4.5: Properties of Enantiomers and Optical Activity
It is essential to understand the difference between chiral and achiral interactions and the implications thereof in optical activity and their applications. Just as our feet, which are chiral, interact uniquely with chiral objects, such as a pair of shoes, but identically with achiral socks, enantiomers of a molecule exhibit different properties only when they interact with other chiral media. An example of a significant implication from this facet is the phenomenon known as optical activity, where enantiomers interact differently with plane-polarized light, resulting in the rotation of the polarized light in a specific direction.
The polarized light consists of electric field vectors oscillating in a single plane. These are rotated by a definite amount, characteristic of the molecular solution through which the polarized light passes. One enantiomer will rotate the plane in the counterclockwise direction and is called laevorotatory, whereas the other enantiomer will rotate the plane in the clockwise direction and is called dextrorotatory. The observed rotation is a function of the specific rotation of the solution, the concentration of the solute, and the cell path length at a specific temperature. The (+)- and (−)- enantiomers possess the same magnitude of specific rotation, albeit with opposite signs. The observed rotation from a solution helps estimate the relative abundance of one enantiomer, defined as the enantiomeric excess or ‘ee.’
The specific optical rotation [α] of a liquid substance is the angle of rotation measured using the polarimetry technique as:
Here ‘α’ is the observed rotation, ‘l’ is the length of the observed layer in mm, and ‘c’ is the concentration. In the International Pharmacopoeia, the specific optical rotation is expressed as:
Here, the superscript ‘T’ is the temperature, and the subscript ‘λ’ is the wavelength of light.