Amines: Chemical and Physical Properties

Amines, in chemistry, are organic compounds that consist of and are generally based on single or multiple atoms of nitrogen. Structure-wise, amines look very similar to ammonia. Amines are derivatives of Ammonia wherein single or multiple hydrogen atoms can be easily replaced by substituent groups such as aryl or alkyl.

These notes cover all the essential topics of physical properties of amines class 12. Some examples of amines are trimethylamine, biogenic amine, aniline, and amino acids.

Here, you will learn about the physical properties of amines, their classification, and other related topics.

What are Amines?

Amines are an important class of organic compounds in chemistry. They are derivatives of ammonia and have a pungent smell.

Amines form when substituent groups such as aryl or alkyl replace single or multiple hydrogen atoms in ammonia. Amines contain a nitrogen atom along with a lone pair of electrons.

It is vital to understand their classification to understand amines’ physical properties, which we will discuss in the next part of the article.

Classification of Amines

We can generally divide amines into four different categories. These categories are:

Primary Amines

In primary amines, an aryl or alkyl replaces one atom of hydrogen. Methylamine (CH3NH2) is an example of primary amines.

Secondary Amines

Secondary amines are those where aryls or alkyls replace two atoms of hydrogen. Dimethylamine (CH3NHCH3) are examples of secondary amines.

Tertiary Amines

In tertiary amines, aryl or alkyl replace all three hydrogen atoms. Trimethylamine [N(CH2CH3)3] is an example of tertiary amines.

Cyclic Amines

It is important to note that only tertiary or secondary amines can be cyclic. An example of cyclic amines is a three-member ring aziridine.

Amines Physical Properties

Here are some important physical properties of amines that you must know:

• Lower aliphatic amines have a fishy odour and are in the gaseous state. Primary amines with 3-4 carbon atoms at room temperature are liquid. However, primary amines with more than four carbon atoms are solid at room temperature.

• Aryl amines such as aniline are mostly colourless. However, they acquire colour when kept open. It happens because of atmospheric oxidation.

• Lower aliphatic amines can form hydrogen bonds using water molecules. Therefore, amines are soluble in water. As the size of hydrophobic alkyl increases, there is a rise in the molar mass of amines, decreasing its solubility in water. Thus, higher amines are not soluble in water.

• In the case of tertiary amines, the intermolecular association is not present due to the lack of free hydrogen atoms essential for bonding.

• Organic solvents such as benzene, ether, and alcohol easily dissolve amines. Amines have lower polarity than alcohols, so they form strong intermolecular hydrogen bonds.

• Both primary and secondary amines are involved in intermolecular associations. It is because of the hydrogen bonding between the hydrogen and the nitrogen of another molecule.

• In the case of primary amines, the intermolecular association is more eminent than the secondary amines. It happens because of the presence of two hydrogen atoms.

• The order of boiling of amines is as follows: primary amines > secondary amines > tertiary amines.

Chemical Properties of Amines

Just like Ammonia, amines also act as the base. The basicity of amines depends on several factors, and it differs from molecule to molecule. Here are some factors on which the basicity of amines depends:

• The presence of lone pair of electrons from the nitrogen

• The degree of solvation of amines generally depends on the solvent used in the chemical reaction.

• The electronic properties of the substituent group, such as the alkyl group, increase the basicity, whereas the aril group reduces it.

• The nitrogen atom of amine has a lone electron pair that can bind the hydrogen ion to form an ammonium ion (R3NH+).

• Simple amines are soluble in water mainly because of hydrogen bonding between the lone electron pair of electrons and protons.

Physical Properties of Aromatic Amines

These amines have nitrogen atoms connected to the aromatic ring structure. Here are some physical properties of aromatic amines:

• They are colourless, toxic/poisonous solids or liquids with an unpleasant odour. They turn into a brown colour due to oxidation with air or light.

• The boiling and melting points of aromatic amines increase as their molecular mass increases. The boiling and melting point of aromatic amines is higher than the aliphatic amines.

• They are completely soluble in organic solvents and are only slightly soluble in water.

• Hydrogen bonding greatly affects the properties of both primary as well as secondary amines. Therefore, amines have a higher boiling point than phosphine but lower than corresponding alcohols.

For instance, ethylamine and methylamine are gases under normal conditions. However, the corresponding alcohols, ethyl alcohol and methyl alcohol, are liquids. Amines in the gaseous state have a typical ammonia odour, and amines in a liquid form have a fishy smell.

• The solubility of aromatic amines decreases as the carbon atoms increase.

Conclusion

Amines are an important concept in organic chemistry. They are organic compounds having functional groups with a nitrogen atom and a lone pair of electrons. Some examples of important amines are amino acids, trimethylamine, biogenic amine, and aniline.

This article covers physical properties of amines for class 12. It also touches upon the classification of amines; it is crucial to know the classification to better understand the properties of amines.