Problem 52 - A Complete Resource Book in Chemistry for JEE Main

Question

Unpleasant smell of carbylamine is produced when chloroform and alcoholic KOH are heated with (a) any amine (b) any aliphatic amine (c) any aromatic amine (d) any primary amine

Step-by-Step Solution

Answer

The unpleasant smell of carbylamine is produced with (d) any primary amine.

Step 1: Understand the Reaction

The question involves the production of carbylamine, which is an organic compound that is known for its unpleasant odor. This is produced through the carbylamine reaction, also called the Hofmann isocyanide synthesis. This is a chemical reaction that involves a primary amine, chloroform, and an alcoholic solution of potassium hydroxide (KOH).

Step 2: Define the Reactants

Chloroform is a compound with the formula CHCl₃, and alcoholic KOH means potassium hydroxide dissolved in alcohol. The type of amine is crucial, and we have options of primary, secondary, and tertiary amines in different forms, such as aliphatic or aromatic.

Step 3: Recognize the Suitable Amine

For the carbylamine reaction to occur, the amine used must be a primary amine. This is because only primary amines have the necessary structure to react with the other chemicals to form carbylamine. Secondary and tertiary amines do not participate in this reaction to produce isocyanides or carbylamines.

Step 4: Identify the Correct Option

Given these steps and analysis, the correct choice from the options provided is (d) any primary amine. Primary amines are the only type of amines that will produce the unpleasant-smelling carbylamine when reacted with chloroform and alcoholic KOH.

Key Concepts

Primary Amine

Primary amines are organic compounds that contain an amino group \(\text{-NH}_2\) bonded to a carbon atom. This type of amine is characterized by having one hydrogen atom of ammonia replaced by an alkyl or aryl group. Here are some key points about primary amines:

• They are foundational structures in organic chemistry, involved in many chemical reactions.
• Primary amines are typically identified by their structure \(RNH_2\), where \(R\) represents an alkyl or aryl group.
• In the context of the carbylamine reaction, primary amines are essential because they possess the right structure to participate in this specific synthesis.

The defining feature of primary amines is that they can form stable reactions with other compounds, a property utilized in the Hofmann isocyanide synthesis to produce the unpleasant-smelling carbylamine.

Hofmann Isocyanide Synthesis

The Hofmann isocyanide synthesis, often referred to as the carbylamine reaction, is a well-known chemical reaction used to form isocyanides or carbylamines. This synthesis involves primary amines, chloroform, and an alcoholic solution of potassium hydroxide.
Here’s how the process works:

• Primary amines \(\text{(RNH}_2\text{)}\), when treated with chloroform \(\text{(CHCl}_3\,)\), serve as the main reactants for generating isocyanides.
• The reaction requires an alcoholic KOH which acts as a strong base, facilitating the deprotonation of chloroform to create dichlorocarbene \(\text{(:CCl}_2)\) as an intermediate.
• The dichlorocarbene interacts with the primary amine to eventually produce the isocyanide and is responsible for the foul odor associated with carbylamines.

The reaction is used not only for its distinctive outcome but also as a diagnostic tool in organic chemistry to confirm the presence of primary amines.

Chloroform

Chloroform is a simple organic compound with the molecular formula \(\text{CHCl}_3\). It is a halogenated hydrocarbon and has been used historically as an anesthetic. However, in modern laboratories, it has more controlled and specific applications.
In the carbylamine reaction:

• Chloroform acts as a source of dichlorocarbene, an unstable and highly reactive intermediate.
• When heated with alcoholic KOH, chloroform undergoes deprotonation, leading to the formation of dichlorocarbene.
• This reactive species then combines with the primary amine to form the isocyanide compound.

Chloroform is integral to this reaction, providing the reactive carbene needed for the synthesis of carbylamines.

Alcoholic KOH

Alcoholic potassium hydroxide is a solution where KOH is dissolved in alcohol rather than water. This serves a specific purpose in chemical reactions, including those that involve organic synthesis.
In the carbylamine reaction, alcoholic KOH:

• Acts as a strong base, necessary for the deprotonation of the reactants.
• The alcohol in the solution can facilitate and speed up the reaction by providing a suitable medium for the formation of intermediates like dichlorocarbene.
• Ensures the correct environment thus avoiding the hydrolysis of dichlorocarbene, which would prevent the formation of the desired carbylamine.

The use of alcoholic KOH is specifically chosen to aid in the effective synthesis of isocyanides using primary amines and chloroform.