Question

The racemic mixture of any compound is optically inactive due to:
(A) the presence of plane of symmetry
(B) internal compensation
(C) external compensation
(D) none of the above

Answer 1

Hint: A racemic mixture (also known as racemate) is the mixture of two enantiomers of a chiral compound. Chiral compounds are those which have no plane of symmetry and have non-superimposable mirror images.

Complete step by step answer:
The two non-superimposable mirror images of a chiral compound are called enantiomers. These can interact with plane-polarized light to rotate its plane. One enantiomer rotates the plane of light clockwise and the other rotates anticlockwise. So, if the two enantiomers are present in equal amounts in any mixture ( called racemic mixture) there is equal extent of rotation of plane-polarised light in both the directions and the end result will be no net rotation. This process is called external compensation and is responsible for the optical inactivity of the racemic mixture.
So, the final answer will be option ‘C’ i.e. external compensation.
Additional information: It can be concluded that two enantiomers of a compound are optically active whereas a racemic mixture is optically inactive. The enantiomer that rotates the plane-polarised light in clockwise direction (to right) is called dextrorotatory represented by sign + or D and the enantiomer that rotates the plane-polarised light in anti-clockwise direction (to left) is called levorotatory represented by sign - or L. Human hands and gloves are the best example of enantiomerism, both are non-superimposable mirror images of each other.

Note:
The terms chiral compounds, enantiomers and racemic mixture should not be confused. It should be noted that a racemic mixture has an equal amount of both enantiomers and if the amount of enantiomers is not equal then the mixture would be optically active. A chiral compound can have many isomers depending on the spatial arrangement of groups on the chiral atom and each isomer can have enantiomers but only those spatial arrangements can be called enantiomers which look like mirror images of each other.