Q.What is the best reagent to convert nitrile to primary amine?

The key idea is that a nitrile ($\ce{R-C#N}$) undergoes reduction to give a primary amine ($\ce{R-CH2NH2}$), with no over-alkylation.

Reasoning:

1. Catalytic hydrogenation ($\ce{H2}$, Ni or Pt) works but can be slow and may reduce other sensitive groups.
2. Lithium aluminium hydride ($\ce{LiAlH4}$) in dry ether is a strong, reliable reducing agent that cleanly converts the triple bond to a methylene group, adding two hydrogens to the carbon and two to the nitrogen. …

The conversion of a nitrile to a primary amine is a classic reduction reaction. A nitrile has a carbon-nitrogen triple bond. To get a primary amine, you need to add two molecules of hydrogen ($H_2$) across that triple bond — one to break the $\pi$ bonds and one to saturate the resulting imine intermediate. The challenge is doing this cleanly without over-reducing or creating side products.

Why $LiAlH_4$ is the best choice

Lithium aluminium hydride is a powerful, non-selective reducing agent. It donates hydride ions ($H^-$) that attack the electrophilic carbon of the nitrile group. The reaction proceeds through an imine intermediate ($R-CH=NH$), which is then further reduced to the amine. The final step is a careful hydrolysis (adding water or dilute acid) to liberate the free amine.

Why not other reagents?

1. Catalytic hydrogenation ($H_2$/Pd, Pt, or Ni) — This also works, but it is less convenient for lab-scale preparations. It requires high pressure and temperature, and it can be tricky to stop at the amine stage without over-reducing to the alkane. Also, if the molecule contains other reducible groups (like a double bond or a nitro group), those will be reduced too. $LiAlH_4$ is more selective for the nitrile in many cases.

2. Sodium borohydride ($NaBH_4$) — This is a milder reducing agent. It does not reduce nitriles under normal conditions. $NaBH_4$ is only strong enough to reduce aldehydes, ketones, and acid chlorides. So it is completely ineffective here.

3. Tin/HCl or Iron/HCl — These are used for reducing nitro groups to amines, not for nitriles. They will not touch a nitrile triple bond.

4. $H_2O$ or $OH^-$ — These hydrolyze the nitrile to an amide or carboxylic acid, not an amine.

Concept: Reduction of Nitriles to Primary Amines

A nitrile ($R-C \equiv N$) can be converted to a primary amine ($R-CH_2-NH_2$) by reduction. The key is that the triple bond is reduced, adding two hydrogen atoms to the carbon and one to the nitrogen.

Method: Catalytic Hydrogenation or Metal Hydride Reduction

Best reagent: Lithium aluminium hydride ($LiAlH_4$) in dry ether, followed by aqueous work-up.

Why this is best:

• $LiAlH_4$ is a strong reducing agent that cleanly reduces the nitrile to a primary amine without over-reduction.
• It works under mild conditions and gives high yields.
• Catalytic hydrogenation ($H_2$/Ni or $H_2$/Pd) can also work but may require high pressure and can sometimes stop at the imine stage.

Steps for the $LiAlH_4$ method:

1. Take the nitrile ($R-C \equiv N$) in anhydrous ether (or THF).
2. Add $LiAlH_4$ slowly (exothermic reaction). The hydride ($H^-$) attacks the electrophilic carbon of the nitrile.
3. Stir the mixture for several hours at room temperature or gentle reflux.
4. Quench carefully with water, then dilute acid (e.g., $H_2SO_4$ or $HCl$) to hydrolyse the aluminium complex. …

Here’s a breakdown of the common mistakes students make when converting a nitrile to a primary amine, along with how to avoid each.

The Core Concept

The standard, high-yield method to reduce a nitrile ($R-C \equiv N$) to a primary amine ($R-CH_2-NH_2$) is:

• Catalytic hydrogenation using $H_2$ in the presence of a catalyst (e.g., $H_2$ / Ni or $H_2$ / Pt).
• Chemical reduction using $LiAlH_4$ (lithium aluminium hydride) in dry ether, followed by hydrolysis.

The best reagent for exam purposes is often $LiAlH_4$ because it works cleanly without over-reduction or side reactions (unlike some catalytic methods).

Common Mistakes & How to Avoid Them

1. Choosing NaBH₄ instead of LiAlH₄

• The mistake: Students assume all hydride reducing agents work the same way.
• Why it’s wrong: $NaBH_4$ is too weak to reduce the nitrile triple bond. It only reduces aldehydes, ketones, and esters — not nitriles.
• How to avoid: Remember the reducing power hierarchy: $LiAlH_4$ > $NaBH_4$. For nitriles, always pick $LiAlH_4$ unless the question specifies a catalytic method.

2. Forgetting that catalytic hydrogenation stops at the imine intermediate

• The mistake: Writing $H_2$ / Pd alone and expecting a clean primary amine.
• Why it’s wrong: With $H_2$ / Pd, the reaction often stops at the imine ($R-CH=NH$) stage, giving a mixture.
• How to avoid: Use $H_2$ / Ni (Raney nickel) or $H_2$ / Pt — these go all the way to the amine. Better yet, use $LiAlH_4$ to avoid this confusion.

3. Confusing nitrile reduction with amide reduction

• The mistake: Using $H_2O$ / $H^+$ (acid hydrolysis) to get an amine.
• Why it’s wrong: Acid hydrolysis of a nitrile gives a carboxylic acid ($R-COOH$), not an amine.
• How to avoid:
  • Nitrile + $H_2O$ / $H^+$ → Carboxylic acid
  • Nitrile + $LiAlH_4$ → Primary amine Draw the mechanism once to lock this in.

4. Forgetting to specify “dry ether” for LiAlH₄

• The mistake: Writing $LiAlH_4$ without mentioning the solvent.
• Why it’s wrong: $LiAlH_4$ reacts violently with water and protic solvents. In exams, they often deduct marks for missing the solvent.
• How to avoid: Always write: $LiAlH_4$ / dry ether, then $H_2O$ (hydrolysis). …