An Introduction to Environmental Chemistry

280 Chapter Seven

Box 7.3 Chiral compounds

Molecules that cannot be superimposed on

their mirror images are said to be chiral

(from Greek kheir—hand). A pair of

molecules that fulfil this condition are called

enantiomers (from the Greek enantio—

opposite). Consider a molecule comprising a

central carbon atom to which the following

groups are attached in a tetrahedral array:

–CH 3 , –H, –Br, –COOH (Fig. 1.). In this figure

the molecule is represented as a tetrahedron.

Bonds represented by lines are in the plane

of the page; bonds represented by wedged

lines are rising from the plane of the page

towards the viewer, and bonds represented

by dashed lines are receding from the plane

of the page away from the viewer. The

mirror image of this molecule cannot be

superimposed upon the original molecule. Try

rotating the mirrored-molecule around the

vertical bond; you will find that as the groups

rotate round they never arrive in a position

that would allow them to be superimposed

on the original molecule. Thus, this molecule

is chiral and the carbon atom to which the

groups are attached is termed the chiral

centre. By contrast, a molecule comprising a

central carbon atom to which the following

groups are attached in a tetrahedral array,

–CH 3 , –CH 3 , –Br, –COOH (Fig. 1.), has a mirror

image that can be superimposed upon the

original molecule. This molecule is achiraland

the molecule and its mirror image are not

enantiomers.

It is possible to describe enantiomers

without drawing the structures by assigning

a particular enantiomer the letter ‘R’ or ‘S’.

This system ascribes priority to the groups

attached to the chiral centre in accordance

with their atomic weight. The higher the

atomic weight the higher the priority

assigned. In a simple case where, for

example, the central carbon atom is attached

to -Cl, -Br, -I and –H, then priority is given in

the order I>Br>Cl>H on account of the

atomic weights being 127, 80, 35 and 1,

respectively. Where for example a –CH 3 group

and a –COOH group are attached to the

central carbon atom, i.e. the central carbon

atom is attached to another carbon atom in

both cases, then the groups attached to these

carbon atoms need to be considered. In this

example priority is given to -COOH as ‘O’ has

a greater atomic weight than ‘H’. Having

assigned priority to the groups, the group of

lowest priority is projected away from the

viewer. The viewer then establishes if the

Rotation

axis

Mirror

Cannot overlap upon rotation

CHIRAL

COOH

C

H 3 C

Br

H

COOH

C

Br

H

Rotation

axis

Mirror

Can overlap upon rotation

ACHIRAL

COOH

C

H 3 C

Br

CH 3

COOH

C

Br

CH 3 H 3 C CH 3

Fig. 1Contrasting examples of chiral and achiral molecules.

(continued)