Concept 02 / 04Amines
Physical Properties and Basic Character of Amines
Physical properties of amines (hydrogen bonding, boiling points, solubility), basicity of amines and its measurement by Kb and pKb, order of basicity of aliphatic amines in the gas phase and in water (inductive effect, solvation, steric hindrance), and the much weaker basicity of aromatic amines with substituent effects.
Physical properties. Lower aliphatic amines are gases with a fishy odour; higher ones are liquids or solids. Aniline is a colourless liquid that darkens on storage (atmospheric oxidation). Primary and secondary amines form intermolecular hydrogen bonds (N-H...N), tertiary amines cannot; so boiling points follow for isomers (butan-1-amine , N-ethylethanamine , N,N-dimethylethanamine ). Amines boil lower than alcohols of similar mass, because N is less electronegative than O and N-H...N bonds are weaker. Lower amines are water-soluble (hydrogen bonds with water); solubility falls as the hydrophobic part grows.
Basicity. The lone pair on nitrogen accepts a proton: , with and . A smaller means a stronger base (see figure). Amines form salts with acids (), which are water-soluble and regenerate the amine with NaOH.
Aliphatic amines. Alkyl groups release electrons (), making the lone pair more available and stabilising the ammonium ion, so aliphatic amines are stronger bases than ammonia.
In the gas phase (only the inductive effect): .
In water, two more factors matter: solvation of the ammonium ion by hydrogen bonding (more N-H bonds, better stabilisation: ) and steric hindrance to solvation and protonation by bulky groups. The balance gives:
methyl series: ;
ethyl series: .
Aromatic amines. Aniline () is a much weaker base than ammonia (). Its lone pair is delocalised into the ring by resonance (five resonance structures for aniline, only two Kekule forms for the anilinium ion, which has no lone pair to delocalise), so protonation costs resonance stabilisation. The ring carbon also withdraws electrons.
Substituents: electron-releasing groups (, , ) at ortho or para increase basicity (p-toluidine ); electron-withdrawing groups (, , , ) decrease it strongly (p-nitroaniline about ). In benzylamine (), the group isolates the nitrogen from the ring, so it behaves like an aliphatic amine. N-alkylation has smaller effects in anilines (N-methylaniline , N,N-dimethylaniline ).
Key Takeaways:
- Boiling points: amines (hydrogen bonding); lower than alcohols.
- Gas phase basicity: .
- In water: ; .
- Aniline is weaker than (resonance); weakens, and strengthen.
- Pitfall: tertiary amines are not the strongest bases in water, despite having the most alkyl groups.
Brain-Teaser 1: Arrange in decreasing basic strength in water: , , , .
Solution: .
Brain-Teaser 2: Arrange aniline, p-nitroaniline and p-toluidine in increasing basicity, and explain.
Solution: p-Nitroaniline < aniline < p-toluidine. The nitro group withdraws the lone pair further into the ring (), making it even less available; the methyl group donates electrons (, hyperconjugation), making the lone pair more available.
Brain-Teaser 3: Why are amines less soluble in water than alcohols of similar molar mass, and why does aniline dissolve in dilute HCl?
Solution: Nitrogen is less electronegative than oxygen, so amine N-H bonds form weaker hydrogen bonds with water than alcohol O-H bonds do. Aniline, though only slightly soluble in water, reacts with HCl to form anilinium chloride, an ionic salt that is water-soluble.
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