Main Menu 12 5 The Acidity of an

Main Menu 12. 5 The Acidity of an a-Hydrogens b-hydrogen p. Ka ≈ 50 dd+ a-hydrogen p. Ka ≈ 19 -20 a hydrogen of a carbonyl group is more acidity than a typical hydrogen in hydrocarbons. 1

p. Ka of a-Hydrogen of Carbonyl Derivatives p. Ka = 25 p. Ka = 20 p. Ka = 17 Remember as: 19 -20 b a p. Ka = 13 b-ketocarbonyl p. Ka = 11 p. Ka = 9 Remember as: 9 -10 2

Resonance Effect on the p. Ka of a. Hydrogen Aldehydes or ketones Esters or amides b-ketocarbonyl 3

Enol-Keto Tautomerization keto enol Examples: 4

Mechanism of Enol-Keto Tautomerization in Acidic Condition (HCl) (R)-3 -Phenyl-2 -butanone Enol form Keto form 5

Mechanism of Enol-Keto Tautomerization in Basic Condition (OH-) Enolate (R)-3 -Phenyl-2 -butanone Enol form Keto form 6

Consequence of Enol-Keto Tautomerization: Racemization In the presence of acid catalyst (or base catalyst), keto-enol tautomerization occurs. Since the a-carbon of 2 adapts an SP 2 hybridization, there is no selectivity between the protonation from the frontside and back-side of 2. 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. 7

Learning Check 1. Which one of the following is not the enol form of 2 -butanone? 8

Learning Check Main Menu 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 9