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Optical Activity

The molecular structure and the geometry of a compound dictate many of its properties. The structural integrity of certain molecules makes them capable of rotating the plane of polarized light. In order to make use of this phenomenon, we must have a source of plane polarized light. When light waves are passed through polarizing materials, the electric field vector of the processed light oscillates in one plane. This is plane polarized light.
To examine the chiral properties, experimenters pass plane polarized light through solutions of chiral compounds. The rotation of light is noted with a detector. If the rotation perceived by an observer looking through the solution toward the source of light is clockwise, positive (+) sign is used to denote the optical activity. For counterclockwise rotation, negative (–) sign is used to denote the optical activity. The positive rotation is often referred as d (dextrorotatory) and negative rotation as l (levorotatory).

Some Generalizations Regarding Optical Activity

Consider two solutions – A and B. Solution A contains a pure enantiomer, and solution B contains the enantiomer of the compound in solution A. Let's say that solution A exhibited positive rotation.


Since the solution A containing a pure enantiomer exhibits positive rotatory properties, its enantiomer will have negative rotatory properties. What can we conclude about these compounds? Well, we can be certain that both compounds present in the solutions A and B are chiral, since they exhibit optical activity.


The specific rotation [α] of a compound at a given wavelength is denoted by


image\Ch 19 page 264 g1.png


whereαis the observed rotation,

c is the concentration in g/ml,

and l is the length (in decimeters) of the polarimeter tube (the optically active solution is taken in the polarimeter tube for the analysis) that is used.


Properties of Enantiomers and Diastereomers

Enantiomers have identical physical properties. So they cannot be distinguished based on their melting points, boiling points, and densities. But enantiomers do differ in terms of optical activity. Enantiomers rotate place polarized light with the same magnitude, but in opposite directions. Because enantiomers have identical physical properties, separation of enantiomers using conventional methods such as simple distillation and recrystallization is not possible. Enantiomers react the same way with achiral molecules, but react differently with chiral molecules. This difference in reactivity of enantiomers toward chiral molecules is utilized in separating enantiomers. The process is called resolution. The enantiomeric mixture is reacted with a chiral molecule to form a pair of diastereomers. Because diastereomers have different physical properties, they can easily be separated. Diastereomers usually have different solubilities. Followed by the separation of the diastereomers that are formed, the original reaction is reversed to get the original enantiomer corresponding to the diastereomer.

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