Amines are organic compounds having functional groups which contain a nitrogen atom with lone pair of electrons. They are the derivatives of ammonia where one or more hydrogen atom can be replaced by substituent groups such as alkyl or aryl. Amino acids, biogenic amine, trimethylamine and aniline are examples of some significant amines. Amines are classified into four categories namely, primary, secondary, tertiary and cyclic. When one of the three hydrogen atoms is replaced by an alkyl or aryl group, the amine is primary. In case of replacing two out of three hydrogen atoms we get secondary amine and when all the three hydrogen atoms are replaced tertiary amines are formed. Only secondary or tertiary amines can be cyclic, 3 – member ring aziridine is an example of cyclic amine.
Physical properties of amines
The lower aliphatic amines are gaseous in nature with fishy smell. Primary amines with three or four carbon atoms are liquids at room temperature whereas higher ones are solids. Aniline and other arylamines are generally colourless but they get coloured if stored in open due to atmospheric oxidation. Lower aliphatic amines can form hydrogen bonds with water molecules hence they are soluble in water. Increase in size of hydrophobic alkyl part increases the molar mass of amines which results in a decrease in its solubility in water. Higher amines are insoluble in water. Organic solvents like alcohol, benzene and ether readily dissolve amines. Alcohols have higher polarity as compared to amines and hence they form stronger intermolecular hydrogen bonds.
Primary and secondary amines are often engaged in the intermolecular association as a result of hydrogen bonding between nitrogen of one and hydrogen of the other molecule. The intermolecular association is more prominent in case of primary amines as compared to secondary due to the availability of two hydrogen atoms. In tertiary amines there is no intermolecular association due to absence of free hydrogen atom for bonding. The order of boiling point of amines is as follows: Primary > Secondary > Tertiary.
This article covers the basic details of physical properties of amines. For any further query on this topic install Byju’s the learning app.
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