Tautomerism

Tautomerism and Desmotropism refer to the same phenomenon. Both terms describe the dynamic equilibrium between two structural isomers (tautomers) that can interconvert by the transfer of a proton and the rearrangement of a double bond. These two isomers differ in the position of a hydrogen atom and a double bond.

1.0What is Tautomerism

Tautomerism, introduced by Laar, refers to a special type of isomerism, where two or more structural isomers (known as tautomers) exist in dynamic equilibrium. This process involves the rapid oscillation of an atom, typically hydrogen, between two polyvalent atoms within a molecule. It is also known by other names such as desmotropism, kryptomerism, allotropism, or dynamic isomerism.

Tautomerism is a special type of isomerism in which two (or more) isomers, known as tautomers, exist in dynamic equilibrium and can interconvert by the transfer of a proton (hydrogen atom) along with the movement of a double bond. These isomers typically differ only in the position of a hydrogen atom and a double bond. This reversible process of shifting a proton is referred to as tautomerization.

2.0Example of Tautomerism

Keto-Enol Tautomerism: One of the most common examples of tautomerism is keto-enol tautomerism, where a molecule with a carbonyl group (keto form) interconverts with its enol form.

Here, the keto form predominates, but the enol form is present in small amounts due to the relative stability of the carbonyl group in the keto form.

3.0Key Features of Tautomerism

4.0Types of Tautomerism 

1. Diad Tautomerism:

• In diad tautomerism, the migration of a proton (hydrogen) takes place between two adjacent atoms. This is the most common type of tautomerism and is typically observed in systems like keto-enol tautomerism, where the hydrogen shifts between an oxygen atom and an adjacent carbon atom.
• Example: Keto-enol tautomerism in carbonyl compounds such as acetone. 

CH₃COCH₃ (Keto) ⇌ CH₃C(OH)=CH₂ (Enol)

2. Triad Tautomerism:

• In triad tautomerism, the migration of a proton occurs between the first and third atom in the system, skipping the second atom. This type of tautomerism typically involves a more extended system compared to diad tautomerism.
• Example: Nitro-aci-nitro tautomerism, where a hydrogen migrates from a nitrogen atom to an oxygen atom through a conjugated system. 

R-NO₂ (Nitro) ⇌ R-C=N-OH 

3. Space Tautomerism:

• Space tautomerism refers to cases where the migration distance is greater than three atoms. This involves a proton moving across a more complex or larger molecular structure, where atoms are more distant from one another. Space tautomerism is less common compared to diad and triad tautomerism.

5.0Conditions for Tautomerism 

Tautomerism occurs in compounds that can facilitate the migration of a proton (usually from an alpha-carbon) and the shifting of a bond, typically between a carbonyl or nitro group. The presence of alpha-hydrogens (hydrogens attached to the carbon next to the functional group) plays an important role in enabling tautomerism.

(a) Tautomerism in Carbonyl Compounds:

Carbonyl compounds, such as aldehydes and ketones, show keto-enol tautomerism if they have at least one alpha-hydrogen (α-H) attached to the carbon atom next to the carbonyl group. The α-hydrogen is essential because it migrates during the tautomeric shift.

(b) Tautomerism in Nitro Compounds:

Nitro compounds (R-NO₂) exhibit nitro-aci nitro tautomerism when they have at least one α-hydrogen. The proton migrates, leading to the formation of the aci-nitro form.

Conditions: At least one α-hydrogen is required for tautomerism to occur. The migration of the α-hydrogen from the carbon adjacent to the nitro group (-NO₂) to the oxygen in the nitro group results in the formation of the aci-nitro form.

Examples: Nitromethane (CH₃NO₂): Exhibits tautomerism between the nitro form (R-NO₂) and the aci-nitro form (R-C=N-OH).

6.0Mechanism of Tautomerism

Acid-Catalyzed Mechanism

In the acid-catalyzed mechanism, the process begins with protonation of the carbonyl oxygen, followed by the migration of a proton from the alpha-carbon, leading to the formation of the enol. If there are multiple enol forms possible, the more stable enol will be the major product.

Steps:

1. Protonation: The carbonyl oxygen is protonated, making the alpha hydrogen more acidic and easier to remove.
2. Proton Transfer: A proton from the alpha-carbon is transferred, forming the enol.
3. Formation of Enol: The double bond between the carbon and oxygen shifts, creating the C=C bond and the -OH group on the adjacent carbon.

Important Point:

• If more than one enol form is possible, the more stable product will dominate.

Base-Catalyzed Mechanism:

In the base-catalyzed mechanism, the base abstracts a proton from the alpha-carbon, forming an enolate ion, which then protonates to form the enol. This mechanism is more common with strong bases like hydroxide ions.

Steps:

1. Deprotonation: The base abstracts the alpha-hydrogen from the carbon next to the carbonyl group, forming an enolate ion.
2. Formation of Enol: The enolate ion rearranges by donating electrons back to the oxygen, leading to the formation of the enol.

7.0Enol Content

• The keto form is generally more stable than the enol form, but the amount of enol present in equilibrium is referred to as the enol content.
• The stability of the enol form affects how much enol exists at equilibrium.

Factors affecting enol content: