Reduction of Nitriles to Primary Amines (LiAlH4, Catalytic)
Chemistry · Amines · NEET
A nitrile (R-C≡N) is reduced to a PRIMARY amine (R-CH2-NH2) by LiAlH4, or by catalytic hydrogenation (H2/Ni or Pt), or by Na(Hg) with ethanol (Mendius reaction). Two molecules of hydrogen add across the C≡N triple bond. Memory hook: nitrile means "one extra carbon" — the amine you get has ONE MORE carbon than the halide you started from, and the amine is always primary (-NH2).
R-C≡N adds two molecules of hydrogen (LiAlH4, H2/Ni, or Na(Hg)/ethanol) to give the primary amine R-CH2-NH2, gaining one carbon. Warning boxes: isocyanide (R-NC) gives a secondary amine, and partial reducers (SnCl2/HCl, DIBAL-H) stop at the aldehyde.
Your doubts, answered
Does reducing a nitrile give a primary or secondary amine?
Always a PRIMARY amine (-NH2). In R-C≡N the nitrogen already carries no carbon-based substituent except the one it is triple-bonded to. When you add 2 H2 across the triple bond you get R-CH2-NH2. The nitrogen ends up with two hydrogens, so it is primary. NEET loves this: nitrile reduction = primary amine, no exceptions.
Which reagents reduce a nitrile to an amine?
Three you must know: (1) LiAlH4 (lithium aluminium hydride), (2) catalytic hydrogenation H2 with Ni, Pt or Pd, and (3) Na(Hg) with ethanol, called the Mendius reaction. All three give R-CH2-NH2. Note: Sn/HCl and Br2/NaOH do NOT reduce nitriles to amines.
Does the number of carbons change?
Yes. The carbon of the C≡N group stays in the product as the CH2. So the amine has ONE MORE carbon than the alkyl halide you began with (R-X → R-CN → R-CH2-NH2). This is the 'ascent of the amine series' idea — nitriles are the standard way to step UP one carbon.
How is reduction of a nitrile different from reduction of an isocyanide?
This is the classic trap. Nitrile R-C≡N reduces to a PRIMARY amine R-CH2-NH2. Isocyanide (isonitrile) R-N≡C reduces to a SECONDARY amine R-NH-CH3, because in an isocyanide the carbon is attached to N, so the N ends up bonded to two carbons. Same reagent (LiAlH4), different amine class.
When does a nitrile give an aldehyde instead of an amine?
With partial/selective reducing agents. SnCl2/HCl (Stephen reaction) or DIBAL-H reduce the nitrile only to an imine, which on hydrolysis gives an ALDEHYDE, not an amine. Full reducing agents like LiAlH4 or H2/Ni go all the way to the primary amine. Read the reagent carefully.
⚠️ The NEET trap ✗ Reduction of an isocyanide (R-NC) with LiAlH4 gives a primary amine, just like a nitrile. ✓ Isocyanide (R-NC) gives a SECONDARY amine (R-NH-CH3) because the nitrogen is bonded to two carbons. Only the nitrile (R-CN) gives a PRIMARY amine (R-CH2-NH2). 🧠 CN = primary, NC = secondary. Read the order of letters: in R-C≡N the extra atom is Carbon (becomes CH2, N free with 2H). Flip it to R-N≡C and N is trapped between two carbons.
Solved Amines NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Ethylamine, CH3CH2NH2. The C≡N becomes CH2-NH2, giving a primary amine with one more carbon than methyl started with.
Is nitrile reduction an example of ascent or descent of the amine series?
Ascent. The nitrile route (R-X → R-CN → R-CH2-NH2) adds one carbon, so it increases the chain length. Descent (losing a carbon) is done by Hoffmann bromamide degradation.
Do Na(Hg)/ethanol and H2/Ni give the same product as LiAlH4 for a nitrile?
Yes. All three fully reduce R-C≡N to the same primary amine R-CH2-NH2. Na(Hg)/C2H5OH is called the Mendius reaction.
Why does the nitrile need two molecules of hydrogen?
The C≡N triple bond must be reduced in two stages: C≡N to C=N (imine) and then C=N to CH-NH2. Each step adds one H2, so two H2 in total are added across the triple bond.
