Optical Isomerism
Introduction
Optical isomerism is a form of stereoisomerism, a concept in chemistry, that arises due to the presence of non-superimposable mirror images. These mirror images, known as enantiomers, are a type of optical isomer. The phenomenon is named "optical isomerism" because the isomers can rotate plane-polarized light in different directions and in equal magnitude but opposite in direction.
History
The concept of optical isomerism was first observed by a French scientist named Jean-Baptiste Biot in 1815. Biot discovered that certain organic substances could rotate the plane of polarized light. Later, in 1848, Louis Pasteur separated the left and right crystal forms of tartaric acid, demonstrating that the organic molecule was responsible for optical activity.
Optical Activity
Optical activity is a measure of how much a substance can rotate plane-polarized light. Substances that can rotate the plane of polarized light are said to be optically active and belong to a class of compounds known as chiral compounds. The direction in which the light is rotated determines whether the compound is a dextrorotatory (d- or (+)) or a levorotatory (l- or (-)) isomer.
Chirality
Chirality is a property of a molecule that is not superimposable on its mirror image. Chiral molecules usually contain a carbon atom attached to four different groups, also known as a chiral center or stereocenter. The arrangement of these groups determines the configuration of the molecule and its optical activity.
Enantiomers
Enantiomers are pairs of molecules that are non-superimposable mirror images of each other. Each member of the pair is a chiral molecule and they are optically active. They rotate plane-polarized light in equal magnitude but opposite directions. Enantiomers have the same physical and chemical properties except for their effect on plane-polarized light and reactions in a chiral environment.
Racemic Mixtures
A racemic mixture, or racemate, is an equimolar mixture of two enantiomers. In such a mixture, the optical activities of the two enantiomers cancel each other out, and the mixture is said to be optically inactive. Racemic mixtures are significant in the pharmaceutical industry as the biological activity of different enantiomers can vary.
Separation of Enantiomers
The process of separating enantiomers is known as resolution. It is a challenging task as enantiomers have identical physical and chemical properties in an achiral environment. Techniques for resolution include the use of a chiral resolving agent, chiral chromatography, and preferential crystallization.
Applications of Optical Isomerism
Optical isomerism has significant implications in various fields, including the pharmaceutical industry, agrochemicals, and materials science. The different enantiomers of a chiral drug can have different therapeutic effects. Similarly, in agrochemicals, one enantiomer may be more effective than the other. In materials science, the optical properties of chiral materials can be exploited for various applications.