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

Question

\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2}+\mathrm{CHCl}_{3}+3 \mathrm{KOH}\) \(\mathrm{A}+\mathrm{B}+3 \mathrm{H}_{2} \mathrm{O}\) Here compound \(\mathrm{A}\) and \(\mathrm{B}\) are respectively? (a) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{CN}\) and \(3 \mathrm{KCl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\) and \(\mathrm{KCl}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NC}\) and \(3 \mathrm{~K} \mathrm{Cl}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NC}\) and \(\mathrm{K}_{2} \mathrm{CO}_{3}\)

Step-by-Step Solution

Answer

Correct answer: (c) 6ig( ext{C}_2 ext{H}_5 ext{NC} 6ig)) and 63 ext{KCl} 6ig)).

Step 1: Understanding the Reaction

The given reaction is an example of the carbylamine reaction, also known as the Hofmann isocyanide synthesis. In this reaction, an aliphatic primary amine reacts with chloroform and a strong base such as potassium hydroxide (KOH) to produce an isocyanide or carbylamine.

Step 2: Identify the Amine Involved

In the given reaction, the amine is ethanamine ( 6ig( ext{CH}_3- ext{CH}_2- ext{NH}_2 6ig)). This is the substrate that will undergo transformation to form the isocyanide.

Step 3: Identify the Reactants

The reactants are chloroform ( 6ig( ext{CHCl}_3 6ig)) and potassium hydroxide ( 63 ext{KOH} 6ig)). Chloroform provides the carbon and part of the isocyanide structure, while the KOH provides the basic conditions necessary for the reaction.

Step 4: Identify the Products

Using the carbylamine reaction rule, the amine 6ig( ext{CH}_3- ext{CH}_2- ext{NH}_2 6ig)) will form an isocyanide 6ig( ext{CH}_3- ext{CH}_2- ext{NC} 6ig)). The byproducts include potassium chloride ( 63 ext{KCl} 6ig)) and water which are formed as a result of the reaction.

Step 5: Finding the Corresponding Option

From the identified products, the main organic compound A formed is 6ig( ext{C}_2 ext{H}_5 ext{NC} 6ig)) and the inorganic byproduct B is 63 ext{KCl} 6ig)). This corresponds to option (c): 6ig( ext{C}_2 ext{H}_5 ext{NC} 6ig)) and 63 ext{KCl} 6ig)).

Key Concepts

Isocyanide Formation

The intriguing transformation of primary amines into isocyanides is central to the Carbylamine Reaction. In this reaction, aliphatic primary amines, like ethanamine \( \text{(CH}_3\text{-CH}_2\text{-NH}_2) \), serve as the primary substrates. These amines, when reacting with chloroform \( \text{(CHCl}_3) \) in the presence of a strong base such as potassium hydroxide \( \text{KOH} \), undergo a sequence of transformations to yield isocyanides. Isocyanides, also known as carbylamines, are characterized by their pungent smell.

• The nitrogen atom of the amine aids in forming a three-membered transition state after attacking the carbon of chloroform.
• Subsequent dehydrohalogenation facilitated by the base results in the removal of hydrogen and halide ions.

In our example, ethanamine reacts to yield ethyl isocyanide \( \text{(C}_2\text{H}_5\text{NC}) \), accompanied by \( \text{KCl} \) and water as byproducts.

Hofmann Isocyanide Synthesis

The Hofmann Isocyanide Synthesis, an established reaction procedure, relies heavily on the principles of nucleophilic substitution and elimination. This method is particularly noteworthy due to its elegant ability to transform simple amines into complex isocyanides with relative ease.

• Chloroform acts as both a reactant and an integral building block for the isocyanide.
• The strong basic conditions provided by potassium hydroxide are crucial to promote dehydrohalogenation.

This synthesis not only reinforces the understanding of organic reaction mechanisms but also demonstrates the utility of using simple starting materials. Here, ethanamine is converted into ethyl isocyanide, proving the effectiveness of Hofmann's method for practical organic synthesis.

Reaction Mechanism Analysis

Analyzing the carbylamine reaction mechanism reveals the dynamics of organic reactivity. First, the amino group in ethanamine attacks the electron-deficient carbon of chloroform. This step leads to a fascinating three-membered intermediate formation, comprising the amine nitrogen, chloroform carbon, and one chlorine atom.

• Potassium hydroxide then plays a crucial role by inducing elimination of HCl, resulting in the formation of dichlorocarbene.
• Subsequent dehydrohalogenation steps facilitate the reforming of the double bond system reminiscent of isocyanide structures.

This step-by-step transformation evidences a superb balance between nucleophilic substitution and base-induced eliminations. The end-product, ethyl isocyanide, emerges through orchestrated sequential reactions, highlighting this mechanism's complexity and elegance. This mechanism is integral to mastering organic synthesis pathways and offers deep insights into reaction dynamics.