12.6 Keto-Enol Tautomerization

12.6 Keto-Enol Tautomerization

  • It is very difficult to remove all traces of acid or base from a reaction vessel.
    • The amount of enol and ketone is determined by the equilibrium.
    • We will only spend a few moments on this important topic since you will encounter it many times as you study organic chemistry.
  • Make sure you don't confuse tautomers and resonance structures.
  • They are two different compounds that are present at equilibrium.
    • They represent one entity.
  • As you progress through the course, you will encounter at least one of the exceptions where the enol is favored.
  • These two compounds are tautomers.
    • The ketone is not favored.
    • The concentration of ketone is very low because the enol is so favored.
  • The enol has more stability than the ketone.
  • You can either cover the concept of aromaticity in your organic chemistry course or you can.
    • It is important for you to know that the tautomerization process favors the aromatic enol over the ketone because of the stability associated with an aromatic ring.
  • Let's look at drawing mechanisms.
    • When drawing the mechanism of tautomerization, we have to decide whether to begin with deprotonation or protons.
    • It is important that the choice is consistent with the conditions of the job.
    • An enol is formed during acid-catalyzed hydration of an alkyne.
    • The tautomerization process begins when the conditions are acidic.
    • It would be a mistake to draw deprotonation first because the resulting intermediate would be inconsistent with acidic conditions.
    • The tautomerization process begins with base-catalyzed tautomerization to give an anionic intermediate.
    • The resulting intermediate would be inconsistent with basic conditions, so it would be a mistake to draw it first.
  • Something else should be considered when drawing mechanisms.
    • Students make an error that causes the entire mechanism to be incorrect for a tautomerization process under acid-catalyzed conditions.
    • Students are tempted to protonate the OH group instead of the p bond during the first step.
  • The base for this deprotonation step is water.
    • We have to stay consistent with the acidic conditions employed.
    • hydroxide is a strong base and it would be inconsistent with acidic conditions to show it as the base here.
    • Water is more likely to be the base for the deprotonation step under acidic conditions.
  • We have to stay consistent with the basic conditions employed.
  • H O+ is inconsistent with basic conditions.
  • The structure of the enol is shown below.
    • We wouldn't be able to store it because it would undergo rapid tautomerization.
  • Draw the tautomer's structure and show how it forms under acidic conditions.
  • If this tautomerization process were to happen under basic conditions, draw a mechanism for it.
  • In acidic conditions, H O+ is the source of the protons.
  • It would be wrong to show hydroxide in your mechanism because the process occurs under acidic conditions and it is inconsistent with acidic conditions.
  • Deprotonation is the first step in basic conditions.
  • The compound cannot be isolated or stored because of its rapid tautomerization.
  • The compound cannot be isolated or stored because of its rapid tautomerization.
  • If the tautomerization process were to occur under acidic conditions, draw a mechanism for it.
  • The compound cannot be isolated or stored because of its rapid tautomerization.