Physical properties Amine

As one might readily guess, the inclusion of a heteroatom such as nitrogen in otherwise exclusively carbon and hydrogen molecules has quite an effect on the properties of amines as compared to alkanes.

General properties

Hydrogen bonding significantly influences the properties of primary and secondary amines as well as the protonated derivatives of all amines. Thus the boiling point of amines is higher than those for the corresponding phosphines (compounds containing phosphorus), but generally lower than the corresponding alcohols. Alcohols, or alkanols, resemble amines but feature an -OH group in place of NR₂. Since oxygen is more electronegative than nitrogen, RO-H is typically more acidic than the related R₂N-H compound.

Methyl, dimethyl, trimethyl, and ethyl amines are gases under standard conditions. Most common alkyl amines are liquids, and high molecular weight amines are, quite naturally, solids at standard temperatures. Additionally, gaseous amines possess a characteristic ammonia smell, while liquid amines have a distinctive “fishy” smell.

Most aliphatic amines display some solubility in water, reflecting their ability to form hydrogen bonds. Solubility decreases relatively proportionally with the increase in the number of carbon atoms in the molecule – especially when the carbon atom number is greater than six. Aliphatic amines also display significant solubility in organic solvents, especially in polar organic solvents. Primary amines react readily with ketone compounds (such as acetone), however, and most amines are incompatible with chloroform and also with carbon tetrachloride as solvent solutions.

Aromatic amines have their lone pair electrons conjugated (“shared”) into the benzene ring, so their tendency to engage in hydrogen bonding is somewhat diminished. The boiling points of these molecules are therefore usually somewhat higher than other, smaller amines due to their typically larger size.
They also often have relatively diminished solubility in water, although they retain their solubility in other organic solvents.

Aromatically conjugated amines are often quite toxic and have the potential to be easily absorbed through the skin, so should always be treated as “hazardous”.