Organic Chemistry

C

ompounds that have the same

molecular formula but are not

identical are called isomers.

Isomers fall into two main classes:

constitutional isomers and stereo-

isomers. Constitutional isomersdiffer

in the way their atoms are connected

(Section 2.0). For example, ethanol and

dimethyl ether are constitutional isomers

because they have the same molecular formula,

but the atoms in each compound are connected differently. The oxygen in

ethanol is bonded to a carbon and to a hydrogen, whereas the oxygen in dimethyl ether

is bonded to two carbons.

Unlike the atoms in constitutional isomers, the atoms in stereoisomers are con-

nected in the same way. Stereoisomers(also called configurational isomers) differ

in the way their atoms are arranged in space. Stereoisomers are different compounds

that do not readily interconvert. Therefore, they can be separated. There are two

kinds of stereoisomers: cis–trans isomers and isomers that contain chirality

(ky-RAL-i-tee) centers.

C 2 H 6 O,

182

Stereochemistry

The Arrangement of Atoms in Space;

The Stereochemistry of Addition Reactions

nonsuperimposable

mirror images

CH 3 CH 2 OH and

ethanol

constitutional isomers

CH 3 OCH 3

dimethyl ether

CH 3 CCH 3

O

and

acetone

CH 3 CH 2 CH

propionaldehyde

O

CH 3 CH 2 CH 2 CH 2 CH 3 and

pentane

CH 3 CHCH 2 CH 3

isopentane

CH 3 CH 2 CH 2 CH 2 Cl

CH 3

Cl

and

1-chlorobutane

CH 3 CH 2 CHCH 3

2-chlorobutane

5