Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction, \(\mathrm{CH}_{3} \mathrm{CHO}+\mathrm{HCN} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CN}\) a chiral centre is produced. This product would be (a) racemic mixture (b) meso compound (c) dextrorotatory (d) laevorotatory

Step-by-Step Solution

Answer

The product would be a racemic mixture (a).

Step 1: Understand the Reaction

The given chemical reaction involves the addition of hydrogen cyanide (HCN) to acetaldehyde (CH₃CHO) which results in the formation of a cyanohydrin compound: CH₃CH(OH)CN. This compound contains a carbon atom that becomes a chiral center due to the presence of four different substituents.

Step 2: Determine the Formation of Stereoisomers

When a chiral centre is formed, it is possible for the product to have different stereochemistry, leading to enantiomers which are non-superimposable mirror images of each other.

Step 3: Analyze the Product

The product CH₃CH(OH)CN can exist as two enantiomers due to the newly formed chiral carbon. These are R and S configurations, which result in a racemic mixture if produced in equal amounts.

Step 4: Identify the Type of Mixture Produced

Since the reaction forms a chiral center and the compound can have R and S forms in equal possibility, the product would naturally be a racemic mixture unless a specific catalyst or condition is introduced to prefer one enantiomer over the other.

Key Concepts

Stereoisomers

Stereoisomers are types of isomers that have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space. This difference in spatial arrangement influences the physical and chemical properties of the molecules.

• Structural differences: These differences can occur because of chiral centers, like in the reaction you are studying. A chiral center is usually a carbon atom bonded to four different groups, allowing for multiple possible spatial arrangements.
• Types of stereoisomers: Two main types are enantiomers and diastereomers. In the context of your exercise, enantiomers are formed when a chiral carbon creates non-superimposable mirror images of each other.

Understanding stereoisomers is crucial because they often have different interactions in biological systems. This occurs due to their differing three-dimensional shapes, which can influence how they interact with enzymes and receptors.

Racemic Mixtures

A racemic mixture is a specific type of mixture containing equal amounts of left- and right-handed enantiomers of a chiral molecule. This is important because, in such mixtures, the optical activity of one enantiomer cancels out the activity of the other, resulting in a net optical rotation of zero.

• Formation of racemic mixtures: When a reaction leads to a chiral center, often, both enantiomers are produced equally unless specific conditions or catalysts favor one.
• Characteristics: Racemic mixtures are optically inactive, meaning they don't rotate plane-polarized light. This is one way to identify such mixtures in a laboratory setting.
• Examples: In pharmaceutical applications, sometimes, only one enantiomer of a drug is therapeutically active, while the other might be inactive or even produce adverse effects. Therefore, understanding and controlling racemic mixtures is critical in drug development.

Recognizing racemic mixtures aids chemists in predicting the behavior of chiral compounds and refining synthesis strategies to obtain the desired enantiomer selectively.

Enantiomers

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. In the context of the reaction you are analyzing, when a chiral center forms, two enantiomers usually form- R and S configurations.

• Properties: Enantiomers have identical physical properties except for their optical activity. They rotate plane-polarized light in opposite directions; one is dextrorotatory (rotates light clockwise) and the other is levorotatory (rotates light counterclockwise).
• Significance in nature: Nature tends to favor one enantiomer over the other. For example, most amino acids found in proteins are left-handed (L-configuration).
• Applications: Understanding enantiomers is vital in the field of pharmacology, as different enantiomers of a compound can have dramatically different biological effects.

Mastering the concept of enantiomers is key to predicting and explaining how specific reactions produce different forms, and how these forms interact in real-world scenarios.