Problem 151 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following questions two statements (Assertion) (A) and Reason (R) are given. Mark (a) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct and \(\mathrm{R}\) is the correct explanation of \(\mathrm{A}\). (b) If both A and \(R\) are correct but \(R\) is not the correct expalnation of \(\mathrm{A}\). (c) \(\mathrm{A}\) is true but \(\mathrm{R}\) is false. (d) \(\mathrm{A}\) is false but \(\mathrm{R}\) is true. (e) \(\mathrm{A}\) and \(\mathrm{R}\) both are false. (Assertion): The reaction of \(\mathrm{HCN}\) with \(\mathrm{EtCH}(\mathrm{Me})\) \(\mathrm{C}^{13} \mathrm{HO}\) gives two optically active isomers in unequal amounts. (Reason): Formation of cyanohydrins at \(\mathrm{C}-13\) gives rise to \(\mathrm{d}\) and 1 isomers.

Step-by-Step Solution

Answer

(a) Both A and R are correct and R is the correct explanation of A.

Step 1: Evaluate the Assertion (A)

The Assertion (A) states that the reaction of HCN with EtCH(Me)C¹³HO produces two optically active isomers in unequal amounts. This assertion is correct because the addition of HCN to an aldehyde with a chiral center, such as C-13 in this case, leads to the formation of a mixture of cyanohydrins that are enantiomers. These enantiomeric pairs are often not formed in equal amounts due to factors such as chiral induction or kinetic favorability of one enantiomer over the other.

Step 2: Evaluate the Reason (R)

The Reason (R) states that formation of cyanohydrins at C-13 gives rise to d and l isomers. This is correct because the reaction involves the addition of HCN across the carbonyl group, resulting in a new chiral center. This leads to the formation of two enantiomers (d and l configurations).

Step 3: Check if Reason (R) Explains Assertion (A)

Reason (R) correctly explains why two optically active isomers are formed during the reaction: it highlights the formation of d and l isomers due to the new chiral center at C-13. This directly correlates with the Assertion about the formation of two isomers, albeit in unequal amounts.

Key Concepts

Optical Isomerism

Optical isomerism is a fascinating and crucial concept in chemistry, especially important in the context of organic molecules. It occurs when two or more compounds have the same formula but differ in the spatial arrangement of atoms, leading to different interactions with polarized light.
Optical isomers, also known as enantiomers, can rotate plane-polarized light: one isomer rotates it clockwise (dextrorotary - d), and the other counterclockwise (levorotary - l).

• These properties arise when a molecule has a chiral center, usually a carbon atom bonded to four different groups.
• Such chiral centers result in non-superimposable mirror images, leading to different isomers.
• Understanding optical isomerism is crucial in fields like pharmaceuticals, where different isomers can have markedly different effects in biological systems.

When studying reactions like the formation of optically active cyanohydrins, recognizing the conditions that lead to optical isomerism is key for predicting the outcomes.

Cyanohydrin Formation

Cyanohydrin formation is an essential reaction in organic chemistry, involving the addition of hydrogen cyanide (HCN) to aldehydes or ketones.
This reaction creates a new chiral center, making it a focal point for studying optical activity.

• The general mechanism starts with the nucleophilic addition of the cyanide ion to the carbonyl carbon, leading to a negatively charged intermediate.
• A proton from the surrounding media then protonates this intermediate, forming the final stable cyanohydrin.
• Cyanohydrins are key intermediates in synthesis, often used to introduce additional functional groups into molecules.
• The reaction's stereochemical outcome can lead to optically active products, which are crucial when considering synthetic pathways.

Recognizing how these formations occur and their resultant chiral centers forms the basis for understanding more complex organic reaction mechanisms.

Chiral Centers

A chiral center in a molecule is a carbon atom that is bonded to four distinct substituents. This specific arrangement leads to chirality, a property vital in chemistry.
Chiral centers are fundamental in determining the optical activity of a compound.

• Every molecule with a chiral center can potentially have two enantiomers, which are mirror images that cannot be superimposed onto each other.
• The presence of a chiral center is essential for the molecule to exhibit optical isomerism.
• In practical terms, identifying and analyzing chiral centers help predict the behavior and reactivity of molecules under different conditions.
• For instance, understanding the chiral center in the context of cyanohydrin formation helps elucidate how enantiomers are formed during the reaction of HCN with an aldehyde like EtCH(Me)C¹³HO.

This understanding is crucial in various applications, particularly where the biological activity of enantiomers differs.

Enantiomers

Enantiomers are a type of stereoisomer, specifically optical isomers, that are non-superimposable mirror images of each other.
These play an essential role in chemistry due to their distinct and often significant properties.

• Despite having identical physical and chemical properties, enantiomers interact differently with chiral environments, such as biological systems, making them essential in pharmaceutical design.
• Enantiomers are typically categorized based on their ability to rotate plane-polarized light in different directions: one rotates light clockwise (d), the other counterclockwise (l).
• These differences affect how enantiomers function biologically, with one often being beneficial or active and the other inactive or even harmful.
• Understanding the formation of enantiomers, such as through cyanohydrin formation, helps predict outcomes of reactions that produce chiral centers.

Studying enantiomers' unique characteristics is critical in fields like medicinal chemistry and biochemistry.

Organic Reaction Mechanisms

Organic reaction mechanisms describe the step-by-step pathway through which reactants transform into products.
Understanding these mechanisms allows chemists to predict and control the outcomes of reactions accurately.

• The mechanism of cyanohydrin formation, for example, involves nucleophilic addition followed by protonation, crucial for understanding the creation of new chiral centers.
• Detailed knowledge about these stepwise processes allows chemists to manipulate reaction conditions, optimizing for desired products, such as specific enantiomers.
• Mechanisms often show intermediate species and their transformations, crucial for predicting reaction kinetics and thermodynamics.
• A solid grasp of organic reaction mechanisms equips chemists with the tools to innovate and synthesize complex organic compounds effectively.

This foundational understanding enhances chemists' ability to design synthetic pathways and engineer chemicals safely and efficiently.