Problem 67 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Complete the following with appropriate structures:(i) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2} \stackrel{\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}, \text { heat }}{\longrightarrow} 2\) products (ii) \(\mathrm{CH}_{3} \mathrm{CONHC}_{6} \mathrm{H}_{5} \stackrel{\mathrm{Br}_{2}, \mathrm{Fe}}{\longrightarrow} 2\) products (iii) \(\triangle \mathrm{NH}_{2}+\langle 0\rangle-\mathrm{COCl}\) base \(?\) Nc1ccccc1N

Step-by-Step Solution

Answer

(i) N-ethylacetamide, acetic acid; (ii) p- and o-bromoacetanilide; (iii) N-carbamoyl/arene derivative.

Step 1: Understanding the Reaction Conditions

In each reaction, the given conditions (reagents and heat) must be analyzed to predict the products. We consider typical reactions such as acylation in (i), aromatic substitution in (ii), and formation of isocyanates/carbamoyl chlorides in (iii).

Step 1: Reaction (i) - Acylation with Acetic Anhydride

The reagent \((\mathrm{CH}_{3}\mathrm{CO})_{2}\mathrm{O}\) suggests an acylation reaction. Ethylamine \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}\) reacts with the acetic anhydride to form N-ethylacetamide and acetic acid as products. The reaction proceeds through nucleophilic attack of the amine on the acyl group of acetic anhydride.

Step 2: Write the Balanced Equation for Reaction (i)

\[\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2} + (\mathrm{CH}_{3}\mathrm{CO})_{2}\mathrm{O} \rightarrow \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NHCOCH}_{3} + \mathrm{CH}_{3}\mathrm{COOH}\] Two products: N-ethylacetamide and acetic acid.

Step 3: Reaction (ii) - Aromatic Substitution with Bromine

The conditions \(\mathrm{Br}_{2}, \mathrm{Fe}\) suggest electrophilic aromatic substitution on the benzene ring in \(\mathrm{CH}_{3}\mathrm{CONHC}_{6}\mathrm{H}_{5}\). The acetanilide molecule allows bromination primarily at the para position, forming p-bromoacetanilide, and a minor product ortho-bromoacetanilide.

Step 4: Write the Balanced Equation for Reaction (ii)

\[\mathrm{CH}_{3}\mathrm{CONHC}_{6}\mathrm{H}_{5} + \mathrm{Br}_{2} \rightarrow \mathrm{CH}_{3}\mathrm{CONH}\text{−}p\text{−}\mathrm{C}_{6}\mathrm{H}_{4}\mathrm{Br} + \mathrm{CH}_{3}\mathrm{CONH}\text{−}o\text{−}\mathrm{C}_{6}\mathrm{H}_{4}\mathrm{Br}\] Two products: p-bromoacetanilide and o-bromoacetanilide.

Step 5: Reaction (iii) - Reaction of Arene with Carbonyl Chloride

This reaction involves aniline (aromatic amine) with COCl subscript to base? The smiling face suggests forming an N-carbonyl derivative, usually forming a N-substituted carbamate or amide, depending on the reaction specifics and base presence.

Step 7: Conclusion for Reaction (iii)

Here, benzene-1,4-diamine ( SMILES: Nc1ccccc1N ) would typically react to form a carbamoyl chloride or isocyanate derivative, suggesting that a detailed reagent context, usually base like pyridine, facilitates the reaction completion.

Key Concepts

Acylation

Acylation is an important reaction in organic chemistry where an acyl group is introduced into a compound. This process commonly involves reagents like acetic anhydride \((\mathrm{CH}_3\mathrm{CO})_2\mathrm{O}\) in our example, to react with amines or alcohols. For reaction (i) in the problem, acylation occurs with ethylamine \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_2\). Here, ethylamine acts as a nucleophile, attacking the electrophilic carbon in the acyl group of the acetic anhydride. This nucleophilic attack leads to the formation of an amide bond, producing N-ethylacetamide \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{NHCOCH}_3\) as one of the products, along with acetic acid \(\mathrm{CH}_3\mathrm{COOH}\) as a byproduct.

• Nucleophilic attack: The nitrogen atom in ethylamine donates its lone pair to the carbon in the acyl group.
• Amide bond formation: This results in the substitution of the leaving group \(\mathrm{CH}_3\mathrm{COOH}\) and linking the ethyl group to the acyl group.
• Products: We end up with N-ethylacetamide and acetic acid.

This type of reaction is vital for synthesizing various pharmaceuticals and polymers by modifying functional groups in organic compounds.

Aromatic Substitution

In aromatic substitution reactions, an electrophile replaces a hydrogen atom on an aromatic ring. The specific type in reaction (ii), known as electrophilic aromatic substitution, involves bromination using \(\mathrm{Br}_2\) and an iron catalyst \(\mathrm{Fe}\). Acetanilide \(\mathrm{CH}_3\mathrm{CONHC}_6\mathrm{H}_5\) serves as the substrate in our example.

The amide group in acetanilide is ortho- and para-directing. This means that the bromine prefers to attach to the ortho and para positions relative to the amide group. The para position is less hindered, often making it the main site of substitution. Thus, the primary product is para-bromoacetanilide, with a small amount of ortho-bromoacetanilide formed as well.

• Electrophilic attack: Bromine is activated by iron, becoming a powerful electrophile that attacks the benzene ring.
• Substitution products: The aromatic stabilization is retained, and hydrogen is replaced with bromine at specific positions.
• Main product: The major product is para-bromoacetanilide due to steric and electronic factors.

This type of structure-directed selectivity is commonly leveraged in organic synthesis to construct complex molecules efficiently.

Isocyanate Formation

Isocyanate formation is a critical step often seen in polymer science and pharmaceuticals, deriving from reactions involving aromatic amines and acid chlorides. In reaction (iii), aniline (an aromatic amine, depicted with two amino groups in the SMILES code \(\text{Nc}1\text{ccccc}1\text{N}\)) reacts with carbonyl chloride (COCl) using a base.

The general mechanism involves the nucleophilic attack of the nitrogen in the amine on the carbon of the carbonyl chloride's \(\ce{-COCl}\) group. This facilitates the loss of \(\ce{HCl}\) and leads to the formation of an isocyanate group, \(\ce{R-N=C=O}\). Isocyanates can further react to form urethanes and polyurethanes:

• Nucleophilic attack: The nitrogen's lone pair attacks the carbon in carbonyl chloride.
• Isocyanate formation: This intermediate loses \(\ce{HCl}\), resulting in the isocyanate \(\ce{-N=C=O}\).
• Subsequent reactions: Isocyanates react further with alcohols or amines to form urethanes or ureas.

Using a base such as pyridine helps to remove the \(\ce{HCl}\) and drive the reaction toward completion. This transformation is essential for synthesizing a wide range of compounds in materials and pharmaceuticals.