Optical Isomerism

Optical Isomers Definition

Optical isomerism arises due to chirality or asymmetry of the molecule. Optical isomers resemble one another in chemical properties and most of their physical properties but differ in their behavior towards polarized light. The isomer, which rotate the plane of polarized light clockwise is called dextro rotatory isomer (d - isomer) and the one which rotate the plane of polarized light anticlockwise is called laevo rotatory isomer (l - Isomer).

The necessary condition for a molecule to be optically active is asymmetry or chirality of the molecule. Chirality is not just the presence of the asymmetric carbon atom but asymmetry of the molecule as a whole. Most of the chiral molecule contains at least one asymmetric carbon atom (Chiral Carbon atom). Still, there are some organic molecules which exhibit optical isomerism with out having chiral carbon (example: Substituted biphenyls). Some of the organic molecules are optically inactive even though they contain chiral carbon. This is due to internal compensation.

Examples of Optical Isomerism

Optical Isomers of Tartaric acid (HOOC-CHOH-CHOH-COOH)

Optical Isomers of lactic acid

Example of optical Isomer : Tartaric acid

Optical Isomers of Tartaric acid (HOOC-CHOH-CHOH-COOH)


Two chiral carbon atoms are present in tartaric acid. The difference in spatial arrangements of various groups in tartaric acid results in d-tartaric acid, l-tartaric acid and an active form known as meso form. In addition to these, racemic modification, another inactive form also exist.

Dextro tartaric acid rotates the plane of polarization of light to right. The rotation due to upper half is strengthened by the rotation of lower half. Laevo tartaric acid is a mirror image of d-form, which rotate the palne of polarization to left.

Racemic tartaric acid is an equimolar mixture of d and l -isomers. It is optically inactive due to external compensation, it can be resolved into d and l forms.

Meso tartaric acid is an inactive variety and the rotation of upper half is compensated by the rotation due to lower half. It cannot be resolved into active constituents. It is therefore inactive due to internal compensation. Mesotartaric acid possess a plane of symmetry.


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Optical Isomers Example: lactic acid

Optical Isomers of lactic acid


In lactic acid CH₃ - CHOH - COOH, second carbon is chiral.

There are two optically active isomers of Lactic acid: d-lactic acid and l-lactic acid. In addition to these optically active varieties there is an optically inactive form which results when dextro and laevo (levo) varieties are present in equal quantities. It is called racemic mixture or (+-) lactic acid.

The racemic mixture is 50:50 mixture of d and l -isomers and hence have zero optical rotation as the rotation due to one enantiomer cancels the rotation due to the other. That is racemic mixture is optically inactive due to external compensation. The process of conversion of an enantiomer in to a racemic mixture is known as racemisation. Racemisation can be brought about by the action of heat, light and chemical reagent.

Dextro rotatory lactic acid may be obtained from meat extract and is known as sarcolactic acid. With muscular activity glycogen present in muscles break down to sarcolactic acid. During rest sarcolactic acid is converted back to glycogen.

Leavo rotatory lactic acid may be obtained by the fermentation of sucroseby Bacillus Acidi laevolactiti. Ordinary lactic acid in sour milk or manufactured by fermentation or by synthetic method is racemic mixture.