Problem 224 - A Complete Resource Book in Chemistry for JEE Main

Question

Reaction of cyclohexanone with dimethylamine in the presence of catalytic amount of an acid forms a compound if water during the reaction is continuously removed. The compound formed is generally known as (a) a Schiff's base (b) an enamine (c) an imine (d) an amine

Step-by-Step Solution

Answer

The compound is an enamine (b).

Step 1: Identify the Reactants

The reaction involves cyclohexanone with dimethylamine in the presence of an acid. Cyclohexanone is a ketone, and dimethylamine is a secondary amine.

Step 2: Determine the Reaction Type

This reaction is a typical example of a condensation reaction, where a ketone and a secondary amine react. Such reactions typically lead to the formation of enamines or imines, depending on the type of amine.

Step 3: Analyze the Product Formation

In the reaction, since a secondary amine (dimethylamine) is used, the product formed is an enamine. In a Schiff base formation, a primary amine reacts with a carbonyl group, which is not the case here.

Step 4: Confirm by Water Removal

The problem states that water is continuously removed, which aligns with enamine formation, as water is a by-product of this reaction.

Step 5: Choose the Correct Answer

Based on the analysis, the compound formed in the reaction is an enamine, corresponding to option (b).

Key Concepts

Cyclohexanone Reaction

Cyclohexanone is a six-membered cyclic ketone. When it reacts with amines, such as dimethylamine, it undergoes a type of organic reaction. Here, the ketone (cyclohexanone) is the key player, as its carbonyl group (c=O) is very reactive. This carbonyl group can react with amines to form different types of compounds.

• Carbonyl Reactions: The carbonyl group is very electrophilic, meaning it can attract nucleophiles, such as amines.
• Role in Enamines: In enamine formation, the carbonyl group reacts with secondary amines.

Understanding this reaction is essential, as it lays the groundwork for understanding more complex organic chemistry reactions.

Dimethylamine Reaction

Dimethylamine is a common secondary amine characterized by the presence of two methyl groups. In organic chemistry, secondary amines like dimethylamine are known for their ability to participate in reactions with ketones, like cyclohexanone, to form enamines.

• Basic Structure: It has the formula \(\text{(CH}_3\text{)}_2\text{NH}\), where two methyl groups are attached to an amino (NH) group.
• Role in Condensation Reactions: Secondary amines are excellent nucleophiles and react with electrophilic carbonyl carbon atoms in ketones.

In the context of cyclohexanone reacting with dimethylamine, the secondary amine acts as a nucleophile, attacking the electrophilic carbon, leading to enamine formation.

Condensation Reaction

A condensation reaction is a fundamental process in organic chemistry where two molecules combine, resulting in the formation of a larger molecule and the loss of a small molecule like water. In the reaction between cyclohexanone and dimethylamine, a condensation reaction leads to enamine formation.

• Mechanism: The carbonyl group of cyclohexanone reacts with dimethylamine, forming an intermediate compound that loses water, forming an enamine.
• Water Removal: Continuous removal of water is crucial, as water can shift the equilibrium back towards reactants.

The process is common in synthesizing various organic compounds and is particularly important in forming compounds like enamines.

Secondary Amines

Secondary amines are a category of amines where two of the hydrogens in ammonia (NH3) are replaced by alkyl or aryl groups. An example is dimethylamine. These amines are known for their unique chemical behavior compared to primary and tertiary amines.

• Structure: Secondary amines have the general formula \(\text{R}_2\text{NH}\), where \(\text{R}\) represents a hydrocarbon group.
• Reactivity: Due to their structure, secondary amines are good nucleophiles, making them reactive with carbonyl groups in ketones.

When reacting with cyclohexanone, secondary amines are a key component for forming enamines, as they engage in nucleophilic attack on the carbonyl carbon.