Physical Properties

Full Notes:

Amines exhibit a variety of physical properties that depend on the type of amine (1°, 2°, 3°), molecular weight, and the presence of hydrogen bonding. These properties influence their solubility, boiling point, and general appearance.

Physical State and Odour

Shorter chain aliphatic amines (e.g., methylamine, ethylamine) are colourless gases at room temperature.

These have a characteristic ammonia-like or fishy odour.

As molecular mass increases:

• Amines become liquids and eventually solids.
• Aromatic amines, like aniline, are colourless liquids but gradually darken on exposure to air due to oxidation.

Solubility

Shorter chain amines are soluble in water due to hydrogen bonding with water molecules.

Solubility decreases with increasing size of the hydrocarbon part (hydrophobic effect).

All types of amines are generally soluble in organic solvents such as alcohol, ether, and benzene.

Note: Hydrogen bonding between amines and water is weaker than that in alcohols because nitrogen is less electronegative than oxygen.

Boiling Points and Hydrogen Bonding

Primary and secondary amines can form intermolecular hydrogen bonds due to the presence of N–H bonds.

Tertiary amines lack hydrogen atoms attached to nitrogen = no intermolecular H-bonding.

As a result:

• Boiling points: Primary > Secondary > Tertiary, for comparable molar masses.
• Boiling points of amines are higher than hydrocarbons and lower than alcohols of similar molecular mass.

Summary

• Lower aliphatic amines are gaseous and fishy while higher members are liquids or solids.
• Primary and secondary amines hydrogen bond which raises their boiling points.
• Tertiary amines do not hydrogen bond so they boil lower.
• Water solubility is good for small amines and decreases with chain length.
• Amines dissolve well in common organic solvents.