Oxygen Containing Compounds - Keto Acids and Esters

Description; nomenclature

  • keto acids and keto esters
  • α-keto acid = 2-oxo acid. For example: α-ketopropanoic acid = 2-oxopropanoic acid
  • β-keto acid = 3-oxo acid.
  • α-keto ester = 2-oxo ester.
  • β-keto ester = 3-oxo ester. For example: methyl β-ketobutanoate = methyl 3-oxobutanoate

Important reactions

  • decarboxylation
    • beta-keto decarboxylation
    • beta-keto esters → beta-keto acids → enols → ketos
    • decarboxylation of beta-keto acids is facile because the enol stabalizes the reaction intermediate.
  • acetoacetic ester synthesis
    • acetoacetic ester synthesis
    • acetate vs acetoacetate
    • Acetoacetic ester is synthesized by Claisen condensation of ethyl acetate in a process called acetoacetic ester condensation
      • 2 x ethylacetate → ethyl acetoacetate
      • acetoacetic ester = β-keto ester
    • "Acetoacetic ester synthesis" is a reaction where acetoacetic ester is used to synthesize a new ketone.
      1. Acidic alpha proton comes off, resulting carbanion attacks new R group.
      2. Hydrolysis of ester turns it into a β-keto carboxylic acid.
      3. β-keto acids undergo decarboxylation because the β-keto group stabilizes the resulting carbanion via enol formation. Enol converts back to keto form, and the net result of this reaction is that an R group is made to attach to the α carbon of acetone.

General principles

  • acidity of alpha hydrogen and beta-keto ester
    • acidic hydrogen of beta-keto ester
    • Any hydrogen alpha to (adjacent to) a carbonyl group is more acidic than a regular hydrogen. The alpha hydrogen of a beta-keto ester is even more acidic because it's adjacent to 2 carbonyl groups.
    • The reason for the acidity is the stabilization of the deprotonated species by the enolate ion resonance structures.
  • keto-enol tautomerism
    • tautomerism
    • enol ends with an "ol" so it has an alcohol group.
    • keto is a ketone group.
    • enol → keto because the keto form is more stable.
    • You can change a ketone to an enolate ion by abstracting the alpha hydrogen. However, that's not tautomerism.