Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

303

Enantiomers
There is additional complexity in the struct

ures of organic molecules that possess carbon

atoms with four

different

groups attached to them. Such carbon atoms are said to be

stereocenters

. The central carbon shown in red in molecule

in Figure 13.13 is a A

stereocenter because it contains four different groups: H, OH, CH

, and Cl. Molecule 3

B in

Figure 13.13 is its

mirror image

. Molecules

and A

B

are different molecules! That they are

different can be seen by rotating molecule B by 180

o about the C-C bond (

B1

→

B2

in

Figure 13.13). The rotation makes the OH and CH

groups of the two molecules ( 3

A and

B2

) superimposable. However, the H and the

Cl groups are still reversed in the two

molecules. Consequently, molecules

and A

are not superimposable. B

Molecules with a

stereocenter cannot be superimposed on their mirror images

. Pairs of molecules that are

non-superimposable mirr

or images are called

enantiomers

. Enantiomers are stereoisomers

because their spatial arrangements are differe

nt while their connectivities are identical.

Molecules

and A

are enantiomers. B

Switching any two groups about a stereocenter results in its mirror image. Thus, each

stereocenter can have only two enantiomers, and a molecule with n stereocenters has 2

n^

stereoisomers. For example, a molecule with fifteen stereocenters can have 2

15 = 32,768

stereoisomers - a very large number of different molecules with the same formula and connectivity!

Enantiomers have the same melting and boiling points, but they often have very

different biological activities

In fact, some biological processes proceed only for.

one

enantiomer. For example, the

enantiomers of carvone (Figure 13.14) have different odors.

One enantiomer has the odor of spearmint, while the other has the odor of caraway. This is because each enantiomer is

recognized by different receptors in the nose.

A tragic example of different biological reactivity of enantiomers is the drug

thalidomide (Figure 13.15). One stereoisomer acts as a sedative and anti-nausea drug, while its enantiomer prevents the growth

of blood vessels and modifies the immune

system, which can lead to severe birth defects

if taken by pregnant women. Unfortunately,

a form of this drug containing both enantiomers was prescribed to pregnant women in Europe between 1959 and 1962, and the result was thousands of badly deformed newborns. Today, however, the very properties

of the isomer that caused birth defects in

babies is being exploited as a possible cure for leprosy, AIDS, cancer, and other diseases.

CH

3

C

HO

ClH

OH

CH C

3

Cl

H

mirrorplane

CH

3

C

HO

HCl

A

B=

B1

B2

Figure 13.13 Enantiomers Molecules A and B are mirror images. B1 and B2 are not different molecules; they simply show B before and after a rotation of 180

o^

about the C-C bond. Molecules A and B are non-superimposable mirror images, so they are enantiomers.

C

O

H

C

H

O

stereocenter

odor of caraway

odor of spearmint

(a) (b)

Figure 13.14 Enantiomes of Carvone with different odors Note that the ring counts as two different groups due to the groups in it or attached to it. Going around the ring clockwise from the stereocenter, the double bond is encountered first in (a), but the C=O group is encountered first in (b).

NH

C

HN

O

O

sedative and antinausea drug

HN causes severe birth defects

C

HN

O

O

O O

O O

Figure 13.15 Thalidomide Two enantiomers of thalidomide with dramatically different biological activity. The stereoc

enter is the carbon in red.

© by

North

Carolina

State

University