Organic Chemistry

Section 5.6 Naming Enantiomers: The R,SSystem of Nomenclature 191

It is easy to tell whether two molecules are enantiomers (nonsuperimposable) or
identical molecules (superimposable) if you have molecular models of the molecules—
just see whether the models superimpose. If, however, you are working with structures
on a two-dimensional piece of paper, the easiest way to determine whether two mole-
cules are enantiomers or identical molecules is by determining their configurations. If
one has the Rconfiguration and the other has the Sconfiguration, they are enan-
tiomers. If they both have the Rconfiguration or both have the Sconfiguration, they
are identical molecules.
When comparing two Fischer projections to see if they are the same or different,
never rotate one 90° or turn one over, because this is a quick way to get a wrong an-
swer. A Fischer projection can be rotated 180° in the plane of the paper, but this is the
only way to move it without risking an incorrect answer.

PROBLEM 8

Indicate whether each of the following structures has the Ror the Sconfiguration:

a. c.

b. d.

PROBLEM 9 SOLVED

Do the following structures represent identical molecules or a pair of enantiomers?

a.

b.

c. d.

SOLUTION TO 9a The first structure shown in part (a) has the Sconfiguration, and the

second structure has the Rconfiguration. Because they have opposite configurations, the

structures represent a pair of enantiomers.

PROBLEM 10

Assign relative priorities to the following groups:

a.

b.

c.

d. CH CH 2 CH 2 CH 3 CH 3

¬CH(CH 3 ) 2 ¬CH 2 CH 2 Br ¬Cl ¬CH 2 CH 2 CH 2 Br

¬CH“O ¬OH ¬CH 3 ¬CH 2 OH

¬CH 2 OH ¬CH 3 ¬CH 2 CH 2 OH ¬H

CH 3 CH 2 CH 3 and

Cl

H

HCl

CH 3

CH 2 CH 3

C

H

CH 2 Br

OH and

CH 3

C

HO

H

CH 3

CH 2 Br

C

CH 3

CH 2 Br

Cl and

CH 2 CH 3

C

CH 3 CH 2

Cl

CH 3

CH 2 Br

C

HO

CH 3

H and

CH 2 CH 2 CH 3

C

CH 3 CH 2 CH 2

OH

CH 3

H

Cl

C

CH 3 CH 2

CH 2 Br

CH 2 CH 2 Cl

OH

HO

C

CH 3

CH(CH 3 ) 2

CH 2 CH 3

CH 2 Br