32 Chirality
32.1 Introduction
Chirality(pronounced kie-RAL-it-tee) is the property ofhandedness. If you attempt to
superimpose your right hand on top of your left, the two do not match up in the sense that
your right hand’s thumb overlays your left hand’s pinky finger. Your two hands cannot be
superimposed identically, despite the fact that your fingers of each hand are connected in
the same way. Any object can have this property, including molecules.
An object that ischiralis an object that can not be superimposed on its mirror image.
Chiral objects don’t have aplane of symmetry. An achiral object has a plane of symmetry
or a rotation-reflection axis, i.e. reflection gives a rotated version.
Optical isomersorenantiomersare stereoisomers which exhibit chirality. Optical iso-
merism is of interest because of its application in inorganic chemistry, organic chemistry,
physical chemistry, pharmacology and biochemistry.
They are often formed when asymmetric centers are present, for example, a carbon with
four different groups bonded to it. Every stereocenter in one enantiomer has the opposite
configuration in the other.
Whenamoleculehasmorethanonesourceofasymmetry, twoopticalisomersmaybeneither
perfect reflections of each other nor superimposeable: some but not all stereocenters are
inverted. These molecules are an example ofdiastereomers: They are not enantiomers.
Diastereomers seldom have the same physical properties. Sometimes, the stereocentres are
themselves symmetrical. This causes the counterintuitive situation where two chiral centres
may be present but no isomers result. Such compounds are calledmeso compounds.
Figure 80 Chiral relationship.
A mixture of equal amounts of both enantiomers is said to be aracemic mixture.