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 \(3 \mathrm{KCl}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NC}\) and \(\mathrm{K}_{2} \mathrm{CO}_{3}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{NC}\) and \(3\mathrm{KCl}\).

Step 1: Identify the Reaction Type

This is an example of a Hoffmann degradation reaction, where a primary amine reacts with chloroform and a strong base (such as KOH) to form an isocyanide (or carbylamine) compound. Here, the primary amine is \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\).

Step 2: Analyze the Reagents

The reagents given are chloroform \(\mathrm{CHCl}_{3}\) and potassium hydroxide \(\mathrm{KOH}\). In this reaction, \(\mathrm{KOH}\) promotes the conversion of the amine into an isocyanide through dehydration and substitution steps.

Step 3: Determine the Products by Reaction Mechanism

When \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) reacts with \(\mathrm{CHCl}_{3}\) and \(3\mathrm{KOH}\), an isocyanide is formed, specifically \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{NC}\). The side products include \(3\mathrm{KCl}\) and water.

Step 4: Choose the Correct Pair of Products

The reaction results in \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{NC}\) and \(3\mathrm{KCl}\), which matches option (c) of the given answers.

Key Concepts

Primary Amine

Understanding primary amines is crucial when exploring the Hoffmann degradation reaction. Primary amines are organic compounds that contain an amino group

• They have a general formula of \(RNH_2\). Here, \(R\) represents an alkyl or aryl group.
• In this reaction, the primary amine is \( extit{CH}_3- extit{CH}_2- extit{NH}_2\).

This specific compound is also known as ethylamine, which consists of an ethyl group (\( extit{CH}_3- extit{CH}_2-\)) attached to an amino group \(\(-NH_2\)\).
Primary amines are essential in organic reactions because they can react in various ways, including forming bonds with carbon atoms. In the Hoffmann degradation reaction, the primary amine serves as the starting material, reacting with other reagents to produce more complex products.

Isocyanide Formation

Isocyanide formation is a fascinating outcome of the Hoffmann degradation reaction.

• Isocyanides, also called carbylamines, are compounds with the functional group \(\(-N\equiv C\)\).
• They exhibit unique properties like a distinct, pungent smell.
• In the given reaction, the primary amine \(\( extit{CH}_3- extit{CH}_2- extit{NH}_2\)\) reacts to form the isocyanide compound \(\( extit{C}_2 extit{H}_5 extit{NC}\)\).

The process of forming an isocyanide involves multiple steps. First, the base, such as potassium hydroxide (KOH), deprotonates the amine. Then, the resulting ion interacts with chloroform (CHCl₃). In the reaction, the chloroform contributes a dichlorocarbene group, which eventually gets converted into the isocyanide. This transformation is an excellent example of how primary amines can be fully utilized in forming complex organic molecules.

Reaction Mechanism

A detailed look at the reaction mechanism sheds light on how the Hoffmann degradation reaction occurs.

• The reaction is initiated by a deprotonation step, where the strong base (KOH) removes a hydrogen from the primary amine.
• This step creates a negatively charged amide ion (\( extit{RNH}^-\)), which is highly reactive.

Subsequently, the negatively charged amide ion attacks the dichlorocarbene that forms from the decomposition of chloroform (CHCl₃). This step is crucial as it leads to the formation of the isocyanide.
Potassium hydroxide not only assists in the deprotonation of amines but also plays a role in generating the intermediate species needed for further reaction progression.

• The final products, in this case, include isocyanide \(\( extit{C}_2 extit{H}_5 extit{NC}\)\) and salt byproducts like potassium chloride \(\(3 extit{KCl}\)\).

Understanding each step of the mechanism helps in predicting and manipulating the outcome of similar organic reactions.