Problem 31 - A Complete Resource Book in Chemistry for JEE Main

Question

The major product in the reaction $$ \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{Br} \stackrel{\mathrm{AgCN}}{\longrightarrow} ? \text { is } $$ (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CN}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{Ag}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{NC}\) (d) none of these

Step-by-Step Solution

Answer

The major product is (c) \( \mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{NC} \).

Step 1: Identify the Reaction Type

The reaction involves the substitution of a bromine atom in an ethyl bromide (\( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{Br} \)) by a nucleophile. This is a nucleophilic substitution reaction where silver cyanide (\( \mathrm{AgCN} \)) is being used.

Step 2: Understand the Nature of Silver Cyanide

Silver cyanide is different from simple cyanides like \( \mathrm{KCN} \). In \( \mathrm{AgCN} \), the cyanide ion (\( \mathrm{CN}^- \)) has a dual character, meaning it can act as both a cyanide (\( \mathrm{C} \equiv \mathrm{N}^- \)) and an isocyanide (\( \mathrm{NC}^- \)). The bond in \( \mathrm{AgCN} \) is more covalent compared to other cyanides.

Step 3: Determine Actual Product Formation

In reactions involving \( \mathrm{AgCN} \), the isocyanide linkage is more favorable due to the covalent nature of the bond. Therefore, the rearrangement leads to the formation of the isocyanide product: \( \mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{NC} \).

Step 4: Select the Major Product

Based on the above steps, the major product of this reaction is an isocyanide, which is option (c) \( \mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{NC} \).

Key Concepts

Silver Cyanide Reactions

Silver cyanide (AgCN) plays a fascinating role in nucleophilic substitution reactions. It’s not as straightforward as you might think. Unlike typical cyanide salts like potassium cyanide (KCN), silver cyanide engages in reactions uniquely due to its covalent characteristic. This involves the cyanide ion \( \text{CN}^- \), which is known to form a stable bond with silver through a covalent connection. The presence of this bond alters how silver cyanide behaves in organic reactions.

During nucleophilic substitution reactions, where a nucleophile replaces a leaving group, AgCN often leads to isocyanide formation. This happens because the covalent bond in AgCN results in a stronger linkage to the silver, thus preferring the isocyanide configuration.

Isocyanide Formation

To comprehend why isocyanide rather than cyanide forms as a product in reactions with AgCN, it’s crucial to look at the structure and behavior of the cyanide ion itself. The cyanide ion can connect to organic molecules in two forms: as a cyanide (\( \text{C} \equiv \text{N}^- \)) or as an isocyanide (\( \text{NC}^- \)).

In the case of AgCN, the covalent nature of the Ag-C bond makes it energetically more favorable for the reaction to proceed with linkage reversal. Instead of attaching via the carbon end, the nitrogen end connects with the organic substrate, forming an isocyanide (\( \text{R-NC} \)). This results in the product as seen in our example: \( \text{CH}_3-\text{CH}_2 \text{NC} \).

The isocyanide group is distinguishable due to its unique \( \text{NC} \) connectivity, which contributes to distinct chemical properties and reactivity compared to typical cyanides.

Organic Chemistry

Understanding the behavior of silver cyanide and isocyanide formation gives us insight into broader organic chemistry principles. Organic chemistry centers around the concept of how different atoms within a molecule interact, form, and break bonds, leading to various chemical structures and properties.

Reactions like these exemplify the dynamic nature of organic molecule transformations. They highlight important fundamentals such as sterics, electronic effects, and the nature of bonding, which are key to predicting reaction pathways and products.

In nucleophilic substitution reactions specifically, the leaving group (like Br in ethyl bromide) is replaced by a nucleophile (AgCN in this case), showing how slight adjustments in bond character can drastically change the outcome, resulting in either a cyanide or an isocyanide linkage. Mastering these basics paves the way for more advanced explorations in organic synthesis and other chemical applications.