Presentation on theme: "12.5 The Acidity of an -Hydrogens 1 -hydrogen -- ++ Main Menu hydrogen of a carbonyl group is more acidity than a typical hydrogen in hydrocarbons."— Presentation transcript:
12.5 The Acidity of an -Hydrogens 1 -hydrogen -- ++ Main Menu hydrogen of a carbonyl group is more acidity than a typical hydrogen in hydrocarbons. -hydrogen pKa ≈ pKa ≈ 50
Resonance Effect on the pKa of - Hydrogen 3 Aldehydes or ketones Esters or amides -ketocarbonyl
Enol-Keto Tautomerization 4 keto enol Examples:
Mechanism of Enol-Keto Tautomerization in Acidic Condition (HCl) 5 (R)-3-Phenyl-2-butanone Keto form Enol form
Mechanism of Enol-Keto Tautomerization in Basic Condition (OH - ) 6 (R)-3-Phenyl-2-butanone Keto form Enol form Enolate
Consequence of Enol-Keto Tautomerization: Racemization 7 Further keto-enol tautomerization will lead to the formation of equal amount of 1 and its enantiomer, ent-1 after a period of time. The resulting mixture, a racemic mixture, has no chirality. In the presence of acid catalyst (or base catalyst), keto-enol tautomerization occurs. Since the -carbon of 2 adapts an SP 2 hybridization, there is no selectivity between the protonation from the front- side and back-side of 2.
8 Learning Check 1. Which one of the following is not the enol form of 2-butanone?
9 Learning Check 2. What should be the enol forms of ethyl phenyl ketone? (a)I, II (b) II, III (c) I, III (d)III, IV (e)None of the above Main Menu