Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{R}-\mathrm{Cl}+\mathrm{AgCN} \longrightarrow \mathrm{A} \stackrel{\text { reductions }}{\longrightarrow} \mathrm{B}\) \(A\) and \(B\) respectively are (a) \(\mathrm{RCN}, \mathrm{RCH}_{2} \mathrm{NH}_{2}\) (b) \(\mathrm{RNC}, \mathrm{RNHCH}_{3}\) (c) \(\mathrm{RCN}, \mathrm{RNHCH}_{3}\) (d) \(\mathrm{RNC}, \mathrm{RCH}_{2} \mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (b) \( \mathrm{RNC}, \mathrm{RNHCH}_{3} \).

Step 1: Understanding the Reaction with AgCN

The reaction of an alkyl halide, \( \text{R-Cl} \), with silver cyanide, \( \text{AgCN} \), leads to the formation of an isocyanide. This happens because \( \text{AgCN} \) acts as a nucleophile, and the \( \text{CN}^- \) ion from \( \text{AgCN} \) tends to bond through the nitrogen atom, forming \( \text{RNC} \). Therefore, the compound \( A \) is \( \text{RNC} \) (an isocyanide).

Step 2: Reduction of the Isocyanide

The reduction of an isocyanide (\( \text{RNC} \)) typically involves the addition of hydrogen atoms to the nitrogen and carbon atoms. This reduction results in a secondary amine \( \text{RNHCH}_{3} \). The compound \( B \), therefore, is \( \text{RNHCH}_{3} \).

Step 3: Comparing Options with Final Products

With \( A \) as \( \text{RNC} \) and \( B \) as \( \text{RNHCH}_{3} \), compare these findings with the options provided: - Option (a): \( \text{RCN}, \text{RCH}_{2} \text{NH}_{2} \) - Option (b): \( \text{RNC}, \text{RNHCH}_{3} \) - Option (c): \( \text{RCN}, \text{RNHCH}_{3} \) - Option (d): \( \text{RNC}, \text{RCH}_{2} \text{NH}_{2} \)The pair that matches our findings is option (b): \( \text{RNC}, \text{RNHCH}_{3} \).

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a vital reaction in organic chemistry. It involves a nucleophile, a species rich in electrons, attacking an electron-poor carbon, usually found in a molecule with a leaving group like a halogen. In our exercise, \(\text{R-Cl}\), an alkyl halide, is a classic example of having a good leaving group, chloride (Cl). When nucleophilic substitution takes place,

• The nucleophile replaces the leaving group.
• In our reaction, silver cyanide (\(\text{AgCN}\)) acts as the nucleophile.
• The \(\text{CN}^-\) ion attacks the carbon bonded to chlorine, eventually replacing the chlorine.

The magic happens in whether the carbon or nitrogen of the cyanide ion creates the bond, leading us deeper into the concept of isocyanides.

Reduction Reactions

Reduction reactions in organic chemistry involve the addition of hydrogen or the removal of oxygen from a molecule. In reduction, electrons are transferred to the molecule, decreasing its oxidation state. For isocyanides, like \(\text{RNC}\), the reduction is achieved typically by adding hydrogen. During this process:

• The nitrogen and carbon receive hydrogen atoms.
• This act transforms the structure, converting a linear isocyanide to a more complex organic molecule.

In our specific exercise, the reduction of \(\text{RNC}\) results in a secondary amine called \(\text{RNHCH}_3\). This conversion showcases how simple changes can significantly alter molecular structures and properties.

Isocyanides

Isocyanides are unique organic compounds characterized by the presence of the \(-\text{NC}\) group, where nitrogen is bonded directly to carbon. They are the less common form of isomers compared to nitriles. The formation of isocyanides is quite intriguing:

• The nucleophile \(\text{CN}^-\) in \(\text{AgCN}\) prefers to bond through nitrogen rather than carbon.
• This preference arises due to the complex bonding environment and the nature of silver cyanide.

When \(\text{R-Cl}\) reacts with \(\text{AgCN}\), the linkage through nitrogen leads to the formation of \(\text{RNC}\). Isocyanides are used in various chemical syntheses and have unique odors, contributing to their distinct presence.

Alkyl Halides

Alkyl halides, also known as haloalkanes, are molecules where a halogen atom is bonded to an alkyl group. They serve as foundational compounds in organic chemistry, acting as a reactive center for numerous reactions like substitutions and eliminations.In our given problem:

• The alkyl halide used is \(\text{R-Cl}\), where R is an alkyl group, and Cl is the halogen.
• The presence of a halogen makes these compounds reactive, often serving as a good leaving group.
• During nucleophilic substitution, the chloride ion is replaced by another group or atom through a reaction with a nucleophile.

This feature of alkyl halides is pivotal for constructing various organic frameworks, making them indispensable in organic reactions.