Question

A 50/50 mixture of d and l isomers of a given chiral compound has optical rotation zero. Such a mixture is said to be:
(A) optically inactive and is called a racemic mixture.
(B) optically inactive and is called a racemic modification.
(C) optically active and is called a racemic mixture.
(D) optically active and is called a racemic modification.

Answer 1

Hint: Think about the concept of optical activity and chirality. Find out what is d and l isomers. The question says, a 50/50 mixture of d and l isomers has zero optical rotation. Recollect the term used to describe such a mixture and justify whether it is optically active or inactive to get the answer.

Complete step by step solution:
- Optical activity is the inherent property of a chiral compound to rotate the plane of polarized light either to left or to the right.
- The chiral isomer which rotates the plane of polarized light to left is called laevo-rotatory (l or -) isomer and the chiral isomer which rotates the plane of polarized light to the right is known as dextro-rotatory (d or +) isomer.
- A chiral compound is a compound having an asymmetric carbon atom. Asymmetric carbon atom is a carbon atom containing four different substituents bonded to it.
- Optical activity is a chiral compound detected by passing the plane polarized light through the sample and then observing the plane of transmitted light.
- If a compound rotates 50% of the plane polarized light to the left direction and 50% of the plane polarized light to the right direction, then the planes get cancelled and therefore, that compound has zero optical activity. Such a compound will contain a mixture of two isomers, one is l-isomer and another one is d-isomer in equal proportion. Such a mixture is known as racemic mixture.
- A racemic mixture is said to have zero optical activity and therefore, it is optically inactive.

Therefore, the answer is option (A).

Note: Remember the notations of optical activity ‘d’ and ‘l’ only indicate whether a compound is optically active or not. ‘D’ and ‘L’ notations used in stereochemistry are completely different. For example, glucose is dextrorotatory but has both D and L isomers which are enantiomers of each other.