Question

Explain the stereoisomerism of tartaric acid. How many optical isomers are present in tartaric acid? What are the differences between meso tartaric acid and racemic tartaric acid?

Accepted Answer

Stereoisomers have the same molecular and structural formula as well as the identical positions of bonds but only differ in spatial arrangements of atoms or groups. Tartaric acid has molecular formula $C_{4} H_{6} O_{6}$ and present carboxylic acid and alcohol, functional groups. There are two ways in which the atoms can be arranged in Fischer projection. One doesn't have any symmetry due to which it is optically active and its mirror image compound is also optically active. Both by combination makes enantiomer pair, which have the same physical and chemical properties only differ in the direction of rotation of plane polarized light.

There are three optical isomers of tartaric acid is present. The d and l are enantiomers to each other and both d and l with meso form are diastereoisomers as they are not mirror images with meso form. Diastereoisomers have different physical as well as chemical properties due to which it is easy to separate them than enantiomers which have the same physical and chemical properties in the achiral environment. Meso form is optically inactive.

The racemic mixture contains an equal amount of d and l isomer due to which it is optically inactive. The rotation caused by the d isomer will get canceled by the rotation of l isomer, this is called external compensation. In the meso form, there is a plane of symmetry present which equally divides the molecules into two equal halves which are mirror images of each other. Due to this, the rotation caused by the upper will get cancelled by lower half, this is called internal compensation. Hence, both the racemic and meso form are optically inactive.

Explain the stereoisomerism of tartaric acid. How many optical isomers are present in tartaric acid? What are the differences between meso tartaric acid and racemic tartaric acid?

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