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Ch_322b_17.01 - Chapter 17.1 lecture note

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Chapter 17 Aldehydes and Ketones II: Aldol Reactions 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
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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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