Tautomerism | General Organic Chemistry Class 11

Tautomerism

Dynamic Functional Isomerism | General Organic Chemistry

1. What is Tautomerism?

Tautomerism is a special type of functional isomerism where two isomers exist in a dynamic equilibrium with each other due to the migration of an atom (usually Hydrogen) between two polyvalent atoms.

Also known as: Desmotropism, Cryptomerism, or Dynamic Isomerism.
It involves 1,3-migration of a proton (Triad System).

2. Conditions for Tautomerism

For a compound to show tautomerism, it must possess:

1. An electronegative atom bonded to a multiple bond (e.g., $C=O, C=N, N=O$).
2. At least one $\alpha$-Hydrogen atom attached to an $sp^3$ hybridized carbon adjacent to the functional group.

Example: Acetaldehyde ($CH_3-CHO$) has 3 $\alpha$-H atoms $\rightarrow$ Shows Tautomerism.
Benzaldehyde ($C_6H_5-CHO$) has NO $\alpha$-H $\rightarrow$ No Tautomerism.

3. Keto-Enol Tautomerism

The most common form involving the migration of a proton from $\alpha$-Carbon to Carbonyl Oxygen.

$$ \underset{\text{Keto Form}}{H-C-C=O} \rightleftharpoons \underset{\text{Enol Form}}{C=C-O-H} $$

Mechanism:

• Acid Catalyzed: Protonation of O $\rightarrow$ Removal of $\alpha$-H.
• Base Catalyzed: Removal of $\alpha$-H (forming Enolate ion) $\rightarrow$ Protonation of O.

4. Stability of Keto vs Enol Form

Generally, the Keto form is much more stable (>99%) than the Enol form because the $C=O$ bond is stronger than the $C=C$ bond. However, certain factors can stabilize the Enol form:

Factors Stabilizing Enol:

1. Aromaticity: In Phenol, the enol form is 100% stable because the keto form loses aromaticity.
2. Intramolecular H-Bonding & Conjugation: In $\beta$-dicarbonyl compounds (e.g., Acetylacetone), the enol form is stabilized by a 6-membered cyclic transition state involving H-bonding.
   Order of Enol Content: Acetylacetone (76%) > Acetoacetic Ester (7%) > Acetone (<1%).
3. Solvent: Non-polar solvents favor the enol form (to maintain internal H-bonding). Polar protic solvents (water) favor the keto form.

5. Other Types of Tautomerism

• Nitro-Aci Nitro: $R-CH_2-N(=O)-O \rightleftharpoons R-CH=N(OH)-O$.
• Imine-Enamine: $R-CH_2-CH=NH \rightleftharpoons R-CH=CH-NH_2$.
• Nitroso-Oxime: $R-CH-N=O \rightleftharpoons R-C=N-OH$.

6. Tautomerism vs Resonance

Tautomerism	Resonance
Involves movement of atoms (H)	Involves movement of $\pi$ electrons only
Distinct compounds (can be separated)	Imaginary structures (cannot be separated)
Different functional groups	Same functional group
Equilibrium exists	No equilibrium (Hybrid exists)

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