Primary and secondary amines have hydrogen atoms bonded to an nitrogen atom and are therefore capable of intermolecular hydrogen bonding as shown in Part (a) of Figure 15.6 “Hydrogen Bonding.  The hydrogen bonding between amine molecules is not as strong as hydrogen bonding between alcohol molecules because nitrogen is less electronegative than oxygen, resulting in smaller partial charges.   Thus primary and secondary amines boil at higher temperatures than alkanes but at lower temperatures than alcohols of similar molar masses. For example, compare the boiling point of methylamine (CH₃NH₂; −6°C) with those of ethane (CH₃CH₃; −89°C) and methanol (CH₃OH; 65°C).

Tertiary amines have no hydrogen atom bonded to the nitrogen atom and so cannot participate in intermolecular hydrogen bonding. They have boiling points comparable to those of ethers, reflecting dipole/dipole intermolecular attractions. (Table 15.5 “Physical Properties of Some Amines and Comparable Oxygen-Containing Compounds”).

Figure 15.6 Hydrogen Bonding. (a) Amine molecules are associated through hydrogen bonding. (b) An amine molecule can form a hydrogen bond with water molecules.  Note color scheme:  pink = C, blue = H, orange = O, purple = N.

 

All three classes of amines can engage in hydrogen bonding with water as shown in part (b) of Figure 15.6 “Hydrogen Bonding”. Amines of low molar mass are quite soluble in water; the borderline of solubility in water is at five or six carbon atoms per nitrogen atom.

To Your Health: Amines in Death and Life

Amines have “interesting” odors. The simple ones smell very much like ammonia. Higher aliphatic amines smell like decaying fish. Or perhaps we should put it the other way around: Decaying fish give off odorous amines. The stench of rotting fish is due in part to two diamines: putrescine and cadaverine. They arise from the decarboxylation of ornithine and lysine, respectively, amino acids that are found in animal cells. (For more information about lysine, see Chapter 18 “Amino Acids, Proteins, and Enzymes”, Section 18.1 “Properties of Amino Acids”.)

Aromatic amines generally are quite toxic. They are readily absorbed through the skin, and workers must exercise caution when handling these compounds. Several aromatic amines, including β-naphthylamine, are potent carcinogens.