Problem 168 - A Complete Resource Book in Chemistry for JEE Main

Question

Racemic mixture is formed by mixing two (a) isomeric compounds (b) chiral compounds (c) meso compounds (d) enantiomers with chiral carbon

Step-by-Step Solution

Answer

A racemic mixture is formed by mixing (d) enantiomers with chiral carbon.

Step 1: Understanding Racemic Mixture

A racemic mixture refers to a mixture containing equal amounts of left- and right-handed enantiomers of a chiral molecule. Essentially, it has no optical activity because the activities of the two enantiomers cancel each other out.

Step 2: Analyzing Each Option

Analyze the options provided: (a) Isomeric compounds can include cis-trans isomers and other structural isomers which are not necessarily enantiomers. (b) Chiral compounds have non-superimposable mirror images, which don't guarantee equal proportions or cancellation of optical activity. (c) Meso compounds are achiral despite having chiral centers. (d) Enantiomers with chiral carbon: Enantiomers are non-superimposable mirror images of each other and when mixed equally, they form a racemic mixture.

Step 3: Selecting the Correct Option

Based on the analysis, the only option that directly relates to the properties of a racemic mixture is option (d) enantiomers with chiral carbon, as they form non-superimposable mirror images which cancel each other's optical activity in a 50:50 mixture.

Key Concepts

Chiral Compounds

Chiral compounds are molecules that are not superimposable on their mirror images. This means, if you were to reflect a chiral molecule in a mirror, it would look different than itself. Most often, chiral compounds contain at least one carbon atom bonded to four different groups. This carbon atom is referred to as a chiral center or stereocenter.

One fascinating property of chiral compounds is their ability to exist in two forms, known as enantiomers. These enantiomers are distinct from each other, even though they have the same chemical structure. As a result, they can have different interactions with other chiral molecules, like those found in biological systems.

It's important to differentiate between chiral and achiral objects: Achiral objects have no chiral centers and their mirror images are superimposable. Even if a molecule has chiral centers, if it can be divided into identical halves, it can be achiral. These molecules are often referred to as meso compounds.

Enantiomers

Enantiomers are a type of stereoisomer where molecules are nonsuperimposable mirror images of each other. This unique relationship stems from the presence of chiral centers within the molecules.

An enantiomer's most intriguing characteristic is how it affects polarized light. Enantiomers rotate polarized light to an equal degree but in opposite directions. If one enantiomer is referred to as "left-handed" (levorotatory), its counterpart will be "right-handed" (dextrorotatory). However, the direction of rotation has no bearing on their other chemical properties.

When discussing enantiomers, it's essential to know that, in a racemic mixture, equal amounts of both enantiomers are present. This results in no net optical rotation since each enantiomer cancels out the optical activity of the other. This is why racemic mixtures are optically inactive.

Optical Activity

Optical activity is a fundamental concept related to chiral compounds and enantiomers. It refers to a compound's ability to rotate plane-polarized light. If a solution rotates light to the right, it is called dextrorotatory, and if it rotates light to the left, it is called levorotatory.

This phenomenon is only possible when a molecule lacks an internal plane of symmetry, as seen in chiral molecules. Each enantiomer of a chiral molecule will rotate light in oppositive directions. The measure of this rotation is called specific rotation and is determined using a polarimeter.

In a mixture or solution, if one enantiomer predominates, the solution will exhibit optical activity. Conversely, racemic mixtures, which contain equal parts of each enantiomer, show no optical activity. This lack of optical activity is due to the complete cancellation of the light rotations caused by each enantiomer.