Chapter 18 (Lecture #18)

Chapter 18 (Lecture #18) - Chapter 18! Reactions at the...

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Acidity of the ! -Hydrogens of Carbonyl Compounds Hydrogens ! to the carbonyl group are much more acidic than ordinary aliphatic hydrogens that have pKa values > 50. C-C-R'' O = H R R' + H 2 O R-C-C-R'' O R' : - + H 3 O + Ka ~ 10 -20 pKa ~ 20 Chapter 18 Reactions at the α -Carbon of Carbonyl Compounds Enols and Enolates
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Stability of Enolate Anions The anion formed by loss of a proton ! to a carbonyl group is called an enolate anion because of a resonance structure that is the conjugate base of an enol. Enolate anions are stabilized by delocalization of the negative charge through an extended " -system. descriptions of an enolate anion R-C-C-R'' O = R' : - R-C=C-R'' : : :O: - R' resonance stabilization C C R' R O R'' - ! -delocalization
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The Keto-Enol Tautomers and the Enolate Anion Protonation of the enolate anion can occur either at the oxygen or carbon atom since both carry partial negative charge. R-C-C-R'' O = R' : - R-C=C-R'' : : :O: - R' resonance structures of the enolate anion C C-R'' :O: R' R !" !" enolate anion + H + - H + R-C=C-R'' :O- H R' enol,pKa ~ 10 + H + - H + R-C-C-R'' O R' : : H keto, pKa ~ 20 Deprotonation of either the enol (pKa ~10) or the keto (pKa ~20) gives the enolate anion.
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The Keto-Enol Equilibrium The keto and enol structures are constitutional isomers that are also called tautomers . The keto and enol structures interconvert rapidly in the presence of catalytic amounts of acids or bases. This equilibration is referred to as tautomerization . The keto form, generally, is heavily favored in the equilibrium, as shown by the examples below. compound keto-enol equilibrium acetaldehyde CH 3 CH O = CH 2 =CH OH ~100% acetone CH 3 CCH 3 O CH 2 =CCH 3 OH >99% 1.5 x 10 -4 % cyclohexanone O OH 98.8% 1.2% The dominance of the keto form in the above equilibria reflects the greater strength of the ! -bond of the carbonyl (~87 kcal/mol) compared with the ! -bond of the alkene (~60 kcal/mol).
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! -Dicarbonyl Compounds: High Enol Content Compounds with two carbonyl groups separated by a methylene, ! -dicarbonyls , often have one carbonyl mostly in the enol form. CH
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Chapter 18 (Lecture #18) - Chapter 18! Reactions at the...

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