19.6:
Physical Properties of Amines
Amines are polar compounds with a lone pair on the nitrogen atom that governs the overall molecular dipole moment. They act as hydrogen bond acceptors in hydroxylic solvents.
Typically, low-molecular-weight amines—with fewer than five carbon atoms—are miscible in water or water-soluble. In comparison, high-molecular-weight amines are either sparingly or completely insoluble in water since the non-polar hydrocarbon chains are incompatible with polar solvents.
Both primary and secondary amines are hydrogen-bond donors and form intermolecular hydrogen bonds. Hydrogen bonding directly influences the observed trends in the boiling points.
Thus, primary amines—with two N–H bonds—have higher boiling points than isomeric secondary and tertiary amines.
Intermolecular hydrogen bonds in amines are comparatively weaker than the ones in alcohols.
So, primary and secondary amines have lower boiling points than their alcohol analogs but higher boiling points than the compounds incapable of hydrogen bonding.
Amines have characteristic odors. Simple amines have a typical fishlike smell, while some diamines have a pungent odor; the odor influences the common names of such amines.
19.6:
Physical Properties of Amines
Amines with low molecular weight are usually gaseous at room temperature, while those with high molecular weight are liquid or solids in nature. Usually, low molecular weight amines have a rotten fish-like smell. Diamines typically have a pungent smell. For instance, cadaverine and putrescine, depicted in Figure 1, are two molecules responsible for decaying tissue.
Figure 1. The chemical structures of putrescine and cadaverine.
Amines are polar. Accordingly, primary and secondary amines with N–H bonds enable intermolecular hydrogen bonding interactions. This interaction results in a higher boiling point of amines than alkanes or ethers of comparable molecular weight, which only have weaker interactions such as van der Waals forces or dipole–dipole interactions. On the other hand, alcohols have higher boiling points than amines because nitrogen is less electronegative than oxygen, resulting in weaker intermolecular hydrogen bonding between nitrogen and hydrogen than oxygen and hydrogen. This scheme is captured in Figure 2.
Figure 2. The influence of the functional group on the boiling points of different compounds.
As illustrated in Figure 3, the tertiary amines have lower boiling points than primary or secondary amines. As opposed to primary or secondary amines, the absence of the N–H bond in tertiary amines eliminates the chances of hydrogen bonding.
Figure 3. The influence of hydrogen bonding on the boiling point.
Low molecular weight amines are also soluble in water—the hydrogen bonding between amines and water, as shown in Figure 4, results in this solubility. The water solubility of amines decreases as the molecular weight increases. Generally, amines with fewer than five carbon atoms are water-soluble. Due to the interaction between the electron pair of the nitrogen atom and the hydrogen atom of water, even tertiary amines are soluble in water.
Figure 4. The hydrogen bond between an amine and water.
In general, amines are significantly toxic and can induce poisoning or death if ingested because of their physiological activity. For example, arylamines such as benzidine and β-naphthylamine, presented in Figure 5, are carcinogenic and should be handled cautiously.
Figure 5. Well-known carcinogenic arylamines.
Moreover, skin secretions of the poison dart frog Phyllobates terribilis from South America have been observed to consist of two toxic amine compounds: batrachotoxin and batrachotoxinin A. These amines are so poisonous that even an amount as low as 200 µg is enough to induce irreversible cardiac arrest in human beings.