Problem 181 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following chemical reactions: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}+\mathrm{CHCl}_{3}+3 \mathrm{KOH} \longrightarrow(\mathrm{A})+(\mathrm{B})+3 \mathrm{H}_{2} \mathrm{O}\) the compounds \((\mathrm{A})\) and \((\mathrm{B})\) are respectively \(\quad\) [2007] (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\) and \(3 \mathrm{KCl}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NC}\) and \(\mathrm{K}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{NC}\) and \(3 \mathrm{KCl}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{CN}\) and \(3 \mathrm{KCl}\)

Step-by-Step Solution

Answer

(b) \( \text{C}_2 \text{H}_5 \text{NC} \) and \( \text{K}_2 \text{CO}_3 \)

Step 1: Understanding the Reaction

The chemical reaction given is \( \text{CH}_3\text{CH}_2\text{NH}_2 + \text{CHCl}_3 + 3 \text{KOH} \longrightarrow (\text{A}) + (\text{B}) + 3 \text{H}_2\text{O} \). This is known as a Hofmann carbylamine reaction, where a primary amine (in this case, \( \text{CH}_3\text{CH}_2\text{NH}_2 \)) reacts with chloroform \( \text{CHCl}_3 \) and a base (\( \text{KOH} \)) to form an isocyanide (isocyanide formation). The side product is a potassium halide.

Step 2: Applying the Reaction Mechanism

The Hofmann carbylamine reaction mechanism involves the formation of an isocyanide. The ethyl amine \( (\text{CH}_3\text{CH}_2\text{NH}_2) \) reacts with \( \text{CHCl}_3 \) in the presence of strong base \( \text{KOH} \). The reaction proceeds with the elimination of \( 3 \text{KCl} \), leading to the creation of an isocyanide compound \( \text{C}_2\text{H}_5\text{NC} \) (ethyl isocyanide).

Key Concepts

Isocyanide Formation

In chemistry, isocyanides, also referred to as isonitriles or carbylamines, are interesting compounds known for their strong odor and unique structure. The general formula for an isocyanide is RNC, where R represents an organic substituent. The Hofmann carbylamine reaction is a classic method used to synthesize these compounds. This reaction involves the conversion of a primary amine into an isocyanide.
The reaction process begins with the terminal nitrogen in the primary amine attacking carbon in chloroform, forming an intermediate species. Subsequently, deprotonation occurs, facilitated by a strong base such as potassium hydroxide (KOH). Through a series of steps involving the loss and rearrangement of atoms, isocyanides are formed as the final product.
The significance of isocyanide formation lies in its use as a versatile building block in the synthesis of many organic molecules. The resulting isocyanides from this reaction, like ethyl isocyanide (\(\text{C}_2\text{H}_5\text{NC}\)), are crucial intermediates in various synthetic applications and pharmaceutical developments.

Primary Amine Reaction

Primary amines are organic compounds characterized by the presence of an amino group (\(-\text{NH}_2\)) attached to a carbon atom. They are key reactants in the Hofmann carbylamine reaction and play a central role in producing isocyanides. These amines provide the nitrogen atom necessary for forming the isocyanide group.
In the specific case of the Hofmann carbylamine reaction, the primary amine (such as ethylamine, \(\text{CH}_3\text{CH}_2\text{NH}_2\)) acts as a nucleophile. This means that it donates a pair of electrons to a positively charged species, in this case, a carbon atom in chloroform. This initial nucleophilic attack is what sets the entire reaction into motion, leading towards isocyanide formation.
Primary amines are not only reactive in producing isocyanides but also appear in numerous other chemical transformations. They are important in forming polymers, drugs, and dyes, showcasing their versatility and crucial nature in chemistry.

Reaction Mechanism

The mechanism of the Hofmann carbylamine reaction is a multi-step process that elegantly transforms specific substrates into isocyanides. Understanding each step can provide insight into how different molecular transformations occur.
The reaction initiates with the nucleophilic attack by the nitrogen atom from the primary amine on the chloroform molecule. This results in the formation of a dichlorocarbene intermediate, which is often transient and highly reactive. The strong base, such as KOH, then facilitates a series of proton transfer and elimination steps.

• The OH- from KOH deprotonates the amine, making the remaining amine group more nucleophilic.
• This promotes the formation of a carbanion that attacks the carbon of chloroform, forming an intermediate that will eliminate chloroform as HCl.
• The final step involves the rearrangement of atoms, breaking some bonds, and forming new ones to yield the desired isocyanide product and potassium chloride as a byproduct.

This detailed mechanism highlights the importance of both substrate reactivity and the role of catalysts or bases that guide the reaction to completion. It showcases how a controlled sequence of electron movements can transform simple starting materials into complex, functionalized products.