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

Question

Which of the following reactions does not yield an amine? (a) \(\mathrm{R}-\mathrm{X}+\mathrm{NH}_{3} \longrightarrow\) (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) \(\mathrm{R}-\mathrm{CONH}_{2}+4[\mathrm{H}] \stackrel{\mathrm{LiAlH}_{4}}{\longrightarrow}\)

Step-by-Step Solution

Answer

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

Step 1: Analyze Reaction (a)

Reaction (a) involves the nucleophilic substitution of an alkyl halide (R-X) with ammonia (NH₃). This reaction typically leads to the formation of a primary amine \[ \mathrm{R}-\mathrm{NH}_{2} \], as ammonia acts as a nucleophile and substitutes the halide.

Step 2: Analyze Reaction (b)

Reaction (b) involves the reduction of an oxime (R-CH=NOH) in the presence of sodium (Na) and ethanol (C₂H₅OH). Oximes are typically reduced to primary amines in such conditions, forming \[ \mathrm{R}-\mathrm{NH}_2 \].

Step 3: Analyze Reaction (c)

Reaction (c) involves the hydrolysis of a nitrile (R-CN) in acidic conditions, which results in the formation of a carboxylic acid\[ \mathrm{R}-\mathrm{COOH} \]. No amine is formed in this process.

Step 4: Analyze Reaction (d)

Reaction (d) involves the reduction of an amide \[ \mathrm{R}-\mathrm{CONH}_2 \] with lithium aluminium hydride (LiAlH₄). This reduction typically yields a primary amine \[ \mathrm{R}-\mathrm{NH}_2 \].

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution reactions are a foundational concept in organic chemistry. In these reactions, a nucleophile, which is a species that is rich in electrons, seeks out a positively charged site on a molecule to donate its electrons and form a new chemical bond. For example, when ammonia (\(\mathrm{NH}_{3}\)) encounters an alkyl halide (\(\mathrm{R}-\mathrm{X}\), where X is a halogen), the nitrogen atom in ammonia behaves as a nucleophile. It substitutes the halogen atom in the alkyl halide, resulting in the formation of a new compound called a primary amine, \(\mathrm{R}-\mathrm{NH}_{2}\).

• The nucleophile attacks the carbon atom bonded to the halogen.
• Amino groups (\(\mathrm{NH}_{2}\)) are introduced into the molecule.
• This type of reaction is essential in creating various amines in synthetic chemistry.

Understanding nucleophilic substitution is key to mastering many organic synthesis reactions.

Reduction Reactions

Reduction reactions involve the gain of electrons by a molecule, atom, or ion. In organic chemistry, this often translates to the addition of hydrogen or the removal of oxygen. For instance, in the reduction of oximes (such as \(\mathrm{R}-\mathrm{CH} = \mathrm{NOH}\)), a reagent like sodium in ethanol can donate electrons and hydrogen atoms to the molecule, converting it into a primary amine (\(\mathrm{R}-\mathrm{NH}_2\)).

• Reduction generally increases the number of C-H bonds.
• Oxidation and reduction are complementary processes.
• Reduction can transform functional groups into more saturated and typically less reactive forms.

These reactions are fundamental for converting precursors into desired functional forms, such as amines.

Nitrile Hydrolysis

Nitrile hydrolysis is a process where nitriles (\(\mathrm{R}-\mathrm{CN}\)) are transformed into carboxylic acids (\(\mathrm{R}-\mathrm{COOH}\)) through the introduction of water and an acid. In acidic conditions, the carbon-nitrogen triple bond in the nitrile is broken as water molecules add across the bond. This results in the formation of an amide intermediate, which further hydrolyzes to produce a carboxylic acid.

• Occurs under acidic conditions or can be base-catalyzed.
• Involves conversion to an amide before fully converting to a carboxylic acid.
• Typically, no amine is produced in this reaction.

This pathway is important for altering nitriles into more oxidized forms and does not produce amines.

Hydride Reduction

Hydride reduction is a powerful method used to reduce molecules. It involves the use of metal hydrides, such as lithium aluminum hydride (\(\mathrm{LiAlH}_{4}\)), to deliver hydride ions (\(\mathrm{H}^-\)) to a substrate. A classic example is the reduction of amides (like \(\mathrm{R}-\mathrm{CONH}_{2}\)) into amines (\(\mathrm{R}-\mathrm{NH}_2\)). The hydride ion donates electrons that facilitate the breaking of bonds within the substrate.

• \(\mathrm{LiAlH}_{4}\) is a strong reducing agent capable of reducing a variety of carbonyl compounds.
• The reduction process often increases the number of hydrogen atoms in the molecule.
• This method is crucial for producing primary amines from carbonyl-containing precursors.

Using hydride reduction, chemists can synthesize a wide array of amines from different initial compounds.