similar

groups on the same side. We will use both prespective formulas and Fischer projections

to depict the arrangement of groups bonded to an asymmetric carbon.

PROBLEM 18

The following compound has only one asymmetric carbon. Why then does it have four

stereoisomers?

PROBLEM 19

a. Stereoisomers with two asymmetric carbons are called _____ if the configuration of

both asymmetric carbons in one isomer is the opposite of the configuration of the asym-

metric carbons in the other isomer.

b. Stereoisomers with two asymmetric carbons are called _____ if the configuration of

both asymmetric carbons in one isomer is the same as the configuration of the asym-

metric carbons in the other isomer.

c. Stereoisomers with two asymmetric carbons are called _____ if one of the asymmetric

carbons has the same configuration in both isomers and the other asymmetric carbon

has the opposite configuration in the two isomers.

PROBLEM 20

a. How many asymmetric carbons does cholesterol have?

b. What is the maximum number of stereoisomers that cholesterol can have?

c. How many of these stereoisomers are found in nature?

cholesterol

CH 3

CH 3

H H

H 3 C H

H 3 C

H 3 C

HO

CH 3 CH 2 CHCH 2 CH CHCH 3

Br

*

erythro enantiomers

perspective formulas of the stereoisomers of 3-chloro-2-butanol (eclipsed)

12

Cl

H

Cl

H 34

H 3 C

H

Cl H

OH

CH 3

C

H

OH

CH 3

CC

H 3 C

C

CH 3

H

H

HO

Cl

C

CH 3

CC

H 3 C

H

HO

C

H 3 C

threo enantiomers

Tutorial:

Identification of

asymmetric carbons