26.2 Amines – Physical Properties

Properties of Amines

Primary and secondary amines have hydrogen atoms bonded to an nitrogen atom and are therefore capable of hydrogen bonding (part (a) of Figure 26.2a.), although not as strongly as alcohol molecules (which have hydrogen atoms bonded to an oxygen atom, which is more electronegative than nitrogen). These amines boil at higher temperatures than alkanes but at lower temperatures than alcohols of comparable molar mass. 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 (Table 26.2a.).

Source: “15.11: Physical Properties of Amines” In Basics of GOB Chemistry (Ball et al.), CC BY-NC-SA 4.0.

All three classes of amines can engage in hydrogen bonding with water (Figure 26.2a.). Amines of low molar mass are quite soluble in water; the borderline of solubility in water is at five or six carbon atoms (Table 26.2a.).

Spotlight on Everyday Chemistry: Hazards with Amines

Amines have “interesting” odours. 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.

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 (Figure 26.2b.), are potent carcinogens.

 

 

Cigarettes and cigarette smoke also have amine-based compounds.  Read more in Infographic 26.2a.