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.

A chiral object and its mirror image, such as our feet, interact differently with other chiral objects, such as a pair of shoes.

For example, our left and right feet can only fit in the left and right shoes, respectively, and not vice versa. In contrast, an achiral object such as a sock can be worn equally well on either foot.

Similarly, enantiomers of a molecule exhibit different properties only when they interact with other chiral media.

For instance, enantiomers interact differently with plane-polarized light, a phenomenon known as optical activity.

Polarized light has electric field vectors oscillating in a single plane, which is rotated by a certain amount when the polarized light passes through a solution of an enantiomer.

Here, the polarized light can be construed as a superposition of chiral left- and right-handed circular polarizations of light. When the polarized light passes through the solution, the enantiomer molecules interact more with one circular polarization. This results in the rotation of the polarized light in a specific direction.

For example, (R)-2-butanol rotates the plane in the counterclockwise direction and is called laevorotatory. The other enantiomer, (S)-2-butanol, rotates the plane in the clockwise direction and is called dextrorotatory.

At a given temperature, the degree of observed rotation of a solution of an enantiomer depends on the specific rotation of the enantiomer, the concentration of the enantiomer, and the pathlength of the cell.

Enantiomers, such as (R)-2-butanol and (S)-2-butanol, have the same magnitude of specific rotation but with opposite signs. As such, an equimolar mixture of enantiomers exhibits no net rotation of polarized light. Such a mixture is referred to as a racemic mixture.

The observed rotation from a sample can be used to calculate the relative abundance of one enantiomer over the other, defined as enantiomeric excess or ee. While a pure solution of one enantiomer has an ee of 100%, racemic mixtures have an ee of 0%.