Problem 223 - 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

(b) an enamine.

Step 1: Identify the Reactants and Conditions

The reaction involves cyclohexanone and dimethylamine with a catalytic amount of acid. Importantly, water is continuously removed from the reaction mixture.

Step 2: Understand Reaction Type

In the presence of an acid catalyst, a ketone (cyclohexanone) can react with a secondary amine (dimethylamine) to form an enamine. This is a key reaction in organic chemistry where the enamine formation typically requires the removal of water to drive the reaction to completion.

Step 3: Determine the Product Type

In this case, since the amine is a secondary amine (dimethylamine), and water is continuously removed, the reaction favors the formation of an enamine rather than an imine, which generally forms with primary amines.

Step 4: Match with Given Options

Through recall of the mechanism and product type: cyclohexanone reacts with dimethylamine forming an enamine via the removal of water. Thus, when comparing the product to the options provided, the answer is (b) an enamine.

Key Concepts

Cyclohexanone

Cyclohexanone is an organic compound that is part of the ketone family. Its structure is characterized by a six-membered ring which includes a carbonyl group (C=O). This carbonyl group plays a central role in cyclohexanone's reactivity.
In reactions with amines, such as dimethylamine, the carbonyl group typically acts as an electrophile. This means it can attract and bind with nucleophilic compounds like amines.
Key aspects of cyclohexanone include:

• A strong ability to engage in nucleophilic addition reactions due to its carbonyl group.
• Its role in enamine formation, a process vital in organic synthesis.
• Being a liquid at room temperature and having a distinct, somewhat acetone-like odor.

In this context, cyclohexanone provides the structural backbone for the formation of enamines, a crucial intermediate in many synthesis reactions.

Dimethylamine

Dimethylamine is a secondary amine, having two methyl groups attached to a nitrogen atom. This configuration gives it specific properties that are essential in its reactions with ketones like cyclohexanone.
Dimethylamine can donate a pair of electrons, making it a strong nucleophile. Its interaction with the carbonyl group in cyclohexanone leads to bond formation and the eventual displacement of water. This nucleophilic property is crucial because it enables the formation of an enamine, rather than an imine (which involves primary amines).
Important characteristics of dimethylamine:

• It is commonly found as a gas or in aqueous solution, known for its sharp, fishy odor.
• Dimethylamine serves as a pivotal component in forming stabilized intermediates like enamines.
• As a secondary amine, it is particularly effective in enamine, rather than imine, formation.

Thus, dimethylamine's structure and properties make it a suitable partner for cyclohexanone in forming an enamine.

Acid Catalyst

The use of an acid catalyst in reactions like enamine formation is crucial to facilitate and speed up the reaction without being consumed in the process. An acid catalyst works by donating protons (H⁺) that activate the carbonyl group of cyclohexanone, making it more electrophilic and thus more reactive towards nucleophiles like dimethylamine.
Here are the key roles of the acid catalyst:

• It enhances the electrophilicity of the carbonyl group, facilitating its reaction with the amine.
• The acid catalyzes the dehydration step, vital for removing the water generated during the reaction, thus driving the equilibrium towards enamine formation.
• It remains unchanged by the end of the reaction, illustrating its role as a true catalyst.

In the context of this reaction, the acid catalyst ensures that the conditions are optimal for cyclohexanone and dimethylamine to produce an enamine efficiently and effectively.