Problem 26 - A Complete Resource Book in Chemistry for JEE Main

Question

Which amine of the following will not give carbylamine reaction? (a) ethylamine (b) dimethylamine (c) methylamine (d) phenylamine

Step-by-Step Solution

Answer

Dimethylamine will not give a carbylamine reaction.

Step 1: Understand the Carbylamine Reaction

The carbylamine reaction, also known as the isocyanide test, is a chemical reaction used to detect primary amines. In this reaction, a primary amine reacts with chloroform and alcoholic potassium hydroxide (KOH) to form an isocyanide, which has a very unpleasant odor. Secondary and tertiary amines do not undergo this reaction.

Step 2: Identify the Type of Amines

Analyze each given compound to determine whether they are primary, secondary, or tertiary amines. - Ethylamine (a primary amine): R-NH2 - Dimethylamine (a secondary amine): R-NH-R' - Methylamine (a primary amine): R-NH2 - Phenylamine or aniline (a primary amine): C6H5-NH2.

Step 3: Determine Which Amine Fails the Reaction

Since the carbylamine reaction is specific to primary amines only, we check which amine is not a primary amine. Dimethylamine is a secondary amine, so it will not undergo the carbylamine reaction.

Key Concepts

Understanding Primary Amines

Primary amines are a type of amine where the nitrogen atom is bonded to only one alkyl or aryl group and two hydrogen atoms. This can be represented with the general formula \( \text{R-NH}_2 \), where "R" stands for an alkyl or aryl group. Primary amines play a crucial role in the carbylamine reaction. This is because they possess the specific structure needed to engage in this particular chemical interaction. To spot a primary amine, look for these characteristics:

• The nitrogen atom is directly attached to one carbon atom.
• There are two hydrogen atoms bonded to the nitrogen.

This setup makes primary amines uniquely reactive in certain chemical tests, such as the carbylamine reaction, due to the availability of hydrogen atoms bonded to the nitrogen. This makes them distinct from secondary and tertiary amines.

Exploring the Isocyanide Test

The isocyanide test is a classic chemical reaction used to identify primary amines. It involves mixing a primary amine with chloroform and alcoholic potassium hydroxide (KOH). The test is known for producing isocyanides or carbylamines, which have a distinct and unpleasant odor. This is why it is sometimes referred to as the carbylamine reaction. Key aspects of the isocyanide test:

• Only primary amines react to form isocyanides.
• The reaction requires the presence of chloroform (CHCl₃) and alcoholic KOH.
• Secondary and tertiary amines do not produce isocyanides, providing a way to differentiate them from primary amines.

The presence of a repugnant smell confirms the existence of a primary amine, allowing chemists to identify it among various amine types.

Dimethylamine and Its Role

Dimethylamine is an example of a secondary amine, characterized by the nitrogen atom being bonded to two alkyl groups, typically the same or different carbon chains. In the case of dimethylamine, the nitrogen is attached to two methyl groups \( \text{(CH}_3) \) and has the general structure \( \text{R-NH-R'} \). Secondary amines like dimethylamine do not participate in the carbylamine reaction due to their structural makeup, which includes:

• The nitrogen atom lacking the necessary lone hydrogen atoms.
• The presence of two alkyl groups reducing its capacity to react with chloroform and KOH.

This means that dimethylamine will not form an isocyanide and therefore is a key factor in identifying amines that do not engage in the carbylamine reaction. This characteristic helps to readily distinguish secondary amines from their primary counterparts in chemical analysis.