The Foundations of Chemistry

bromochloroiodomethane, CHBrClI (Figure 28-5). This molecule is not superimposable
with its mirror image, so it is chiral,and the two forms are said to be enantiomers of each
other. Any compound that contains four different atoms or groups bonded to the same
carbon atom is chiral; that is, it exhibits optical isomerism. Such a carbon is said to be
asymmetric(meaning “without symmetry”). Most simple chiral molecules contain at least
one asymmetric carbon atom, although there are other ways in which molecular chirality
can occur.
Stereoisomers have the same type and number of atoms, connected in the same order,
but arranged differently in space. Optical isomers (enantiomers) and geometric isomers
are subgroups of stereoisomers. They differ, however, in that geometric isomers have
differentphysical and chemical properties, whereas optical isomers have physical proper-
ties that are identical (e.g., melting point, boiling point, and density). Optical isomers also
undergo the same chemical reactions, except when they interact with other chiral
compounds. Consequently, their properties in biological systems may be very different.
They also often exhibit different solubilities in solvents that are composed of chiral
molecules.

Figure 28-3 Mirror images. Place your left hand in front of a mirror; you will observe that
it looks like your right hand. We say that the two hands are mirror images of each other;
each hand is in every way the “reverse” of the other. Now try placing one hand directly over
the other; they are not identical. Hence, they are nonsuperimposable mirror images. Each
hand is a chiralobject.

28-2 Stereoisomers 1111

Isomers

Stereoisomers

Optical

(enantiomers)

(D- or L-)

Constitutional

(structural)

Geometric

(cis- or trans-)

Mirror plane

Figure 28-4 Models of two mirror-image forms of bromochloromethane, CH 2 BrCl. The

two models are the same (superimposable), so they are achiral. CH 2 BrCl does notexhibit

optical isomerism.