Topic 27 of 34
Track every carbon when preparing an amine
Nitrile and amide reduction keep the original carbonyl or nitrile carbon in the product chain.
A-Level 9476 (2026-2027)
Track every carbon when preparing an amine
Nitrile and amide reduction keep the original carbonyl or nitrile carbon in the product chain.
| Target | Starting compound and conditions | Product accounting |
|---|---|---|
| Ethylamine from an amide | Ethanamide, CH3CONH2; LiAlH4 in dry ether, then aqueous work-up. | CH3CONH2 + 4[H] → CH3CH2NH2 + H2O. Both carbons remain. |
| Ethylamine from a nitrile | Ethanenitrile, CH3CN; LiAlH4 in dry ether then work-up, or H2/Ni. | CH3CN + 4[H] → CH3CH2NH2. The nitrile carbon becomes CH2 next to nitrogen. |
| Phenylamine from nitrobenzene | Sn and concentrated HCl with heating, then aqueous NaOH. | The acidic reduction mixture contains phenylammonium salt; NaOH liberates C6H5NH2. |
For nitrobenzene reduction, the formal redox change is C6H5NO2 + 6[H] → C6H5NH2 + 2H2O. In the acidic reaction medium the amine is protonated, so the later NaOH step is chemically necessary when the target is the free amine. Reduction changes NO2 into NH2; it does not put nitrogen into the aromatic ring.
An amine lone pair accepts H+ to form a salt: C2H5NH2 + HCl → C2H5NH3+Cl-. Phenylamine similarly gives phenylammonium salts. Adding aqueous base reverses protonation. This change between neutral amine and charged salt can help separate an amine from neutral organic impurities.
Worked example
Choose the carbon count before the reagent
To make propylamine by nitrile reduction, should the starting nitrile be ethanenitrile or propanenitrile?
- Nitrile reduction preserves the nitrile carbon as a CH2 carbon.
- Propylamine has three carbons, so the nitrile must also have three.
- Propanenitrile is CH3CH2CN, not CH3CN.
Use propanenitrile, CH3CH2CN. Its reduction gives CH3CH2CH2NH2.