Problem 57 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound that will produce an offensive smell when heated with chloroform and alcoholic potash is (a) \(\left(\mathrm{C}_{2} \mathrm{H}_{3}\right)_{2} \mathrm{NH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{NH}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CN}\)

Step-by-Step Solution

Answer

The compound \\(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{NH}_{2}\\) (option c) will produce an offensive smell.

Step 1: Understanding the Reaction Condition

The question refers to a reaction involving chloroform (CHCl₃) and alcoholic potash (KOH in alcohol). This setup is characteristic of the Carbylamine reaction (also known as Hofmann's isonitrile synthesis), which produces an offensive smell.

Step 2: Identifying Compounds Suitable for the Reaction

The Carbylamine reaction occurs when a primary amine reacts with chloroform and alcoholic potash. This produces an isonitrile (carbylamine), known for its unpleasant odor. Let's identify which provided options are primary amines.

Step 3: Analyzing Each Option

- (a) \(\left(\mathrm{C}_{2} \mathrm{H}_{3}\right)_{2} \mathrm{NH}\) is a secondary amine as it has two ethyl groups attached to the nitrogen atom.- (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) is a tertiary amine with three methyl groups attached to the nitrogen.- (c) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{NH}_{2}\) is a primary amine with one ethyl group attached and capable of undergoing the Carbylamine reaction.- (d) \(\mathrm{CH}_{3} \mathrm{CN}\) is not an amine but a nitrile.

Step 4: Conclusion and Answer Selection

Since primary amines can undergo the Carbylamine reaction, only option (c) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{NH}_{2}\) is a primary amine capable of producing an offensive smell under the given conditions.

Key Concepts

Primary Amines

Primary amines are organic compounds that contain a nitrogen atom attached to one carbon chain or alkyl group. In chemical terms, a primary amine can be represented as R-NH₂. Here, 'R' stands for any hydrocarbon chain, such as a methyl or ethyl group.
When considering the structure of a primary amine, it's important to note that the nitrogen atom forms three bonds: two to hydrogen atoms and one to the carbon chain.
This single hydrocarbon chain attachment differentiates primary amines from secondary and tertiary amines.

• Primary amine example: Ethylamine (\( \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{NH}_{2} \)).
• Secondary amines: These have two hydrocarbon chains attached to the nitrogen.
• Tertiary amines: Here, three hydrocarbon chains are attached.

Their unique structure is crucial for their ability to participate in certain chemical reactions, such as the Carbylamine reaction. Recognizing primary amines becomes essential when predicting the chemical behavior of compounds in reactions.

Offensive Odor

The term 'offensive odor' often refers to smells that are unpleasant or strong, commonly associated with certain chemical reactions. In the context of the Carbylamine reaction, this odor is particularly notable.
When a primary amine reacts with chloroform and alcoholic potash, it produces an isonitrile. Isonitriles are infamous for their potent and unpleasant smells, often described as musty or fishy.
This attribute of producing a strong odor is used as an identification marker in organic chemistry. To summarize, the Carbylamine reaction intentionally utilizes this property as an indicator of reaction occurrence. Understanding this concept helps in identifying compounds that undergo this reaction.

Isonitrile Synthesis

Isonitrile synthesis, often referred to as the Carbylamine reaction, is a chemical process involving the transformation of primary amines into isonitriles. This reaction involves heating a primary amine with chloroform (CHCl₃) in the presence of alcoholic potash (a mixture of alcohol and potassium hydroxide).
The reaction proceeds in the following steps: 1. **Formation of Dichlorocarbene:** - Chloroform reacts with KOH to produce dichlorocarbene (a reactive species). 2. **Interaction with Primary Amine:** - The dichlorocarbene then interacts with the primary amine to form an intermediate product. 3. **Isonitrile Formation:** - Finally, rearrangement and elimination produce an isonitrile. The significance of this synthesis lies in its utility for preparing isonitriles, compounds challenging to produce by other means. Though isonitriles are valuable in various chemical applications, they are most known for their strong and often offensive smell.