Amine | Chemical Compound - Encyclopedia Britannica

Physical properties

Distinctive odours that are fishy to putrid characterize the lower-molecular-weight amines. They are gases at room temperature or are easily vaporized liquids. Aliphatic amines are less dense than water, having densities in the range 0.63 to 0.74 gram per cubic cm; aromatic amines are typically slightly heavier than water (e.g., the density of aniline is 1.02 grams per cubic cm). With increasing size, they become less volatile; the odour decreases and eventually becomes unnoticeable, although some diamines have offensive odours. For example, H2N(CH2)4NH2, called putrescine, and H2N(CH2)5NH2, called cadaverine, are foul-smelling compounds found in decaying flesh. Amines are colourless; aliphatic amines are transparent to ultraviolet light, but aromatic amines display strong absorption of certain wavelengths. Amines with fewer than six carbons mix with water in all proportions. The aliphatic amines are stronger bases than ammonia, and the aromatic ones are substantially weaker. The basicity is measured in terms of the pKb value, which is equal to −logKb, where Kb is the equilibrium constant for the reaction B + H3O+ ⇌ BH+ + H2O (B is the amine). Thus, a larger pKb value indicates greater strength as a base. For comparison, the pKb of ammonia is 4.75. Quaternary ammonium hydroxides are very strong bases, but the chlorides are not basic.

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SUBSCRIBE Properties of some amines

substituent group, R	RNH2		R2NH		R3N
	boiling point (°C)	pKb	boiling point (°C)	pKb	boiling point (°C)	pKb
*Melting point −6.24 °C.
**Melting point 52.8 °C.
***Melting point 127 °C.
CH3	−6	3.38	7	3.36	3.5	4.24
C2H5	16.5	3.37	55.5	3.02	89.4	3.35
n-C4H9	77.8	3.41	161	2.75	214	3.11
sec-C4H9	66	3.44	132	2.99
tert-C4H9	45	3.55	95
C6H5	184.4*	4.59	302**	0.85	365***	<0

Most aliphatic amines are not highly toxic, and many are harmless, natural components of foods and pharmaceuticals. In high concentrations, however, the smaller amines are highly irritating to the skin and especially to the mucous membranes of the eyes, nose, throat, and lungs, to which they can cause acute damage upon prolonged exposure. Larger amines (12 or more carbon atoms) are usually less irritating. Aromatic amines are also irritants and can be absorbed through the skin. They may be dangerous poisons. Aniline, for example, destroys the hemoglobin of blood, and prolonged exposure has been linked with cancer. Amines can be corrosive to some metals and plastics; they should therefore be stored in glass or steel containers.