Read the passage carefully and answer the Questions.

Haloalkanes undergo nucelophilic substitution reactions owing to the polarity of C-X bond. The nucleophile reacts with the haloalkane on the carbon possessing a partial positive charge holding the halogen atom. The halogen atom X is replaced by a nucleophile. Depending on the kinetics and mode of bond breaking, the mechanism can be either $S_N1$ or $S_N2$ reaction. The rate of $S_N1$ reaction is governed by the stability of carbocation and in $S_N2$ reaction, the rate of reaction is governed by steric factor. Chirality is the main factor in both $S_N1$ and $S_N2$. In $S_N1$ reaction, the chirality of alkyl halide is accompanied by racemization of the product, while in $S_N2$ reaction, the product is characterized by inversion of configuration. The structure of alkyl halide and the nature of the solvent also governs the mechanism of the substitution.

What is meant by the term racemic mixture?

An equimolar mixture of a pair of enantiomers.

The correct answer is Option (2) → An equimolar mixture of a pair of enantiomers. **

A racemic mixture (or racemate) is a specific type of mixture in stereochemistry defined by two key characteristics:

1. Composition: It contains equal amounts (equimolar ratio, $50:50$) of two enantiomers. Enantiomers are stereoisomers that are non-superimposable mirror images of each other (e.g., $(+)$ and $(-)$ isomers, or $\text{D}$ and $\text{L}$ isomers).
2. Optical Activity: Due to the equimolar presence of both enantiomers, their opposing effects on plane-polarized light cancel each other out. One enantiomer rotates the light clockwise (dextrorotatory, $+$, $\text{d}$), and the other rotates it counter-clockwise (levorotatory, $-$, $\text{l}$) by an equal magnitude. Therefore, a racemic mixture is optically inactive.

Analysis of Options:

• An equimolar mixture of a pair of enantiomers. (Correct)
• It is optically active. (Incorrect, it is optically inactive due to external compensation.)
• Can be separated by physical methods. (Incorrect, separation requires chemical methods like resolution.)
• Possess different physical properties. (Incorrect, enantiomers have identical physical properties, except for their interaction with plane-polarized light and other chiral molecules.)