Optical Activity

The stereochemistry of molecules has its roots in the work of the nineteenth‐century French physicist Jean Biot. Biot was studying the nature of plane‐polarized light (Figure 1 ) when he discovered that solutions of some organic compounds caused polarized light to rotate. Compounds with this property are called optically active, and the amount and direction of rotation can be determined with a polarimeter (Figure  2).


                  Figure  1

               Figure  2

White light consists of electromagnetic waves oscillating in an infinite number of planes at right angles to the direction of light travel. Plane‐polarized light is ordinary white light that has passed through a polarizer. A polarizer is a filter that blocks the light waves in all planes but one. A common example is a pair of Polaroid sunglasses, which prevent glare by polarizing sunlight. Figure illustrates the concept of plane‐polarized light.

Figure shows how the optical activity of a compound is measured in a polarimeter.

In 1849, while doing recrystallization experiments with tartaric acid salts, Louis Pasteur isolated two distinct kinds of crystals. Upon microscopic examination, he discovered the crystals were nonsuperimposable mirror images of each other. Pasteur was able to separate the different crystals into two piles. Solutions of the pure samples of each kind of crystal showed optical activity. The angle of rotation (α) in a polarimeter was the same for both solutions, but the directions of rotation were opposite. A solution of the original mixture of crystals showed no optical activity. The crystalline structure of the tartrate salt crystals is illustrated in Figure 3 .

           Figure 3

As you can see, the crystals are nonsuperimposable, having the same relationship as a right hand to a left hand. Such opposite configurations are called enantiomers; thus, the two forms of tartrate crystals are enantiomers of each other.

Optical purity. A collection of molecules of one enantiomer is said to be optically pure, while a 1:1 mixture of two enantiomers is a racemic mixture. Between these two extremes, there can be an infinite number of mixtures containing various ratios of two enantiomers. Such chemical mixtures have a specified optical purity. The optical purity of a mixture of enantiomers is defined as the ratio of the rotation of the mixture to the rotation of the pure enantiomers.