Physical Properties of Amines: Amine Structure, Classification, Examples

An important class of organic compounds are the Amines. The study of Amines is very interesting in many ways. Amine chemistry is very important in biology. It plays a vital role in all life forms. Here we will study the Amine structure and their physical properties.

The Amine Structure

Amines contain a nitrogen atom with a lone pair of electrons. They are basically the derivatives of ammonia. If we replace one or more hydrogen atoms by substituent groups such as alkyl or aryl in Ammonia, we get a new class of organic compounds called Amines. Examples of significant amine structure include Amino acids, biogenic amine, trimethylamine, and aniline.

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Classification of Amines

Amines are classified into four categories namely, primary, secondary, tertiary and cyclic. A primary amine is one when you can replace one of the three hydrogen atoms by an alkyl or aryl group. When you can replace two out of three hydrogen atoms, we get secondary amine.

Similarly, when all the three hydrogen atoms are replaced tertiary amines are formed. Only secondary or tertiary amines can be cyclic. The 3 – member ring aziridine is an example of cyclic amine. Let us now look at the physical properties of amines in brief.

Physical Properties of Amines

The lower aliphatic amines are gaseous in nature. They have a 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. However, they get coloured when we store them in open due to atmospheric oxidation.
Lower aliphatic amines can form hydrogen bonds with water molecules. Therefore, such amines are soluble in water.
Increase in the size of hydrophobic alkyl part increases the molar mass of amines. This usually 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 the 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 the absence of free hydrogen atom for bonding.
The order of boiling point of amines is as follows: Primary > Secondary > Tertiary.

Solved Examples for You

Question 1: Primary amines are less soluble than tertiary amines.

True
False

Solution: Option A – True. Primary amines are less soluble than tertiary amines as primary amines can form hydrogen bonds with water but tertiary amines cannot.

Question 2: Arrange the following in the decreasing order of boiling point:
i) $C_{2} H_{5} O H$ ii) $(C H_{3})_{2} N H$ iii) $C_{2} H_{5} N H_{2}$

i > ii > iii
ii > i > iii
iii > i > ii
iii > ii > i

Solution: Option A – i > ii > iii. All the given compounds have intermolecular hydrogen bonding due to which their boiling points are higher. C₂H₅OH possesses the strongest hydrogen bonding because the O is more electron negative than N in other compounds. Therefore it has the highest boiling point. In (CH₃)₂NH and C₂H₅NH₂, the latter has longer alkyl chain which makes hydrogen bonding weaker. Hence the order of boiling points is i > ii > iii.

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