Optical isomers interact with polarized light in different ways. Separate equimolar solu-
tions of each rotate a plane of polarized light (see Figures 25-4 and 25-5) by equal amounts
but in opposite directions. One solution is dextrorotatory(rotates to the right) and the
other is levorotatory(rotates to the left). Optical isomers are called dextroand levoisomers.
The phenomenon by which a plane of polarized light is rotated is called optical activity.
It can be measured with a device called a polarimeter (Figure 25-5) or with more sophis-
ticated instruments. A single solution containing equal amounts of the two isomers is a
racemic mixture.This solution does not rotate a plane of polarized light. The equal and
opposite effects of the two isomers exactly cancel. To exhibit optical activity, the dextro
and levoisomers must be separated from each other. This is done by one of a number of
chemical or physical processes broadly called optical resolution.
Alfred Werner was also the first person
to demonstrate optical activity in an
inorganic compound (not of biological
origin). This demonstration silenced
critics of his theory of coordination
compounds, and in his opinion, it was
his greatest achievement. Louis Pasteur
had demonstrated the phenomenon of
optical activity many years earlier in
organic compounds of biological
origin.25-7 Stereoisomers 989Figure 25-3 The optical isomers
of the cis-diammine-cis-diaqua-cis-
dibromochromium(III) ion.