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Chapter 18 Lecture 1 Enols. Enolate Ions Carbonyl Reactivity Nucleophilic carbonyl oxygen Electrophilic carbonyl carbon a -carbon containing acidic a -protons (the subject of this chapter) Acidity of Aldehydes and Ketones pKa of protons alpha to an aldehyde or ketone carbonyl = 19-21

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chapter 18 lecture 1 enols
Chapter 18 Lecture 1 Enols
  • Enolate Ions
    • Carbonyl Reactivity
      • Nucleophilic carbonyl oxygen
      • Electrophilic carbonyl carbon
      • a-carbon containing acidic a-protons (the subject of this chapter)
    • Acidity of Aldehydes and Ketones
      • pKa of protons alpha to an aldehyde or ketone carbonyl = 19-21
        • Ethene pKa = 44
        • Ethyne pKa = 25
        • Alcohol pKa = 15-18
      • Strong bases can remove a-hydrogens to produce an Enolate Ion

Enolate Ion

Why are carbonyl a-protons acidic?
    • The conjugate base is stabilized by the enolate ion resonance structures
    • The d+ carbon of the carbonyl destabilizes the a C—H bond

C. Formation of Enolate Ions

  • LDA (lithium diisopropyl amide) or other strong bases are used
  • Aprotic solvents are used to prevent solvent deprotonation
  • Enolate Resonance Hybrid
    • The a-carbon and the oxygen of an enolate ion are both nucleophilic
    • Ambident = “two-fanged” = a species that can react at 2 different sites to give 2 different products
The carbon atom is the normal site of reaction by SN2. This type of reaction is called alkylation or C1-alkylation of the enolate ion.
      • The oxygen atom is the normal site of protonation, forming an enol, which will tautomerize to the original ketone.

II. Keto-Enol Equilibria

  • Ketone—Enol Tautomerization
    • This reaction is reversible, and the extent of reaction depends on conditions
    • Base-catalyzed Enol-Keto Equilibration
      • Base removes proton from the enol
      • The mechanism is the reverse of the original enolate formation
Acid Catalyzed Enol-Keto Equilibration
      • Protonation occurs at the double bond
      • Resonance stabilized C is next to O
      • Protonated carbonyl deprotonates to give the keto form
    • Both reaction are fast if the catalyst (B- or H+) are present
    • Keto form is usually dominant
    • Keto to enol tautomerization mechanisms are the reverse of those above
  • Effects of Substituents on Keto-Enol Equilibria
    • Ketone donating substituents stabilize keto form
    • Aldehyde lack of donating substituents pushes equilibria toward enol form
Deuteration of Carbonyl a-Carbons
    • Dissolving an aldehyde or ketone in D2O, DO- (or D+) replaces all of the a-Hydrogens with Deuteriums
    • Even though the keto form dominates, a small % is always tautomerizing to the enol. Over time, reprotonation at C gives the fully deuterated product.
    • Reaction can be followed by 1H NMR as a-H signal disappears
  • Interconversion of a-C stereochemistry

1) Keto-Enol tautomerization proceeds through an achiral intermediate

Loss of optical activity occurs under basic or acidic conditions
  • Halogenation of Aldehydes and Ketones
    • Acid-Catalyzed a-Halogenation of Ketones and Aldehydes
      • In acidic conditions, only one halogen is able to add
      • The reaction rate is independent of X2 concentration, suggesting that the rate determining step depends only on the carbonyl compound
3) Mechanism of acid catalyzed a monohalogenation

4) Why does the reaction stop after only one halogenation?

  • Mechanism requires enolization
  • Electron withdrawing Br prevents protonation needed in first step

O is no longer basic enough

To attack proton. Enolization

Can’t happen.

Base Mediated Halogenation of a-Carbon Goes to Completion
    • Mechanism
    • Electron Withdrawing Br increases a-Hydrogen acidity, favoring complete bromination of all a-Carbons
    • The Iodoform test for Methyl Ketones is base catalyzed halogenation
Alkylation of Aldehydes and Ketones
    • Alkylation of Ketones Using NaH
      • Ketones with only one a-Hydrogen are alkylated in high yield
        • Example:
        • NaH is a strong base yielding enolate ion when reacted with carbonyls
      • Polyalkylation occurs if multiple a-H’s are present
Unsymmetric Ketones give multiple products
  • Enamine Route to Ketone/Aldehyde Alkylation
    • Enamine formation makes C=C bonds electron rich by resonance
    • The nucleophilic a-Carbon can then attack electrophiles
The amine is removed from the alkylated product by acid to give the alkylated ketone or aldehyde
  • The Enamine Alkylation Route is Preferred
    • No multiple alkylations
    • Works on Aldehydes and Ketones