Problem 68 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following compounds, which will react with acetone to give a product containing \(>\mathrm{C}=\mathrm{N}-?\) 1\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) 2\. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) 3\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHC}_{6} \mathrm{H}_{5}\) 4\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHNH}_{2}\) (a) 3,4 (b) \(1,3,4\) (c) 1,3 (d) 1,4

Step-by-Step Solution

Answer

Choice (d) 1,4 is correct; these compounds will react with acetone to form \(>\mathrm{C}=\mathrm{N}-\).

Step 1: Understand Acetone's Reactivity

Acetone is a ketone, and it can react with primary amines or hydrazines to form Schiff bases or hydrazones, characterized by the presence of the group \(>\mathrm{C}=\mathrm{N}-\). Secondary amines and tertiary amines generally do not form such structures.

Step 2: Analyze Reactivity of Each Compound

1. \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2}\) is aniline with a primary amine group, which can form a Schiff base with acetone. 2. \((\mathrm{CH}_{3})_{3}\mathrm{N}\) is a tertiary amine and does not form \(>\mathrm{C}=\mathrm{N}-\) with acetone.3. \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NHC}_{6}\mathrm{H}_{5}\) contains a secondary amine group and doesn't typically form the desired structure with ketones like acetone.4. \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NHNH}_{2}\) is phenylhydrazine and can form a hydrazone with acetone, resulting in the \(>\mathrm{C}=\mathrm{N}-\) group.

Step 3: Choose the Correct Answer

Given the reactivity analysis, the compounds that form a \(>\mathrm{C}=\mathrm{N}-\) group with acetone are: 1. \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2}\) and 4. \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NHNH}_{2}\). Therefore, the correct choice is (d) 1,4.

Key Concepts

Primary Amines

Primary amines are organic compounds characterized by the presence of an amino group \(\text{-NH}_2\) attached to a carbon chain. These are crucial building blocks in chemical reactions, especially when forming Schiff bases with carbonyl compounds like ketones or aldehydes. The general structure of a primary amine is \(\text{R-NH}_2\), where \(\text{R}\) represents a hydrocarbon group.

Here are a few key properties of primary amines:

• They are nucleophilic, meaning they readily donate a pair of electrons to a suitable electron-deficient molecule, like a carbonyl group in acetone.
• The lone pair on the nitrogen atom allows the formation of strong hydrogen bonds, making primary amines reactive.
• Due to their reactivity, primary amines are highly useful in organic synthesis to produce a range of chemical products, including pharmaceuticals and polymers.

Understanding primary amines is essential when working with reactions that require the introduction of nitrogen atoms into organic molecules. With acetone, primary amines can eliminate a water molecule to form a stable product called a Schiff base, marked by the characteristic \(>\text{C=NH}\) group.

Reactivity with Acetone

Acetone is a simple, yet influential ketone represented by the formula \(\text{CH}_3\text{COCH}_3\). It readily participates in reactions with certain compounds, including primary amines, due to its polar carbonyl group. The carbon in acetone's carbonyl group is electrophilic, making it eager to attract nucleophiles like the nitrogen atom in primary amines.

Here's why the reaction between acetone and primary amines is significant:

• When a primary amine reacts with acetone, the nitrogen from the amine donates a lone pair to the carbonyl carbon of the acetone.
• The presence of hydrogen atoms on the nitrogen allows the formation of a stable imine (Schiff base) structure after water is eliminated from the reaction.
• This reaction forms a \(>\text{C=NH}\) linkage, characteristic of Schiff bases, and is a widely used method in pharmaceuticals and biochemistry for labeling or modifying molecular structures.

Only compounds capable of forming such a linkage will result in the \(>\text{C=NH}\) group. In our original exercise, it is the presence of the primary amine that facilitates this critical transformation with acetone.

Hydrazone Formation

Hydrazones are formed when hydrazine derivatives react with carbonyl compounds like ketones. In this process, the reaction partners are often hydrazines, which contain two nitrogen atoms.

Here is how hydrazone formation unfolds:

• The reaction begins with the attack by the hydrazine's nitrogen on the electrophilic carbon atom of the ketone's carbonyl group.
• Following this, a dehydration step occurs, wherein a molecule of water is removed, resulting in the formation of a double bond between the carbon and nitrogen, creating a \(>\text{C=NNH}\) structure, which is the hydrazone.
• Hydrazone formation is particularly important in biochemistry for detecting aldehydes and in various organic synthesis processes.

In the context of the original exercise, phenylhydrazine \(\text{C}_6\text{H}_5\text{NHNH}_2\) reacts with acetone to give a hydrazone, demonstrating its capability to undergo this type of reaction. This showcases the utility of hydrazones in both synthetic and analytical chemistry.