Problem 128 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following (a) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{CO}-\mathrm{NH}_{2}\) (p) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{O}-\) \(\mathrm{Br}_{2} / \mathrm{KOH}\) \(\mathrm{C}(\mathrm{O})-\mathrm{CH}_{3}\) (b) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{COOH}+\) (q) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{NCO}\) \(\mathrm{HN}_{3} \frac{\text { conc. } \mathrm{H}_{2} \mathrm{SO}_{4}}{\text { heat }}\) (c) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{C}(\mathrm{O})-\mathrm{NH}_{2}\) (r) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) \(\frac{\text { (i) } \mathrm{LiAlH}_{4}}{\text { (ii) } \mathrm{H}_{2} \mathrm{O}}\) (d) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{C}(\mathrm{O})-\mathrm{CH}_{3}\) (s) \(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2}\)

Step-by-Step Solution

Answer

(a) - (r), (b) - (q), (c) - (r), (d) - (s).

Step 1: Understanding Reactions for (a) and Matching Reactants

The compound (a) \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CO}-\mathrm{NH}_{2} \) is an amide. The reaction \( \mathrm{Br}_{2} / \mathrm{KOH} \) is used in the Hofmann bromamide degradation reaction to convert an amide to a primary amine with one less carbon atom. This means compound (a) will react to form \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) (p), which results in the formation of \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \). Thus, \( (\text{a}) - (\text{r}) \).

Step 2: Exploring Reactions for (b) and Finding Matches

The compound (b) \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{COOH} \) is a carboxylic acid. It reacts with \( \mathrm{HN}_{3} \) under acidic and heat conditions in the Curtius rearrangement to form an isocyanate \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{NCO} \). Therefore, the product associated with (b) is (q), leading to \( (\text{b}) - (\text{q}) \).

Step 3: Analyzing Reactions for (c) and Establishing Connection

The compound (c) \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{C} \mathrm{(O)}-\mathrm{NH}_{2} \) is also an amide. The reaction \( \text{(i) } \mathrm{LiAlH}_{4}\text{, (ii) }\mathrm{H}_{2}\mathrm{O} \) implies a reduction process where an amide is converted into an amine. Hence, (c) reduces to \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \). Therefore, the correct match is \( (\text{c}) - (\text{r}) \).

Step 4: Determining Reactions for (d) with Appropriate Matches

The compound (d) \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{C} \mathrm{(O)}-\mathrm{CH}_{3} \) is a ketone. This can be reduced using reactions like Wolff-Kishner or Clemmensen reduction to an alkane \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \). Given the provided products, the logical result of the described reaction is the conversion into \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \) (s) upon complete reduction. Thus, \( (\text{d}) - (\text{s}) \).

Key Concepts

Hofmann Bromamide Degradation

The Hofmann Bromamide Degradation is an important reaction in organic chemistry, mainly used to convert an amide into a primary amine. What makes this reaction special is that it results in an amine with one less carbon atom compared to the original amide. This transformation is achieved using

• Bromine (\( \text{Br}_2 \) )
• an alkaline medium (usually potassium hydroxide, \( \text{KOH} \) )

This method is particularly useful when a smaller carbon chain amine is desired.
The mechanism begins with the formation of an N-bromoamide, followed by rearrangement and deprotonation steps leading to the alkylamine. In practical terms, utilizing this reaction allows chemists to strategically shorten carbon chains, a process useful in fine-tuning molecular properties.

Curtius Rearrangement

The Curtius rearrangement is a unique reaction that transforms carboxylic acids into isocyanates. This is accomplished through the formation of an acyl azide intermediate. This reaction requires:

• the addition of hydrazoic acid (\( \text{HN}_3 \) )
• acidic conditions often provided by \( \text{H}_2 \text{SO}_4 \) and heat.

The acyl azide, once formed, rearranges to form an isocyanate upon heating.
Isocyanates are valuable intermediates that can further be used to synthesize various functional groups making Curtius rearrangement versatile.
This reaction is widely utilized in organic synthesis for introducing nitrogen-containing functionalities and creating more complex molecules from simpler ones.

LiAlH4 Reduction

Lithium Aluminium Hydride, commonly abbreviated as LiAlH4, is a powerful reducing agent in organic chemistry. It is especially useful for reducing various functional groups. In the context of amides, LiAlH4 facilitates the conversion of an amide to an amine, eliminating the carbonyl group entirely.
The reduction process takes place in two steps:

• Initial attack on the carbonyl carbon
• followed by liberation of the amine upon hydrolysis.

This means that complex molecules containing amide functionalities can be effectively simplified using LiAlH4. It's important for students to handle this reagent with care due to its high reactivity, which can lead to vigorous reactions, especially in the presence of protic solvents.

Clemmensen Reduction

The Clemmensen Reduction is an essential method for converting ketones and aldehydes into alkanes. This process is especially useful in scenarios where acidic conditions are favored, as it employs zinc amalgam in the presence of hydrochloric acid.
Clemmensen reduction is characterized by its:

• ability to work under strongly acidic environments
• and preference for reducing carbonyl functions without affecting other sensitive functional groups.

Although another method such as the Wolff-Kishner reduction can achieve similar transformations under basic conditions, Clemmensen is superior in substrates intolerant to base. In complex organic syntheses, accruing alkanes reliably from carbonyl compounds via Clemmensen opens multiple pathways for further chemical manipulation.