Chapter 18 Reactions at the a Carbon of Carbonyl Compounds Enols and Enolates
Keto, enol and enolate structures: • What is a keto form ? A structure that contains a carbonyl. E.g. aldehyde, ketone, ester, acid, etc. • What is an enol ? A compound that has an alkene and an OH attached to the same carbon atom. • What is an enolate ? An enol with the proton removed. May be formed by removing a proton from the a carbon atom of a keto form.
The a carbon and a hydrogens: • What is an a carbon ? A carbon atom adjacent to a carbonyl. • What is an a hydrogen ? A hydrogen attached to an a carbon atom. aHydrogens are weakly acidic (pKa = 19 – 20) due to the e-withdrawing C=O.
Reactions at the a Carbon of Carbonyl Compounds:Enols and Enolates This shows a reaction at the carbonyl carbon atom. Tetrahedral intermediate
The Acidity of the a Hydrogens of Carbonyl Compounds: Enolate Anions Comparison of pKas.
Deprotonation: Removal of a H forms a resonance stabilized enolate. Resonance structures for the delocalized enolates
Protonation: Protonation of a carbon. Protonation of oxygen.
Keto and Enol Tautomers • Interconvertible keto and enol forms are called tautomers, and their interconversion is called tautomerization. • The keto and enol forms are in equilibrium (not resonance structures) because a proton transfer occurs.
E.g. Resonance stabilization of the enol form
Reactions via Enols & Enolates 3A. Racemization Racemization at an a carbon takes place in the presence of acids or bases
4A. Regioselective Formation of Enolates • Formation of a Kinetic Enolate: (Dimethoxyethane) This enolate is formed faster because the hindered strong base removes the less hindered proton faster.
Formation of a Thermodynamic Enolate: This enolate is more stable because the double bond is more highly substituted. It is the predominant enolate at equilibrium.
4B. Direct Alkylation of Ketones via Lithium Enolates Lithium diisopropylamide = LDA or LiN(iPr)2.
Recall: • a-hydrogens of b-dicarbonyl compounds are more acidic:
Contributing resonance structures Resonance hybrid
6A. Acylation • Synthesis b-diketones: