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 formed is an enamine (b).

Step 1: Understanding the Reaction Components

The reaction involves cyclohexanone, dimethylamine, and a catalytic amount of acid. Cyclohexanone is a ketone, dimethylamine is a secondary amine, and an acid catalyst facilitates the reaction.

Step 2: Identifying the Reaction Type

In the presence of an acid, a ketone like cyclohexanone reacts with an amine such as dimethylamine. This typically leads to the formation of either an imine or an enamine, depending on the type of amine involved.

Step 3: Considering the Amine Type

Dimethylamine is a secondary amine. Secondary amines, when reacting with ketones or aldehydes, lead to the formation of enamines, provided water is removed continuously from the reaction mixture.

Step 4: Analyzing the Water Removal

The continuous removal of water during the reaction with a secondary amine supports the formation of an enamine. This removal is crucial for driving the equilibrium towards enamine formation rather than reverting back to the starting materials.

Step 5: Conclusion on the Product

Synthesizing all the analysis, the reaction of cyclohexanone with dimethylamine in the presence of catalytic acid and continuous water removal leads to the formation of an enamine.

Key Concepts

Cyclohexanone Reaction

Cyclohexanone is a six-carbon cyclic ketone, a common starting point in organic reactions due to its reactive carbonyl group. The carbonyl group (\(C=O\)) is highly susceptible to nucleophilic attack, which is crucial in many transformation processes. In our case, cyclohexanone undergoes a reaction with a secondary amine, such as dimethylamine, to form an enamine. For this reaction to succeed, certain conditions are essential.

• The presence of an acid catalyst: This is not only important for activating the carbonyl group but also for stabilizing the transition state in the reaction.
• Continuous removal of water: This assists in shifting the reaction equilibrium towards the formation of the enamine product.

Thus, cyclohexanone serves as the keystone molecule in this synthesis, with its reactivity being the prime driver for successful enamine formation.

Secondary Amines

Secondary amines like dimethylamine play a pivotal role in enamine formation. A secondary amine consists of two organic groups attached to the nitrogen atom as opposed to primary amines, which have only one. This structural difference is significant because:

• Secondary amines are more adept at forming enamines compared to primary amines, which form imines.
• In reactions with ketones or aldehydes, the secondary amine attacks the electrophilic carbon of the carbonyl group. This forms a tetrahedral intermediate.
• Following the formation of this intermediate and subsequent shifts in the molecular structure, an enamine, which is characterized by a C=C double bond adjacent to a nitrogen atom, is generated.

The involvement of secondary amines, therefore, determines whether an enamine or an imine will result from a reaction with a carbonyl group.

Acid-Catalyzed Reactions

Acid-catalyzed reactions are fundamental in organic chemistry, especially in facilitating difficult transformations. In the context of cyclohexanone and dimethylamine, the acid catalyst is crucial for several reasons:

• It protonates the oxygen of the carbonyl group in cyclohexanone, increasing the electrophilic character of the carbon, which is vital for subsequent nucleophilic attack.
• It helps stabilize any charged intermediates or transition states that form during the reaction.
• In acid-catalyzed formations like these, controlling the acidity level is important as too much acid may lead to side reactions or degrade sensitive amines.

Thus, the acid catalyst doesn't just facilitate the reaction; it directs the formation pathway, making it possible for dense molecules like enamines to form efficiently and effectively.