Optical activity
33.4 Enantiomers
It is just such specificity that accounts for the optical isomerism ofenantiomeric com-
pounds^2. Enantiomers possess identical chemical structures (i.e. their atoms are the
same and connected in the same order), but are mirror images of one another. Therefore,
their electron clouds are also identical but actually mirror images of one another andnot
superimposable. For this reason, enantiomeric pairs rotate light by the same magnitude
(number of degrees), but they each rotate plane polarized light inoppositedirections. If
one chiral version has the property of rotating polarized light to the right (clockwise), it
only makes sense that the molecule’s chiral mirror image would rotate light to the left
(counterclockwise).
Equal amounts of each enantiomer results in no rotation. Mixtures of this type are called
racemicmixtures, and they behave much as achiral molecules do.
33.5 History
Via a magneto-optic effect, when a beam of polarized light passes through solution, the
(-)-form of a moleculed rotates the plane of polarization counterclockwise, and the (+)-
form rotates it clockwise. It is due to this property that it was discovered and from which
it derives the name optical activity. The property was first observed by J.-B. Biot in
1815, and gained considerable importance in the sugar industry, analytical chemistry, and
pharmaceuticals.
Louis Pasteur deduced in 1848 that the handedness of molecular structure is responsible
for optical activity. He sorted the chiral crystals of tartaric acid salts into left-handed and
right-handed forms, and discovered that the solutions showed equal and opposite optical
activity.
Artificial composite materials displaying the analog of optical activity but in the microwave
regime were introduced by J.C. Bose in 1898, and gained considerable attention from the
mid-1980s.
2 Chapter 34 on page 147