The Foundations of Chemistry

Sometimes the different order of arrangements of atoms results in different functional
groups. Some examples of this type of constitutional isomerism follow.

An alcohol and an ether:

An aldehyde and a ketone:

STEREOISOMERS

In stereoisomersthe atoms are linked together in the same atom-to-atom order,

but their arrangements in space are different.

There are two types of stereoisomers: geometric and optical.

Geometric Isomers

Geometric isomers (or cis–transisomers) differ only in the spatial orientation of groups
about a plane or direction. Two geometric isomers have the same molecular formula, the
same functional groups, the same base chain or ring, and the same order of attachment
of atoms; they differ in orientation either (1) around a double bond or (2) across the ring
in a cyclic compound. If the larger groups are on opposite sides of the ring or the double
bond, the designation transappears in the name; if they are on the same side, the desig-
nation is cis.We learned in Section 27-3 about the geometric isomerism associated with
the double bond in alkenes such as the 1,2-dichloroethenes (Figure 27-9). Similarly, two
or more substituents can be either on the same side or on opposite sides of the ring, as
shown in Figures 28-1 and 28-2. This kind of isomerism is possible when substituents
have replaced an H from a XCH 2 Xunit in a ring. Because substituents on an aromatic
ring are bonded in the plane of the ring, such substitutions do not lead to geometric
isomerism.

28-2

H

H

C C

H

H

C

H

H

C

H

H

butanal, C 4 H 8 O

(butyraldehyde)

H

H

C

H

H

H

C

HH

C

O

CH

butanone, C 4 H 8 O

(methyl ethyl ketone)

O

H

HH

C C

H H

OH

ethanol, C 2 H 6 O

H

H

C O

H

HH

C H

methoxymethane, C 2 H 6 O

(dimethyl ether)

28-2 Stereoisomers 1109

Figure 28-1 Models of (a) cis-dichlorocyclopropane and (b) trans-dichlorocyclopropane.

(a)

(b)