The Foundations of Chemistry

Figure 28-2 Models of (a) cis-1-

chloro-3-methylcyclopentane, (b)

trans-1-chloro-3-methylcyclopentane, and

(c) cis-1-chloro-3-methylcyclopentane

(stereoview).

1110 CHAPTER 28: Organic Chemistry II: Shapes, Selected Reactions, and Biopolymers

Optical Isomers

Many objects are mirror images of each other and cannot be superimposed. Your two

hands are a familiar example of this; each hand is a nonsuperimposable mirror image of

the other (Figure 28-3). “Superimposable” means that if one object is placed over the

other, the positions of all parts will match.

An object that is notsuperimposable with its mirror image is said to be chiral(from

the Greek word cheir,meaning “hand”); an object that issuperimposable with its mirror

image is said to be achiral.Examples of familiar objects that are chiral are a screw, a

propeller, a foot, an ear, and a spiral staircase; examples of common objects that are achiral

are a plain cup with no decoration, a pair of eyeglasses, and a sock.

Optical isomersthat are nonsuperimposable mirror images of each other (chiral)

are called enantiomersof one another. Enantiomers can exist in two forms that

bear the same relationship to each other as do left and right hands.

As an example of this, consider first the two models of bromochloromethane, CH 2 BrCl,

shown in Figure 28-4. They are mirror images of each other, and they can be superim-

posed. Thus, this molecule is achiraland is not capable of optical isomerism. Now consider

(a) (b)

(c)

We speak of a screw or a propeller as

being “right-handed” or “left-handed.”