Concept 01 / 04Amines
Amines: Structure, Classification, Nomenclature and Preparation
Structure of amines (pyramidal nitrogen), primary, secondary and tertiary amines, common and IUPAC names, and preparation by reduction of nitro compounds, ammonolysis of alkyl halides, reduction of nitriles and amides, Gabriel phthalimide synthesis and Hofmann bromamide degradation.
Amines are derivatives of ammonia in which one, two or three hydrogens are replaced by alkyl or aryl groups: primary (), secondary (), tertiary (). Quaternary ammonium salts () have four groups on nitrogen.
Structure. Nitrogen is hybridised with a lone pair in one hybrid orbital, so amines are pyramidal. The C-N-C angle in trimethylamine is . In aniline, the lone pair is partly delocalised into the ring, so the nitrogen is somewhat flattened.
Nomenclature. Common names: alkylamine (methylamine, dimethylamine, triethylamine). IUPAC: the -e of the alkane is replaced by -amine (ethanamine, propan-2-amine); substituents on nitrogen are given the locant N (N-methylethanamine, N,N-dimethylmethanamine). Arylamines: aniline (benzenamine), N-methylaniline, p-toluidine (4-methylaniline). Diamines: ethane-1,2-diamine, hexane-1,6-diamine.
Preparation (see figure).
Reduction of nitro compounds: , or catalytic hydrogenation (/Pd, Pt or Ni). Fe with HCl is preferred industrially: the formed is hydrolysed, regenerating HCl, so little acid is needed.
Ammonolysis of alkyl halides (SN2): , then base frees the amine. The primary amine can react further, giving a mixture of primary, secondary and tertiary amines and the quaternary salt: . Excess ammonia favours the primary amine. Reactivity of halides: RI > RBr > RCl.
Reduction of nitriles: (ascends the series by one carbon, starting from RX + KCN).
Reduction of amides: .
Gabriel phthalimide synthesis: phthalimide is converted by ethanolic KOH into potassium phthalimide, which undergoes SN2 with a primary alkyl halide to give N-alkylphthalimide; alkaline hydrolysis releases the pure primary amine. It cannot make aromatic amines, since aryl halides do not undergo nucleophilic substitution with phthalimide anion.
Hofmann bromamide degradation: an amide treated with bromine in aqueous or ethanolic NaOH gives a primary amine with one carbon fewer (the carbonyl carbon is lost as carbonate): . The alkyl or aryl group migrates from carbon to nitrogen (rearrangement via an isocyanate).
Key Takeaways:
- Amines are pyramidal; classified , , by the number of groups on N.
- Nitro reduction (Fe/HCl), nitrile and amide reduction ().
- Ammonolysis gives mixtures; Gabriel gives pure primary aliphatic amines.
- Hofmann bromamide: amide → amine with one carbon fewer.
- Pitfall: the Gabriel synthesis cannot be used to make aniline.
Brain-Teaser 1: Starting from ethanoic acid, how would you prepare (a) methanamine and (b) ethanamine?
Solution: (a) Convert the acid to ethanamide (via ammonium salt and heat), then Hofmann bromamide degradation (/NaOH) gives (one carbon fewer). (b) Reduce ethanamide with to (same carbon count).
Brain-Teaser 2: How would you convert bromoethane into propan-1-amine?
Solution: Treat with KCN to get propanenitrile (, one carbon added), then reduce with or /Ni to .
Brain-Teaser 3: What mass of bromine is needed to convert of ethanamide completely into methanamine?
Solution: Ethanamide (): . One mole of is needed per mole of amide: of bromine.
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