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 option is (d) \(\mathrm{RNC}, \mathrm{RCH}_{2} \mathrm{NH}_{2}\).

Step 1: Identify the reaction mechanism

When R-Cl reacts with AgCN, there is an ion exchange where Cl- is replaced by CN-. However, the product is not as straightforward because AgCN can lead to two different products: the nitrile (R-CN) or the isonitrile (R-NC). Generally, AgCN tends to form the isonitrile (R-NC) due to the ambident nature of the AgCN nucleophile.

Step 2: Determine the structure of compound A

Considering the nature of AgCN, the more likely product when R-Cl reacts with AgCN is R-NC (isonitrile) rather than R-CN (nitrile). This is due to the ability of the Ag+ ion to form a complex with the nitrogen atom and facilitate the formation of the isonitrile.

Step 3: Identify subsequent reaction (reduction step)

The compound A (R-NC) will undergo reduction by suitable reducing agents. During the reduction of isonitrile, the C-N triple bond is transformed into a C-N single bond, typically yielding a primary amine, in this case RCH₂NH₂.

Step 4: Confirm the identities of compounds A and B

From the previous steps, compound A is identified as an isonitrile, R-NC. After the reduction process, compound B is the primary amine, RCH₂NH₂. Thus, the correct identities of A and B respectively are RNC and RCH₂NH₂.

Key Concepts

Isonitrile Formation

Isonitriles, also known as isocyanides, form through a reaction involving an organic halide like R-Cl and a nucleophile such as silver cyanide, AgCN. When silver cyanide is employed, it acts as an ambident nucleophile, which means it has more than one reactive site. It can react either via the carbon or the nitrogen atom to form different products.
AgCN typically prefers the nitrogen reaction pathway, leading to the formation of isocyanides (R-NC).
This tendency is due to the formation of a complex between the Ag+ ion and the nitrogen, which facilitates the easier formation of the isonitrile bond rather than the nitrile bond in such reactions.

In summary, the ambident nature of AgCN makes it likely for it to produce isonitriles (R-NC) rather than nitriles (R-CN). This gives an edge to isonitriles being the primary product when reacting organic halides with AgCN.

Nitrile vs Isonitrile

Nitriles and isonitriles, although similar in name, exhibit distinct structural and chemical differences. In nitriles, the carbon atom of the cyano group is attached to the organic chain, forming a R-CN structure. On the other hand, isonitriles have the nitrogen atom of the cyano group linked to the organic moiety, resulting in an R-NC configuration.

These differences in structure lead to variations in their chemical properties:

• Bond Formation: RCN forms through nucleophilic substitution where the carbon atom of CN- acts as the reactive center, while RNC results from a nitrogen-centered nucleophilic substitution.
• Stability: Nitriles are generally more stable than isonitriles due to the more favorable carbon-to-carbon bonding as opposed to the easier-to-break carbon-to-nitrogen isonitrile bond.
• Applications: Nitriles find usage primarily in polymer production and as solvents, while isonitriles are often used in the synthesis of pharmaceuticals and unique organic compounds.

This distinction is important when considering the desired outcomes of a reaction involving these compounds.

Reduction of Isonitriles

Reducing isonitriles involves transforming the highly reactive C-N triple bond into a more stable single bond, producing a primary amine. In our exercise, the isonitrile R-NC is reduced to form RCH₂NH₂.
This reduction can be accomplished using various reducing agents such as lithium aluminum hydride (LiAlH4), which is known for its potency in reducing organic compounds.
As a result of this reaction:

• The C-N triple bond is broken.
• Hydrogen atoms are added, converting the isonitrile into a primary amine.
• Significant changes in chemical properties occur, as the reactive isonitrile transforms into a more stable and less reactive amine.

Understanding this reduction process is crucial for students exploring synthetic pathways in organic chemistry, as it highlights the transformation from functional groups with high reactivity to more stable forms suitable for further chemical modifications.