Optical isomerism in tartaric acid.
Tartaric acid (2,3-Dihydroxybutanedioic acid) contains two asymmetric carbon atoms.
Four forms of tartaric acid are known (Fig. 1.1). Two of them are optically active and two are optically inactive. The optically active forms are related to each other as an object to its mirror image. That is, they are enantiomers.
- (+)-Tartaric Acid. It rotates the plane of polarised light to the right (anticlockwise direction) and is called dextrorotatory.
- (-)- Tartaric Acid. It rotates the plane of polarised light to the (anticlockwise direction) and is called laevorotatory. (-)Tartaric acid is the mirror image of (+)-tartaric acid and vice verse.
- Meso-Tartaric Acid. It possesses a plane of symmetry and is consequently optically inactive. This optically inactive form is said to be internally compensated (Optical rotation of one asymmetric carbon is cancelled by that of the other).
- (±)-Tartaric Acid. It does not rotate the plane of polarised light. That is, it is optically inactive. It is an equimolar mixture of (+)-and(-)-forms (racemic mixture).