Classification of Amines: Aliphatic, Aromatic, Primary, Secondary, Tertiary Amines

Classification of Amines: The chemical formula of ammonia is \({\rm{N}}{{\rm{H}}_3}.\) What do you get when you replace one or more hydrogen from ammonia? Do you get a class of organic compound amine? What is the name of the compound if hydrogen of ammonia is replaced by alkyl or aryl group? You will find answers to all these questions in the topic Classification of amines. In this article, you will understand the meaning of the amine, its structure, classification into primary, secondary and tertiary amines, aliphatic and aromatic amines, uses of amines, etc., in detail with suitable examples.

What are Amines?

Amines are the important organic compounds found widely throughout plants and animals as proteins, vitamins, alkaloids, hormones, etc. Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by an alkyl or aryl group.

\({\rm{N}}{{\rm{H}}_3} + {\rm{R}} \to {\rm{RN}}{{\rm{H}}_2} + {\rm{H}}\)
\({\rm{RN}}{{\rm{H}}_2} + {\rm{R}} \to {{\rm{R}}_2}{\rm{NH}} + {\rm{H}}\)
\({{\rm{R}}_2}{\rm{NH}} + {\rm{R}} \to {{\rm{R}}_3}{\rm{N}} + {\rm{H}}\)

Structure of Amines

Amines are derivatives of ammonia in which one or more hydrogen atoms are replaced by alkyl or aryl groups. Therefore, the structure of amine is similar to ammonia.

In amines, the nitrogen atom is \({\rm{s}}{{\rm{p}}^3}\) hybridised. There are four \({\rm{s}}{{\rm{p}}^3}\) hybrid orbitals in which three are singly occupied, whereas the fourth one accommodates the lone pair of electrons. All these \({\rm{s}}{{\rm{p}}^3}\) hybrid orbitals are distributed tetrahedrally.

The nitrogen atom of the amino \(\left( { – {\rm{N}}{{\rm{H}}_2}} \right)\) group is bonded to the carbon atom of the alkyl group and two hydrogen atoms as follows.

• 1. N atom of the amino group is linked to the carbon of the alkyl group by \(\sigma \)-bond due to \({\rm{s}}{{\rm{p}}^3} – {\rm{s}}{{\rm{p}}^3}\) overlap.
• 2. N atom of the amino group is linked to each of the two \({\rm{H}}\) by a \(\sigma \)-bond due to \({\rm{s}}{{\rm{p}}^3} – {\rm{s}}\) overlap.
• 3. The lone pair of \({\rm{N}}\) atom occupies its fourth \({\rm{s}}{{\rm{p}}^3}\) hybrid orbitals.

In the case of secondary and tertiary amines, the hydrogen atoms are successively replaced by alkyl groups. The bonding between the carbon of the alkyl groups and the \({\rm{N}}\) atom is due to \({\rm{s}}{{\rm{p}}^3} – {\rm{s}}{{\rm{p}}^3}\) overlap.

The electron diffraction studies of trimethylamine show that the molecule has a pyramidal structure with a \({\rm{C}} – {\rm{N}} – {\rm{C}}\) bond angle of \({110.9^{\rm{o}}}.\)

Types of Amines

Amines are classified into aliphatic and aromatic amines based on the alkyl or aryl group attached to the nitrogen.

Aliphatic Amine

Amines in which the nitrogen atom is directly bonded to one or more alkyl \(\left( {\rm{R}} \right)\) groups are called aliphatic amines.

Example: Methanamine (methyl amine) \({\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2},\) ethanamine (ethylamine) \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2},\,{\rm{N}}\)-methylmethanamine (Dimethyl amine) \({\left( {{\rm{C}}{{\rm{H}}_3}} \right)_2}{\rm{NH}}\) and \({\rm{N,N}}\)-dimethylmethanamine (trimethylamine) \({\left( {{\rm{C}}{{\rm{H}}_3}} \right)_3}{\rm{N}}{\rm{.}}\)

Aromatic Amine

Aromatic amines are further classified into aryl amine and aryl alkyl amine.

1. Aryl Amine: Amines in which the nitrogen atom is directly bonded to one or more aromatic rings or aryl groups are called arylamines.
Example: Aniline \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{N}}{{\rm{H}}_2},\) Diphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_2}{\rm{NH}},\) Triphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_3}{\rm{N}}{\rm{.}}\)

2. Aryl Alkyl Amine: Amines in which the nitrogen atom is bonded to the sidechain of the aromatic ring is called aryl alkyl amines.
Example: Phenylmethanamine (benzylamine) \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2},\) Dibenzylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}{\rm{C}}{{\rm{H}}_2}} \right)_2}{\rm{NH}}\) and Tribenzylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}{\rm{C}}{{\rm{H}}_2}} \right)_3}{\rm{N}}{\rm{.}}\)

Based on the number of alkyl or aryl groups attached to the nitrogen of the amine, amines are classified into primary, secondary and tertiary amines.

Primary \(\left( {{1^{\rm{o}}}} \right)\) Amines

If one hydrogen of ammonia is replaced by an alkyl group \(\left( {\rm{R}} \right)\) or aryl group \(\left( {{\rm{Ar}}} \right)\) then it gives primary amine.

Aliphatic primary amine is represented as \({\rm{RN}}{{\rm{H}}_2}.\)

Examples: Methanamine (methyl amine) \({\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2},\) ethanamine (ethylamine) \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}.\)

Aryl primary amine is represented as \({\rm{ArN}}{{\rm{H}}_2}.\)

Examples: Aniline \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{N}}{{\rm{H}}_2}.\)

Primary Arylalkyl amine

Example: Phenylmethanamine (benzylamine) \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}.\)

Secondary \(\left( {{2^{\rm{o}}}} \right)\) Amines

If two hydrogen atoms of ammonia are replaced by the same or different alkyl group \(\left( {\rm{R}} \right)\) or aryl group \(\left( {{\rm{Ar}}} \right)\) then it gives secondary amine.

Aliphatic secondary amine is represented as \({\rm{R}} – {\rm{NH}} – {\rm{R}}’.\)

Examples: \({\rm{N}}\)-methylmethanamine (Dimethyl amine) \({\left( {{\rm{C}}{{\rm{H}}_3}} \right)_2}{\rm{NH}}.\)

Secondary Aryl amine is represented as \({\rm{ArNHAr}}.\)

Examples: Diphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_2}{\rm{NH}}.\)

Secondary Arylalkyl amine

Example: Dibenzylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}{\rm{C}}{{\rm{H}}_2}} \right)_2}{\rm{NH}}.\)

Tertiary \(\left( {{3^{\rm{o}}}} \right)\) Amines

If all three hydrogen atoms of ammonia are replaced by the same or different alkyl group \(\left( {\rm{R}} \right)\) or aryl group \(\left( {{\rm{Ar}}} \right)\) then it gives tertiary amine.

Aliphatic tertiary amine

Example: \({\rm{N,}}\,{\rm{N}}\)-dimethylmethanamine (trimethylamine) \({\left( {{\rm{C}}{{\rm{H}}_3}} \right)_3}{\rm{N}}.\)

Aryl tertiary amine

Example: Triphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_3}{\rm{N}}.\)

Arylalkyl tertiary amine

Example: Tribenzylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}{\rm{C}}{{\rm{H}}_2}} \right)_3}{\rm{N}}.\)

Quaternary Ammonium Salts

Besides the primary, secondary, and tertiary amine there are some derivatives in which all the four hydrogens of the ammonium ions \(\left( {{\rm{NH}}_4^ + } \right)\) are replaced by alkyl or aryl groups. These are called quaternary ammonium salts, or Tetraalkyl (or tetraaryl) ammonium salts. They are represented as \({\left[ {{\rm{N}}{{\rm{H}}_4}} \right]^ + }{{\rm{X}}^ – }.\)

Example: Tetra methyl ammonium iodide \({\left[ {{{\left( {{\rm{C}}{{\rm{H}}_3}} \right)}_4}{\rm{N}}} \right]^ + }{{\rm{I}}^ – },\) trimethyl phenyl ammonium chloride \({{\rm{C}}_6}{{\rm{H}}_5}{{\rm{N}}^ + }{\left( {{\rm{C}}{{\rm{H}}_3}} \right)_3}{\rm{C}}{{\rm{l}}^ – }\)

Difference Between Primary \(\left( {{1^{\rm{o}}}} \right)\) Secondary \(\left( {{2^{\rm{o}}}} \right),\) and Tertiary \(\left( {{3^{\rm{o}}}} \right)\) Amines

Distinction Between the Primary, Secondary and Tertiary Amines

Primary, secondary and tertiary amines can be differentiated based on the following tests

Uses of Amines

Amines are very useful compounds. These are used as follows:

• 1. The lower members of the aliphatic amine series are used as solvents.
• 2. Amines are also used as intermediate members in the drug industry and as a reagent in organic synthesis.
• 3. Quaternary salts derived from long-chain aliphatic amines are used as detergents. \({\rm{n}}\)-hexadecyltrimethylammonium chloride is a typical detergent.
• 4. Aromatic amines, such as aniline, are widely used in the manufacture of polymers, dyes, drugs, and intermediates used in the rubber industry. These are also used as synthetic reagents via diazonium salts.

FAQs on Classification of Amines

Q.1. How can amine groups be classified?
Ans: Amines are classified into aliphatic and aromatic amines based on the alkyl or aryl group attached to the nitrogen.
Amines in which the nitrogen atom is directly bonded to one or more alkyl \(\left( {\rm{R}} \right)\) groups are called aliphatic amines.
Example: Methanamine (methyl amine) \({\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2},\) ethanamine (ethylamine) \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}.\)
Aromatic amines are further classified into aryl amine and aryl alkyl amine.
Amines in which the nitrogen atom is directly bonded to one or more aromatic rings or aryl groups are called arylamines.
Examples: Aniline \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{N}}{{\rm{H}}_2},\) Diphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_2}{\rm{NH}}{\rm{.}}\)
Amines in which the nitrogen atom is bonded to the sidechain of the aromatic ring is called aryl alkyl amines.
Example: Phenylmethanamine (benzylamine) \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2},\) Dibenzylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}{\rm{C}}{{\rm{H}}_2}} \right)_2}{\rm{NH}}{\rm{.}}\)

Q.2. What is the most suitable classification of the following amine?
\({\rm{N}},\,{\rm{N}}\)-dimethylmethanamine, Ethanamine, \({\rm{N}}\)-methylmethanamine.
Ans: The given compounds can be classified into primary, secondary and tertiary amine. Here ethanamine is a primary amine, \({\rm{N}}\)-methylmethanamine is a secondary amine and \({\rm{N}},\,{\rm{N}}\)-dimethylmethanamine is a tertiary amine.

Q.3. What are the different types of amines?
Ans: Based on the number of the alkyl or aryl groups attached to the nitrogen of the amine, amines are classified into three types, i.e., primary amine, secondary amine, and tertiary amines.

Q.4. What are primary, secondary, and tertiary amines?
Ans: If one hydrogen of ammonia is replaced by an alkyl group \(\left( {\rm{R}} \right)\) or aryl group \(\left( {{\rm{Ar}}} \right),\) It gives primary amine.
Example: Ethanamine (ethylamine) \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}.\)
If two hydrogen atoms of ammonia are replaced by the same or different alkyl group \(\left( {\rm{R}} \right)\) or aryl group \(\left( {{\rm{Ar}}} \right),\) It gives secondary amine.
Example: Diphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_2}{\rm{NH}}{\rm{.}}\)
If three hydrogen atoms of ammonia are replaced by the same or different alkyl group \(\left( {\rm{R}} \right)\) or aryl group \(\left( {{\rm{Ar}}} \right)\) gives tertiary amine.
Example: Triphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_3}{\rm{N}}\)

Q.5. What is an amine in chemistry?
Ans: Amines are the derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by an alkyl or aryl group.

Q.6. Why is wool an animal Fibre?
Ans: Wool is obtained from the fur of animals such as goats, sheep, camels, etc. Wool contains a protein called keratin. Proteins are made up of amino acids.

Study About Ammonia Formula Here