Question

Racemic tartaric acid is optically inactive due to:
A) Internal compensation
B) External compensation
C) Loss of asymmetric centre
D) A steric hindrance

Answer 1

Hint: To solve this we must know that racemic mixtures contain equal amounts of right and left handed enantiomers of a chiral molecule. The substances which cannot rotate the plane of polarized light are said to be optically inactive. The separation of racemic mixture into its components is known as chiral resolution.


Complete answer:
The pairs of molecules that exist in two forms such that the two forms are non-super imposable mirror images of each other are known as enantiomers. The enantiomers are chemically identical in every order.
We know that racemic mixtures contain equal amounts of right and left handed enantiomers of a chiral molecule. The substances which cannot rotate the plane of polarized light are said to be optically inactive.
We know that racemic mixtures contain equal amounts of right and left handed enantiomers of a chiral molecule. A molecule which cannot be superimposed on its mirror image is known as a chiral molecule.
The racemic mixture of tartaric acid contains d-tartaric acid and l-tartaric acid, where d is dextrorotatory which is a right handed enantiomer and l means laevorotatory which is a left handed enantiomer.
As the racemic mixture of tartaric acid contains d-tartaric acid and l-tartaric acid in equal proportions, the rotation due to one enantiomer is cancelled out by the rotation due to the other isomer. Thus, the net rotation is zero.
We know that molecules having zero optical rotation are optically inactive. Tus, the racemic mixture of tartaric acid is optically inactive. This is due to the external compensation of equal amounts of d-tartaric acid and l-tartaric acid.
Thus, racemic tartaric acid is optically inactive due to external compensation.

Thus, the correct option is (B) external compensation.

Note: Racemic mixture is also known as racemate. The separation of racemic mixture into its components i.e. pure enantiomers is known as chiral resolution. The separation of racemic mixture is done by crystallization, chromatography, using enzymes, etc.