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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.

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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:

1 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

2 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

3 Resonance Effect on the pKa of  - Hydrogen 3 Aldehydes or ketones Esters or amides  -ketocarbonyl

4 Enol-Keto Tautomerization 4 keto enol Examples:

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

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

7 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 8 Learning Check 1. Which one of the following is not the enol form of 2-butanone?

9 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


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