Question

Explain stereoisomerism in tartaric acid. How many optical isomers are possible for tartaric acid? What are the differences between meso tartaric acid and racemic mixture?

Answer 1

Stereoisomerism in Trataric Acid:

Compounds having the same constitutions but the different spatial arrangement of their atoms are known as stereoisomers and this phenomenon is called stereoisomerism.

Tartaric acid can be taken as an example :
$HOOC-\stackrel{\ast }{C}H-\stackrel{\ast }{C}H-COOH$
$\mid$ $\mid$
$OH$ $OH$
It has two asymmetric carbon atoms so using the $2^n$ formula there should be a maximum of $2^2=4$ stereoisomers. These can be represented as :(Refer Fig.)
I and II i.e. the forms are mirror images of each other and are called enantiomers but in erythro form i.e., meso form, the plane of symmetry is present thus it does not show optical activity. Therefore, tartaric acid exists only in three isomeric forms of which one is meso (optically inactive) and the other two are optical active i.e., d (or) (+) and l or (-).

Differences between meso and racemic tartaric acid :

Meso Tartaric Acid	Racemic Mixture
The angle of optical rotation of meso tartaric acid is $0^{\circ }$	Its angle of optical rotation is also $0^{\circ }$
It is melting point is ${140}^{\circ }$	It is melting point is ${260}^{\circ }$
It cannot be separated into two forms.	It can be separated into two forms.
It is optically inactive due to internal compensation.	It is optically inactive due to I to external compensation.