Structures of amines is an important chapter in CBSE class 12th. The chemical formula of amine is \({\text{N}}{{\text{H}}_3}.\). In this article, you will indulge in the structure of amines, amides and heterocyclic amides and many more. Amines are referred as the derivatives of ammonia, it can be obtained by replacing one or all atoms of hydrogen by atoms of alkyl groups. The structure of amines will help students understand the formation and composition of amines in details.

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.

\({\rm{N}}{{\rm{H}}_{\rm{3}}}{\rm{ + R}} \to {\rm{RN}}{{\rm{H}}_{\rm{2}}}{\rm{ + H}}\)
\({\rm{RN}}{{\rm{H}}_{\rm{2}}}{\rm{ + R}} \to {{\rm{R}}_{\rm{2}}}{\rm{NH + H}}\)
\({{\rm{R}}_{\rm{2}}}{\rm{NH + R}} \to {{\rm{R}}_{\rm{3}}}{\rm{N + H}}\)

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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 quite similar to ammonia.

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

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

1.\({\text{N}}\) atom of the amino group is linked to the carbon of the alkyl group by \(\sigma \)-bond due to \({\text{s}}{{\text{p}}^3} – {\text{s}}{{\text{p}}^3}\) overlap.

2. \({\text{N}}\) atom of the amino group is linked to each of the two \({\text{H}}\) by a \(\sigma \)-bond due to \({\text{s}}{{\text{p}}^3} – {\text{s}}\) overlap.

3. The lone pair of \({\text{N}}\) atom occupies its fourth \({\text{s}}{{\text{p}}^3}\) hybrid orbital.

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 \({\text{N}}\) atom is due to \({\text{s}}{{\text{p}}^3} – {\text{s}}{{\text{p}}^3}\) overlap.

The electron diffraction studies of trimethylamine show that the molecule has a pyramidal structure with a \({\text{CNC}}\) bond angle of \({108^ \circ }.\)

Types of Amines

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

Example: Methanamine (methyl amine) \({\text{C}}{{\text{H}}_3}{\text{N}}{{\text{H}}_2},\) ethanamine (ethylamine) \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{N}}{{\text{H}}_2},{\text{N}} – \) methylmethanamine (Dimethylamine) \({\left({{\text{C}}{{\text{H}}_3}} \right)_2}{\text{NH}},{\text{N}},{\text{N}} – \)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.

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 \({{\text{C}}_6}{{\text{H}}_5}{\text{N}}{{\text{H}}_2},\) Diphenylamine \({\left( {{{\text{C}}_6}{{\text{H}}_5}} \right)_2}{\text{NH}},\) Triphenylamine \({\left( {{{\text{C}}_6}{{\text{H}}_5}} \right)_3}{\text{N}}{\text{.}}\)

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) \({{\text{C}}_6}{{\text{H}}_5}{\text{C}}{{\text{H}}_2}{\text{N}}{{\text{H}}_2},\) Dibenzylamine \({\left( {{{\text{C}}_6}{{\text{H}}_5}{\text{C}}{{\text{H}}_2}} \right)_2}{\text{NH}}{\text{.}}\)

Primary \(\left({{1^ \circ }} \right)\) amines

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

An aliphatic primary amine is represented as \({\text{RN}}{{\text{H}}_2}.\)

Examples: Methanamine (methylamine) \({\text{C}}{{\text{H}}_3}{\text{N}}{{\text{H}}_2},\) ethanamine (ethylamine) \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{N}}{{\text{H}}_2}.\)

Aryl primary amine is represented as \({\text{ArN}}{{\text{H}}_2}.\)
Examples: Aniline \({{\text{C}}_6}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}\)

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Arylalkyl primary amine

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

Secondary \(\left({{2^ \circ }} \right)\) amines

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

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

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

Aryl secondary amine is represented as \({\text{Ar}} – {\text{NH}} – {\text{Ar}}.\)
Example: Diphenylamine \({\left({{{\text{C}}_6}{{\text{H}}_5}}\right)_2}{\text{NH}}.\)

Arylalkyl secondary amine

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

Tertiary \(\left({{3^ \circ }} \right)\) amines

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

Aliphatic tertiary amine

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

Aryl tertiary amine

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

Arylalkyl tertiary amine

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

Structure of Heterocyclic Amines

1. The chemical compound containing at least one heterocyclic ring ( at least two different elements in the ring), having at least one amine group, is called heterocyclic amines. The vitamins and carcinogens are heterocyclic amines.
2. Example: Niacin (Vitamin \({\rm{B3}}\)) nicotine, etc.

Structure of Amides

Compounds containing nitrogen atoms attached to the carbonyl carbon are called amides. The carbonyl carbon is bonded to the nitrogen by an amide linkage. If the remaining two bonds on the nitrogen in the amide are attached to the hydrogen, the compound is called a simple amide (primary amide). In substituted amines, one or both the hydrogen are attached to alkyl or aryl groups.

Example: Acetamide and benzamide

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 compounds in the drug industry and as a reagent in organic synthesis.
3. Quaternary salts derived from long-chain aliphatic amines are used as detergents. \({\text{n}}\)-hexadecyltrimethyl ammonium chloride is a typical detergent.
4. Aromatic amines, such as aniline, are widely used in the manufacture of polymers, dyes, drug, and intermediate used in the rubber industry. These are also used as synthetic reagents via diazonium salts.

Summary

In this article, you have understood the structure of amines, types of amines like aliphatic amine and aromatic amine. You are also able to differentiate primary, secondary and tertiary amines. You can recall the structure of amides and heterocyclic amines with examples.

FAQs on Structure of Amines

Frequently asked questions related to amines and structure of amines are listed as follows:

Q.1. How do you name an amine structure?
Ans: In the IUPAC system, the aliphatic amines are called alkanamines. These names are written by replacing ‘e’ of the name of parent alkane with the suffix ‘amine’.
Example: \({\text{C}}{{\text{H}}_3}{\text{N}}{{\text{H}}_2}\) is named methanamine.
The simplest aromatic amine \({\text{C}}{{\text{H}}_6}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}\) is called benzenamine ( Aniline). Other aromatic amines are named as derivatives of benzenamine, and the positions of other groups are indicated by numbers.

Q.2. What is the shape of amine?
Ans: Amine is pyramidal in shape.

Q.3. How do you know if amine is primary, secondary or tertiary?
Ans: The primary, secondary or tertiary amines are distinguished by Hinsberg’s reagent (benzene sulphonyl chloride, \({{\text{C}}_6}{{\text{H}}_5}{\text{S}}{{\text{O}}_2}{\text{Cl}}\)). On heating amine with Hinsberg’s reagent in the presence of excess alkali, primary amine gives a clear solution, which on acidification yields an insoluble material. Secondary amines give an insoluble substance with Hinsberg’s reagent in the presence of excess alkali that remains unaffected on the addition of acid. Tertiary amines do not react with Hinsberg’s reagent and remain insoluble in alkali, and can be dissolved in acids.

Q.4. What are the different types of amines?
Ans: Different types of amines are aliphatic amines and aromatic amines.
Amines in which the nitrogen atom is directly bonded to one or more alkyl \(\left({\text{R}} \right)\) groups are called aliphatic amines.
Examples: Methanamine
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.
Example: Aniline \({{\rm{C}}_{\rm{6}}}{{\rm{H}}_{\rm{5}}}{\rm{N}}{{\rm{H}}_{\rm{2}}}{\rm{,}}\) Diphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_2}{\rm{N}}{{\rm{H}}_2},\) Triphenylamine \({\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_3}{\rm{N}}{\rm{.}}\)
Amines in which the nitrogen atom is bonded to the sidechain of the aromatic ring is called aryl alkyl amines.
Example: Phenylmethanamine (benzylamine) \({{\text{C}}_6}{{\text{H}}_5}{\text{C}}{{\text{H}}_2}{\text{N}}{{\text{H}}_2}\)

Q.5. Explain the structure of amines?
Ans: In amines, the nitrogen atom is \({\text{s}}{{\text{p}}^3}\) hybridised. There are four \({\text{s}}{{\text{p}}^3}\) hybrid orbitals in which three orbitals are singly occupied, whereas the fourth one accommodates the lone pair of electrons. It has a pyramidal structure.

Q.6. Which of the following is similar to the primary amines?
a) \({\left({{{\text{C}}_6}{{\text{H}}_5}} \right)_3}{\text{N}}\)
b) \({\left({{{\text{C}}_6}{{\text{H}}_5}{\text{C}}{{\text{H}}_2}} \right)_2}{\text{NH}}\)
c) \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{N}}{{\rm{H}}_2}\)
Ans: The aniline \({{\rm{C}}_{\rm{6}}}{{\rm{H}}_{\rm{5}}}{\rm{N}}{{\rm{H}}_{\rm{2}}}\) is the primary aromatic amine.

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