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

pKa of  -Hydrogen of Carbonyl Derivatives 2 Remember as: pKa = 25 pKa = 20 pKa = 17 pKa = 25 pKa = 13 pKa = 11 pKa = 9 Remember as: 9-10    -ketocarbonyl

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