Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions does not yield an amine? (a) \(\mathrm{R}-\mathrm{X}+\mathrm{NH}_{3} \longrightarrow \longrightarrow\) (b) \(\mathrm{R}-\mathrm{CH}=\mathrm{NOH}+[\mathrm{H}] \stackrel{\mathrm{Na}, \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}}{\longrightarrow}\) (c) \(\mathrm{R}-\mathrm{CN}+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{H}^{*}}{\longrightarrow}\) (d) \(\mathrm{R}-\mathrm{CONH}_{2}+4[\mathrm{H}] \stackrel{\text { LiAlH }_{4}}{\longrightarrow}\)

Step-by-Step Solution

Answer

Reaction (c) does not yield an amine; it forms a carboxylic acid.

Step 1: Understand the Reaction Types

Identify the type of reaction presented in each option. Options (a) and (b) involve substitution and reduction reactions respectively. Options (c) and (d) involve hydrolysis and reduction respectively. Recognize that these reactions typically transform functional groups into amines.

Step 2: Analyze Reaction (a)

In reaction (a), an alkyl halide (R-X) reacts with ammonia (NH₃). This is a nucleophilic substitution reaction, where the halogen is replaced by an amine group, yielding a primary amine.

Step 3: Analyze Reaction (b)

In reaction (b), oxime (R-CH=NOH) is reduced to an amine group when treated with sodium and alcohol. This is a reduction process and yields a primary amine.

Step 4: Analyze Reaction (c)

In reaction (c), a nitrile (R-CN) is subjected to acidic hydrolysis (H^*), resulting in the formation of a carboxylic acid (R-COOH). This process does not yield an amine; instead, it results in a carboxylic acid.

Step 5: Analyze Reaction (d)

In reaction (d), an amide (R-CONH₂) is reduced with lithium aluminum hydride (LiAlH₄). This reduction transforms the amide directly into an amine due to the reduction of the carbonyl group.

Step 6: Identify the Non-Amine Yielding Reaction

Based on the analyses, only reaction (c) results in the formation of a product that is not an amine. It produces a carboxylic acid instead of an amine.

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a cornerstone reaction in organic chemistry that involves the replacement of a leaving group by a nucleophile. In simpler terms, it's like switching dance partners at a dance: a group attached to a carbon is replaced by another more reactive group. This reaction is fundamental in the creation of many natural and synthetic compounds.

• **Alkyl Halides**: These are common substrates where the leaving group is often a halogen atom (like chlorine or bromine).
• **Ammonia**: Acts as the nucleophile (an electron-rich species) in our case, seeking to bond with a positively charged or electron-poor center, often carbon.

In reaction (a) from our exercise, we see an alkyl halide (\( \mathrm{R}-\mathrm{X} \)) reacting with ammonia (\( \mathrm{NH}_3 \)), leading to the formation of a primary amine. Here, the halide ion acts as a leaving group, while the ammonia, due to its lone electron pair, replaces the halide as the nucleophile. This transformation is key in forming amines from more basic organic compounds.

Reduction Reaction

Reduction reactions are processes where a molecule gains electrons, typically through the addition of hydrogen. In organic chemistry, reduction is a vital concept because it alters the structure of molecules significantly, enabling new chemical properties and functionalities.

• **Oximes to Amines**: When oximes are treated with reducing agents like sodium and ethanol, they lose the double bond to nitrogen, converting into amines. This is what happens in reaction (b) with an R-CH=NOH compound.
• **Amides to Amines**: Amides are reduced using potent reagents like Lithium Aluminum Hydride (\( \text{LiAlH}_4 \)). This results in the cleavage of the carbonyl group, transitioning the compound into an amine.

Both reactions (b) and (d) highlight how diverse and powerful reduction reactions can be in synthesizing amines. This is because they effectively replace multiple bonded oxygen atoms with hydrogen, enabling the nitrogen atom to become part of an amine group. These transformations are crucial in producing amines from both oximes and amides.

Hydrolysis Reaction

Hydrolysis reactions involve the breaking of chemical bonds through the addition of water. This often leads to the transformation of complex organic molecules into simpler units. Hydrolysis reactions take advantage of water's reactivity, particularly when catalyzed by acids or bases.

• **Nitriles to Carboxylic Acids**: In the case of nitriles like \( \mathrm{R}-\mathrm{CN} \), hydrolysis in the presence of acid (\( \mathrm{H}^* \)) does not yield amines. Instead, the triple bonded nitrile group is converted into a carboxylic acid (\( \mathrm{R}-\mathrm{COOH} \)).

In reaction (c) from our exercise, this hydrolysis differs significantly from substitution or reduction because the end product is a carboxylic acid rather than an amine. Therefore, although hydrolysis is a very effective way to transform nitriles, it doesn't contribute to amine formation, which distinguishes it from the other reactions in forming amino groups.