22.1 Keto–Enol Tautomerism

Keto-enol tautomerism is a fascinating dance between two forms of the same molecule. It's like a chemical Jekyll and Hyde, where a compound can switch between a keto form with a C=O group and an enol form with C=C and OH groups.

This switcheroo happens through proton movement and electron rearrangement. Unlike resonance, where forms can't be isolated, keto and enol forms can sometimes be caught red-handed. Factors like alpha hydrogens, stability, and solvents influence which form wins out.

Keto-Enol Tautomerism

Keto-enol tautomerism vs resonance

Top images from around the web for Keto-enol tautomerism vs resonance

• 

  File:Keto-enol.gif - Wikipedia View original

  Is this image relevant?

• 

  Keto–enol tautomerism - Wikipedia View original

  Is this image relevant?

• 

  organic chemistry - alpha-hydrogen atom and keto-enol tautomerism - Chemistry Stack Exchange View original

  Is this image relevant?

• 

  File:Keto-enol.gif - Wikipedia View original

  Is this image relevant?

• 

  Keto–enol tautomerism - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Keto-enol tautomerism vs resonance

• 

  File:Keto-enol.gif - Wikipedia View original

  Is this image relevant?

• 

  Keto–enol tautomerism - Wikipedia View original

  Is this image relevant?

• 

  organic chemistry - alpha-hydrogen atom and keto-enol tautomerism - Chemistry Stack Exchange View original

  Is this image relevant?

• 

  File:Keto-enol.gif - Wikipedia View original

  Is this image relevant?

• 

  Keto–enol tautomerism - Wikipedia View original

  Is this image relevant?

1 of 3

• Keto-enol tautomerism type of isomerism molecule exists in two different forms (keto form and enol form)
  • Keto form contains carbonyl group (C=O)
  • Enol form contains carbon-carbon double bond (C=C) and hydroxyl group (OH) on adjacent carbons
• Keto and enol forms are constitutional isomers have same molecular formula but different bonding arrangements
• Keto and enol forms interconvert through movement of proton and rearrangement of bonding electrons
• Differs from resonance forms
  • Resonance forms have same bonding arrangement but different distributions of electrons (benzene)
  • Resonance forms cannot be isolated as separate compounds while keto and enol forms can be isolated under certain conditions (acetone and propen-2-ol)

Mechanisms of keto-enol tautomerization

• Acid-catalyzed mechanism:
  1. Protonation of carbonyl oxygen in keto form
  2. Formation of enol intermediate through migration of proton from alpha carbon to hydroxyl group
  3. Deprotonation of oxygen to form enol tautomer
• Base-catalyzed mechanism:
  1. Deprotonation of alpha carbon in keto form
  2. Formation of enolate anion intermediate
  3. Protonation of enolate anion at oxygen to form enol tautomer
• Both mechanisms involve migration of proton and rearrangement of bonding electrons (cyclohexanone and cyclohexenol)
• The rate of tautomerization is influenced by kinetics, with the transition state playing a crucial role in determining the reaction pathway

Factors in keto-enol equilibrium

• Presence of alpha hydrogens (hydrogens adjacent to carbonyl group)
  • Compounds without alpha hydrogens cannot undergo keto-enol tautomerization (benzaldehyde)
• Stability of enol form
  • Enols with extended conjugation are more stable and favored at equilibrium (phenol)
  • Intramolecular hydrogen bonding in enol form can increase its stability (2,4-pentanedione)
• Substituents on alpha carbon
  • Electron-withdrawing groups (CN, NO2) stabilize enolate anion and shift equilibrium towards enol form
  • Electron-donating groups (alkyl, OR) destabilize enolate anion and shift equilibrium towards keto form
• Solvent effects
  • Protic solvents (water, alcohols) can stabilize keto form through hydrogen bonding
  • Aprotic solvents (acetone, DMSO) can stabilize enol form by solvating hydroxyl group
• The equilibrium constant determines the relative concentrations of keto and enol forms at equilibrium

Thermodynamics and Equilibrium Principles

• Thermodynamics governs the overall direction and extent of tautomerization
• Le Chatelier's principle explains how changes in conditions can shift the keto-enol equilibrium
• Hammond's postulate relates the structure of transition states to the energies of reactants and products in tautomerization reactions