Physical Properties of Amines 🚀

1 · Physical State & Odour 🐟

  • Lower aliphatic amines are pungent, fish-smelling gases.
  • Primary amines with ≥ 3 carbon atoms become liquids, and still larger ones form solids. :contentReference[oaicite:0]{index=0}
  • Aniline and most arylamines start out colourless but pick up colour on standing as air slowly oxidises them. :contentReference[oaicite:1]{index=1}

2 · Solubility 💧

  • Small-chain amines dissolve well in water by forming hydrogen bonds with H2O.
  • Solubility falls as the alkyl chain length grows because the hydrophobic part gets bigger.
  • High-molar-mass amines are practically insoluble in water, yet they mix freely with organic solvents such as alcohol, ether, and benzene. :contentReference[oaicite:2]{index=2}
  • Predict the trend by noting the electronegativity values: \( \chi_{\text{N}} = 3.0 \) vs. \( \chi_{\text{O}} = 3.5 \). Alcohols are more polar than amines and create stronger H-bonds, so an alcohol like butan-1-ol dissolves better than butan-1-amine. :contentReference[oaicite:3]{index=3}

3 · Intermolecular Hydrogen Bonding 🤝

  • Primary and secondary amines stick together through hydrogen bonds:
    \( \text{R–NH}_2 \;\cdots\; \text{H–NHR} \)
  • Primary amines bond more strongly than secondary ones (they have two N-H hydrogens instead of one).
  • Tertiary amines cannot H-bond to each other because they lack N-H hydrogens. :contentReference[oaicite:4]{index=4}

4 · Boiling-Point Trends 🔥

Among isomers of the same molar mass:

Primary > Secondary > Tertiary

The extra H-bonding in primary amines pushes their boiling points highest. :contentReference[oaicite:5]{index=5}

Boiling Points of Similar-Mass Compounds (Table 9.2)
CompoundMolar MassBoiling Point / K
n-C4H9NH273350.8
(C2H5)2NH73329.3
C2H5N(CH3)273310.5
C2H5CH(CH3)272300.8
n-C4H9OH74390.3

Notice how the alcohol (n-butan-1-ol) outruns every amine because of even stronger H-bonding. :contentReference[oaicite:6]{index=6}

Important Concepts for NEET 🎯

  1. Order of boiling points: Primary > Secondary > Tertiary amines.
  2. Solubility trend with chain length and comparison with alcohols.
  3. Physical state shift (gas → liquid → solid) as carbon count rises.
  4. Role of intermolecular H-bonding in physical properties.
  5. Contrast of boiling points among amine, alcohol, and alkane of the same molar mass.