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Tautomerism | Structural Isomerism

Learning Objective: To learn about tautomerism, a subtype of structural isomerism. 

Skill Level – Intermediate

 

Prerequisites: 

Related – 

 

Chapter: Structural Isomerism

 

Author's Note: It is very easy to confuse between tautomerism and functional isomerism. Tautomerism is a type of functional group isomerism where the hydrogen atom migrates from 1,2 and 1,3 positions to generate structures or isomers that rapidly interconvert and where one such isomer is more stable than the other. This stability of an isomer is harnessed for greater good in organic chemistry. In the later chapters, you will learn how tautomerism explains the aldol condensation reaction or why phenol (alkene and alcohol) exits majorly in the alcohol form. This section touches on all types of basics, the common functional groups, and the conditions under which molecules show tautomerism.

Tautomerism

It is a special type of functional group isomerism in which the isomers exist in dynamic equilibrium with each other; that is, they undergo rapid interconversion from one structure to another due to the migration of the Hydrogen atom. 

The hydrogen atom migrates from one polyvalent atom to another within the same molecule. The bond or the linkage rearrange accordingly.

The isomers are called tautomers, and the phenomenon is called tautomerism. Tautomerism is also called desmotropism (desmos=bonds , propos= turn).

 

Types

Out of several types of tautomerism, the two main types are-

a) Dyad systems 

b) Triad systems

 

a) Dyad systems

This type of tautomerism involves the 1,2-migration of a hydrogen (as proton) from one polyvalent atom to the other within the same molecule.

For example, the Hydrogen atom migrates from carbon to nitrogen between the cyanide and isocyanide functional groups. Therefore, hydrogen cyanide and isocyanide are tautomer.

 

 

However, if an alkyl group is present instead of hydrogen,....

 

b) Triad systems

The Triad system involves the 1,3- migration of hydrogen atom from one polyvalent atom to the other within the same molecule.

Keto-enol tautomerism is the...

...

The higher stability of the enol form...

 

The other tautomerism involving 1,3-hydrogen shifts is-

i) Nitro-acinitro tautomerism..

..

ii) Nitroso-oximino tautomerism...

..

 

Essential Conditions 

Two main conditions for compounds to show tautomerism are-

1) The compound must have an electronegative atom....

 

In contrast, benzaldehyde, and benzophenone, etc., do not show ...

 

Subscribe to learn all subtypes of structural isomerism.

 

Structural Isomerism
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Stereoisomerism - Conformation and Configurational Isomerism

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