Question

Select the reagents that reduce nitriles to primary amines: \[ \text{A. } LiAlH_4 \] \[ \text{B. } Sn+HCl \] \[ \text{C. } H_2-Ni \] \[ \text{D. } Na(Hg)/C_2H_5OH \] \[ \text{E. } Br_2/aq.\ NaOH \]

• A. A, C and D only
• B. A, B and C only
• C. B, D and E only
• D. A, D and E only

Hint:

Nitriles are reduced to primary amines by \(LiAlH_4\) or catalytic hydrogenation using \(H_2/Ni\).

Answer 1

The Correct Option is A

Step 1: Understanding the Topic:
The central theme of this question is the functional group transformation involving the reduction of nitriles (also known as organic cyanides) into primary amines. In organic synthesis, nitriles contain a carbon-nitrogen triple bond ($C \equiv N$). Reduction is the process of adding hydrogen across this multiple bond to reach a saturated state where the carbon and nitrogen are both $sp^3$ hybridized, resulting in the $-CH_2-NH_2$ group. This reaction is highly valued in laboratories because it allows for the extension of a carbon chain by one unit (if starting from an alkyl halide) while simultaneously introducing a primary amino group.
Step 2: Key Formulas and Approach:
The overall chemical transformation can be summarized as: $R-C \equiv N \xrightarrow{Reduction} R-CH_2-NH_2$. To identify suitable reagents, we must look for chemicals that provide hydride ions or facilitate catalytic hydrogenation. We evaluate each option based on its known reactivity:
Hydride-based reduction (using $LiAlH_4$).
Catalytic hydrogenation (using $H_2$ gas with metal catalysts).
Dissolving metal reduction (nascent hydrogen from $Na/EtOH$).

Step 3: Detailed Explanation:

Reagent A (\(LiAlH_4\)/\(H_2O\)): Lithium aluminium hydride is an exceptionally strong reducing agent. The $Al-H$ bonds act as a source of nucleophilic hydride ions ($H^-$) which attack the electrophilic carbon of the nitrile. Two equivalents of hydride are added to the carbon, and the nitrogen eventually becomes protonated during the aqueous workup ($H_2O$). This is a standard and very effective laboratory method for producing primary amines from nitriles.
Reagent B (\(Sn + HCl\)): This combination generates nascent hydrogen and is primarily used for the reduction of the nitro group ($-NO_2$) to the amino group ($-NH_2$) in aromatic systems (forming aniline). It is generally not considered a standard or efficient reagent for the reduction of nitriles to primary amines.
Reagent C (\(H_2/Ni\)): This is a catalytic hydrogenation process. In the presence of a catalyst like Raney Nickel, hydrogen gas adds across the $C \equiv N$ triple bond. This method is industrially preferred because it is clean and can be performed on a large scale to yield primary amines without the generation of bulky metal-salt byproducts.
Reagent D (\(Na(Hg)/C_2H_5OH\)): Known as the Mendius reduction, this method utilizes the reaction between sodium amalgam and ethanol to produce nascent hydrogen. This system is historically significant for specifically reducing nitriles to primary amines.
Reagent E (\(Br_2/aq. NaOH\)): This is the reagent set for the Hofmann Bromamide Degradation. It is used to convert an amide ($R-CONH_2$) to an amine ($R-NH_2$) with one fewer carbon atom. It does not reduce nitriles.
Step 4: Final Answer:
Reagents A, C, and D are valid for this specific reduction. Thus, the correct option is (B).