Amine Physical Properties

Amines have physical properties sitting between alcohols (strong H-bonding) and ethers (no H-bonding). The N-H bond is polar but not as polar as O-H, so hydrogen bonding is moderate. This places amine boiling points between those of comparable alcohols and alkanes.

Boiling Points

For MW ~30:

Methylamine boils about 70°C higher than methane but 70°C lower than methanol - an intermediate H-bonding situation.

Tertiary Amines Cannot Donate H-Bonds

Tertiary amines (R₃N) have no N-H bonds, so they cannot DONATE hydrogen bonds. They can still accept H-bonds (via the nitrogen lone pair), but the overall intermolecular forces are weaker than with primary or secondary amines:

• Trimethylamine ((CH₃)₃N): b.p. 3°C.
• Triethylamine ((CH₃CH₂)₃N): b.p. 89°C.

Compare dimethylamine ((CH₃)₂NH, MW 45, b.p. 7°C) with trimethylamine ((CH₃)₃N, MW 59, b.p. 3°C). Despite a higher MW, trimethylamine boils lower because it cannot H-bond with itself.

Water Solubility

Small amines (1-4 carbons) are miscible with water or highly soluble. N lone pair and N-H bonds H-bond to water molecules. Larger amines (>5 carbons) have diminishing water solubility as the hydrophobic chain grows, following the same pattern as alcohols.

Protonated amines (in acidic solution, RNH₃⁺) are always water-soluble due to their positive charge. This is why “amine hydrochloride” salts (RNH₃⁺Cl⁻) are used in pharmaceuticals - they are dramatically more water-soluble than the free amine.

Odor

Amines have distinctive odors, often unpleasant:

• Trimethylamine: rotten fish smell (found in spoiling seafood, vaginal secretions in bacterial vaginosis).
• Putrescine (1,4-butanediamine): decomposing meat.
• Cadaverine (1,5-pentanediamine): rotting flesh.

These are the compounds that give decomposition its characteristic smell - all amines.