Nitriles (–C≡N) often appear in synthetic pathways because they are a convenient way to add an extra carbon atom to a molecule—usually through the carbon–carbon bond-forming methods discussed in the previous article. Once introduced, the nitrile group can then be transformed into other useful functional groups. At A level, there are two key reactions to understand: reduction and acid hydrolysis.

1) Reduction of Nitrile Group to Form Primary Amine

Reagents/conditions: H₂/Ni (catalytic hydrogenation)
Outcome: The nitrile is reduced to a primary amine, converting –C≡N into –CH₂NH₂.

Important note: Under these conditions, alkenes will also be reduced (C=C → C–C), so hydrogenation of a nitrile is not selective if other unsaturated groups are present.

Balanced equation (general):
R–C≡N + 2 H₂ → R–CH₂NH₂

Example (ethanenitrile → ethylamine):
CH₃–C≡N + 2 H₂ → CH₃–CH₂NH₂

2) Acid Hydrolysis of Nitrile Group to Form Carboxylic Acid

Reagents/conditions: H⁺(aq), heat (e.g. dilute HCl or H₂SO₄)
Outcome: The nitrile is converted to a carboxylic acid, and the nitrogen becomes ammonium.

Balanced equation (ionic form):
R–C≡N + 2 H₂O + H⁺ → R–COOH + NH₄⁺

Alternative (using hydrochloric acid):
R–C≡N + 2 H₂O + HCl → R–COOH + NH₄Cl

Example (ethanenitrile → ethanoic acid):
CH₃–C≡N + 2 H₂O + H⁺ → CH₃–COOH + NH₄⁺

3. Exam Tips

• If a nitrile is formed earlier in a synthetic route, exam questions will often extend it to a carboxylic acid or primary amine.
• Watch for C=C bonds when using H₂/Ni — they will also hydrogenate.
• To avoid missing a carbon when drawing the structure, you can draw the triple bond. You can also number the carbon atoms.

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