Question

Racemic mixtures are optically _______ because of ________ compensation.

Answer 1

Hint: A racemic mixture is a mixture of enantiomeric isomers which contains both isomers in equal proportions. Racemic mixtures exist as d/l pairs. Optical activity of one isomer is cancelled out by the other.

Complete answer:
We can define racemic mixture as mixture which contains two enantiomers in equal proportions. We know that enantiomers are basically mirror images.
We denote racemic mixtures as-‘d’ and ‘l’ pairs where,‘d’ is the dextrorotatory isomer and ‘l’ is the levorotatory isomer.
When we pass a plane of polarised light through a compound, the isomer which rotated the plane of polarised light towards right, or clockwise direction is known as the Dextro isomer and the isomer which rotates the plane of polarised light in the anti-clockwise direction or towards left side is known as the Levo isomer.
We also use positive sign to denote the dextro isomer and negative sign to denote the levo isomer and together the pair is written as$\pm $or d/l pair.
A racemic mixture contains both dextro and levo isomers in 1:1 ratio, thus there is no net change in the plane of polarised light thus we call a racemic mixture optically inactive. Both the isomers rotate the plane of polarised lights in the opposite direction and cancel their rotations as they are present in a 1:1 ratio.
Therefore, a racemic mixture is optically inactive.
As we discussed above, the rotation one isomer is cancelled by the other as they are present in equal amounts. We often refer to this cancellation or compensation as external compensation.
Therefore, the answer is- racemic mixtures are optically inactive due to external compensation.

Note: Generally, dis-symmetrical molecules are optically active. Optical activity does not completely depend on the chirality of the molecule. If the molecule has at least one non-superimposable mirror image, it will exhibit optical activity.