Physical Properties of Amines

Low-molecular-weight amines with fewer than five carbon atoms are water-soluble because the lone pair on nitrogen forms hydrogen bonds with water molecules. High-molecular-weight amines are sparingly or completely insoluble because their non-polar hydrocarbon chains are incompatible with polar solvents, making the hydrophobic effect dominant over hydrogen bonding interactions.

Primary amines with two N–H bonds and secondary amines with one N–H bond form intermolecular hydrogen bonds, resulting in higher boiling points than tertiary amines or compounds lacking N–H groups. However, these hydrogen bonds are weaker than those in alcohols because nitrogen is less electronegative than oxygen, so primary and secondary amines have lower boiling points than their alcohol analogs.

Tertiary amines lack N–H bonds, eliminating the possibility of intermolecular hydrogen bonding. Without this strong intermolecular interaction, tertiary amines rely only on weaker van der Waals forces, resulting in significantly lower boiling points compared to primary and secondary amines of similar molecular weight.

Simple amines have a typical fishlike smell, while diamines such as putrescine and cadaverine have pungent odors. These odors are characteristic enough that they influence the common names of such amines. The specific smell depends on the amine's structure and molecular composition.

Amines are polar compounds because the lone pair on the nitrogen atom governs the overall molecular dipole moment. This polarity enables amines to act as hydrogen bond acceptors in hydroxylic solvents and allows primary and secondary amines to act as hydrogen bond donors through their N–H bonds.

Arylamines such as benzidine and β-naphthylamine are carcinogenic and must be handled cautiously due to their physiological activity. Additionally, some natural amines like batrachotoxin from poison dart frogs are extremely toxic; even 200 µg can induce irreversible cardiac arrest in humans, demonstrating the severe toxicity of certain amine compounds.

Low-molecular-weight amines are typically gaseous at room temperature, while high-molecular-weight amines are liquid or solid in nature. This difference in physical state reflects how molecular weight influences intermolecular forces and volatility. Preparation of amines reductive amination of aldehydes and ketones involves controlling these properties during synthesis.