Isomerism in Amines: What is the structural difference between \({\rm{n}}\)-Butylamine and \(2\)-methylpropanamine? Both the compounds belong to the homologous amine series, but they differ in the structural arrangement of atoms. The arrangement of atoms in space gives compounds their characteristic physical and chemical properties.

Isomerism is the phenomenon that refers to the occurrence of molecules having the same molecular formula but a different structural arrangement of atoms. Isomerism in Amines occurs in \(4\) ways namely, chain, position, functional and metamerism. Let’s explore what type of structural arrangement of atoms occurs in amines and how it affects their chemical and physical properties. 

What are Amines?

Amines are nitrogen-based organic compounds that are considered as derivatives of ammonia. These compounds are formed by replacing one, two, or all three hydrogen atoms in an ammonia molecule with alkyl and/or aryl groups. For example,

Study Structure of Amines Here

In amines, the nitrogen orbitals are \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised with a pyramidal geometry. Each of the three \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised orbitals of nitrogen overlap with carbon or hydrogen orbitals as per the amines’ composition. In all amines, an unshared pair of electrons are found in the fourth orbital of nitrogen. The \({\rm{C–N–E}}\) (where \({\rm{E}}\) is \({\rm{C}}\) or \({\rm{H}}\)) bond angle is reduced from \({\rm{109}}{\rm{.}}{{\rm{5}}^{\rm{o}}}\) to \({\rm{10}}{{\rm{8}}^{\rm{o}}}\) due to this unshared electron pair.

Amines are classed as primary \(\left( {{{\rm{1}}^{\rm{o}}}} \right)\), secondary \(\left( {{{\rm{2}}^{\rm{o}}}} \right)\), or tertiary \(\left( {{{\rm{3}}^{\rm{o}}}} \right)\) based on the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecules.

If one hydrogen atom of ammonia is replaced by \({\rm{R}}\) or \({\rm{Ar}}\), we get a primary amine \(\left( {{{\rm{1}}^{\rm{o}}}} \right){\rm{ – RN}}{{\rm{H}}_{\rm{2}}}\) or \({\rm{ArN}}{{\rm{H}}_{\rm{2}}}\).

We get secondary amine \(\left( {{{\rm{2}}^{\rm{o}}}} \right){\rm{ – R – NH}}{{\rm{R}}^{\rm{‘}}}\) when two hydrogen atoms of ammonia or one hydrogen atom of \({\rm{RN}}{{\rm{H}}_{\rm{2}}}\) are substituted by another \({\rm{alkyl/aryl(R’)}}\) group. The second \({\rm{alkyl/aryl}}\) group could be identical or dissimilar.

A tertiary amine \(\left( {{{\rm{3}}^{\rm{o}}}} \right)\) is formed when all three hydrogen atoms of ammonia or the terminal hydrogen atom of \({\rm{RN}}{{\rm{H}}_{\rm{2}}}\) are replaced by an \({\rm{alkyl/aryl}}\) group.

Simple amines are created when all of the \({\rm{alkyl}}\) or \({\rm{aryl}}\) groups are the same, while mixed amines are formed when they are not.

Isomerism in Amines

Isomerism is the phenomenon that refers to the occurrence of molecules having the same molecular formula but a different structural arrangement of atoms. Amines exhibit four types of isomerism. These are-

Chain Isomerism 

Amines exhibit chain isomerism due to the difference in the nature of the carbon chain of the \({\rm{alkyl}}\) group attached to the amino group. For example, in \({\rm{n}}\)-Butylamine \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}\) and \(2\)-methylpropanamine \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{CH}}\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}} \right){\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{N}}{{\rm{H}}_{\rm{2}}}\), the \({\rm{alkyl}}\) group consists of the butyl and iso-butyl chains.

Position Isomerism

Position isomerism in amines arises due to the difference in the position of the amino group in the carbon chain. For example, \(1\)-aminopropane, \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}\) and \(2\)-aminopropane \({\rm{C}}{{\rm{H}}_3}{\rm{CH}}\left( {{\rm{N}}{{\rm{H}}_2}} \right){\rm{C}}{{\rm{H}}_3}\), \({\rm{N}}{{\rm{H}}_2}\) the \({\rm{alkyl}}\) group is attached to the terminal carbon atom and secondary carbon atom, respectively.

Functional Isomerism 

This isomerism arises when there is a difference in the attachment of the amino group to the carbon chain. For example, \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}\) Propylamine \(\left( {{1^{\rm{o}}}} \right){\rm{C}}{{\rm{H}}_3}{\rm{NHC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3}\) Ethyl methylamine \(\left( {{{\rm{2}}^{\rm{o}}}} \right),\,{\left( {{\rm{C}}{{\rm{H}}_3}} \right)_3}\;{\rm{N}}\) Trimethylamine \(\left( {{{\rm{3}}^{\rm{o}}}} \right)\)

Metamerism

Metamerism in amines arises due to the difference in the nature of the alkyl group attached to the same functional group. For example, in Diethylamine, \({\left( {{{\rm{C}}_2}{{\rm{H}}_5}} \right)_2}{\rm{NH}}\left( {{{\rm{2}}^{\rm{o}}}} \right)\) Methyl propylamine \({{\rm{C}}_{\rm{3}}}{{\rm{H}}_{\rm{7}}}{\rm{NHC}}{{\rm{H}}_{\rm{3}}}\left( {{{\rm{2}}^{\rm{o}}}} \right)\), there is a difference in the alkyl group to which the amino group is attached.

Summary

All those compounds that bear the same molecular formula but differ in chemical properties are known as isomers. Amines are derivatives of ammonia that contain a basic nitrogen atom with a lone pair of electrons. The different spatial arrangement of atoms in amines results in the chain, positional, functional, and metamerism. In this article, we learnt the different isomerism exhibited by amines. We also learned its classification and the difference between primary, secondary and tertiary amines.

FAQs on Isomerism in Amines

Q.1. Do amines show functional isomerism?
Ans: All primary amines exhibit functional isomerism with secondary and tertiary amines and vice versa

Q.2. What is a secondary amine?
Ans: We get secondary amine \(\left( {{2^{\rm{o}}}} \right){\rm{R}} – {\rm{NHR’}}\) when two hydrogen atoms of ammonia or one hydrogen atom of \({\rm{RN}}{{\rm{H}}_{\rm{2}}}\) are substituted by another \({\mathop{\rm alkyl}\nolimits} /{\mathop{\rm aryl}\nolimits} \left( {{\rm{R}}’} \right)\) group. The second \({\rm{alkyl/aryl}}\) group could be identical or dissimilar.

Q.3. Is metamerism possible in amines?
Ans: Secondary amines can show metamerism.

Q.4. How are amines classified?
Ans: Amines are classified as primary \(\left( {{{\rm{1}}^{\rm{o}}}} \right)\), secondary \(\left( {{{\rm{2}}^{\rm{o}}}} \right)\), or tertiary \(\left( {{{\rm{3}}^{\rm{o}}}} \right)\) based on the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecules. In chemical notation, these three classes are represented as \({\rm{RN}}{{\rm{H}}_2},\,{{\rm{R}}_2}{\rm{NH}}\), and \({{\rm{R}}_3}\;{\rm{N}}\), respectively.

Q.5. Define Chain Isomerism.
Ans: Chain isomerism is the phenomenon in which two or more compounds with similar chemical formulas, but consists of distinct carbon atom arrangements in straight or branched chains. The compounds exhibiting chain isomerism are known as chain isomers.

Learn About Chemical Reactions in Amines Here

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