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Tautomerism in chemistry -Learn with Super tricks in 1 Shot!

What is tautomerism?

  •  This is the phenomenon in which a hydrogen atom’s movement and a double bond’s shifting takes place.
  •  A chemical phenomenon in which a single chemical molecule exists in two or more interconvertible forms with different placements of a proton and double bond. These various structures are known as tautomers and such process is known as tautomerism in chemistry. Interconversion between these structures is often quick and occurs in a dynamic equilibrium condition.
  • The most prevalent type of tautomerism is keto enol tautomerism, which occurs when a molecule with a keto group (C=O) is transformed into its enol counterpart (C=C-OH). For example, in acetoacetic acid, the keto form is CH3-CO-CH2-COOH, while the enol form is CH3-C(OH)=CH-COOH.
  • It refers to isomers that can easily interconvert.
  • It is a structural isomerism significant in many chemical and biological processes.
  • Tautomers and contributing structures in chemical resonance are two different terms, don’t confuse them.

Examples:

Enol-keto form:

Tautomerism in chemistry

Lactam-Lactim form:

Lactam and lactim form of tautomerism

Amide-Imidic acid form:

Amide imidic form

Amine-Imine form:

tautomerism between amine and imine

Conditions for tautomerism:

  • That compound must contain an electron-withdrawing group that can abstract the hydrogen atom.
  • The carbonyl group must contain an alpha hydrogen atom.
  • keto enol tautomerism type proceeds in the availability of acidic or basic catalyst.

Structural requirement of Tautomerism in chemistry:

  • Those compounds contain polar molecules and weakly acidic functional groups.
  • Involvement of change in the position of an atom.
  • It does not affect the bond length or such features.
  • Normally, it occurs in planar or non-planar molecules.

Types of tautomerism:

1. Prototrophy:

The process that arises due to the transfer of proton within a molecule i.e. amino acid which is the building block of protein,  is called prototrophy.

prototrophy is the type of tautomerism

2. Annular tautomerism:

The process that arises due to the shifting in a heterocyclic system.

Annular tautomerism

 

3. Ring chain tautomerism:

The phenomenon that arises due to the shifting of protons from the open chain and converting them into a closed chain.

Ring chain tautomerism

4. Non-carbonyl tautomerism:

The phenomenon that arises due to the shifting of protons in noncarbonyl compounds.

Tautomerism between nitroso and Oxime

 

Tautomerism between nitrile and ketenimine

tautomerism between imine and enamine

5. Valence tautomerism:

That phenomenon which arises due to the continuous formation and breaking of single and double bonds in the compound without any migration of groups or atoms.

The molecular geometry of a molecule is changed in valence tautomerism and is not confused with canonical resonance structures.

 

6. Tautomeric form of unsymmetrical ketones:

There is only one form of tautomer that occurs in symmetric form. However, for an unsymmetrical form, there can be two tautomeric forms.

Tautomerism of unsymmetrical ketone

7. Keto enol tautomerism:

  • The process that arises due to the conversion of keto form into enol form by the action of a small amount of acid or base as a catalyst is called keto-enol tautomerism.
  • Tautomers are constitutional isomers that are readily interconvertible. These are differentiated based on the placement of a labile hydrogen atom and the position of the double bond.
  • The equilibrium between tautomers is not only rapid under normal conditions but often strongly favors one of the isomers. For example, Acetone is 99.999% keto tautomer and ~0.001% enol tautomer.

keto enol tautomerism

  • The minor tautomer and the major tautomer are formed due to the chemical behavior of the compound.
  • Traces of acids or bases are used as catalysts for the tautomeric equilibrium.
  • A keto tautomer favors the rapid equilibrium than the enol tautomer. But in some other reactions, enol is the most important tautomer.
Case 1: Intramolecular H-bonding

One example is the 1,3-diketones, such as acetylacetone (2,4-pentanedione), which prefer the enolic form. Intramolecular H-bonding creates a 6-membered cyclic structure that stabilizes the enol form over the keto.

tautomerism in Intramolecular Hydrogen bonding

Case 2: Aromaticity

In some aromatic compounds i.e. phenol, the enol exhibits the aromatic character so it is more favorable than ketone.Tautomerism due to aromaticity

Case 3: Solvent

Polar solvents ( which can form hydrogen bonding ) will make lone pairs less available for intramolecular H-bonding, hence % of keto increases.  aromatic character

tautomerism in Intramolecular Hydrogen bonding due to different solvents

keto: enol

6: 94 (when benzene is used as solvent)

81: 19 ( when water is used as solvent)

Case 4: Conjugation

conjugated enol is more favored for ketone than non-conjugated enol.

 

type of tautomerism: conjugation

Case 5: Substitution

more substituted ketone (A) is more favored for enol than (B) ketone.

tautomerism in substituted products

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