Chap18 - 1/17/12 CHAPTER 18 Enols, Enolates, and the Aldol...

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1/17/12 1 CHAPTER 18 Enols, Enolates, and the Aldol Condensation: α , β -Unsaturated Aldehydes and Ketones Acidity of Aldehydes and Ketones: Enolate ions 18-1 Strong bases can remove α hydrogens leading to an anions called enolate ions or enolates .
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1/17/12 2 The enolate resonance hybrid possesses partial negative charges on both carbon and oxygen and may attack electrophiles at either position. A species that can attack at two different sites to give two different products is called ambident . Keto-Enol Equilibria 18-2 An enol equilibrates with its keto form in acidic or basic solution. Keto-enol tautomerism is the interconversion of the thermodynamically more stable keto form and the enol form of a carbonyl compound.
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1/17/12 3 Substituents can shift the keto-enol equilibrium. For ordinary aldehydes and ketones only traces of the enol form are present. The enol form is less stable by 8-12 kcal mol -1 . However for acetaldehyde , the enol form is about 100 times more stable than that of acetone because the less substituted aldehyde carbonyl is more stable than the more substituted ketone carbonyl . Enol formation leads to deuterium exchange and stereoisomerization. Treatement of a ketone with traces of acid or base in D 2 O solvent leads to the exchange of all the α carbons: The number of α hydrogens in a molecule can be readily determined by following the disappearance of the 1 H NMR signal or by Mass Spectrometry as the hydrogens are sequentially replaced by deuterium.
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1/17/12 4 Enolization also leads to the easy interconversion of stereoisomers at α -carbons. The trans isomers are sterically more stable . Mechanism Of Base-Catylyzed Isomerization
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1/17/12 5 When an α -hydrogen is part of a stereocenter , keto-enol tautomerization can lead to racemization in the presence of a basic or acidic catalyst. Halogenation of Aldehydes and Ketones 18-3 Aldehydes and ketones react with halogens at their α -carbons. The extent of halogenation depends upon whether acidic or basic catalysis is used. What are the mechanisms?
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1/17/12 6 The rate of halogenation is independent of the halogen concentration. The rate determining step involves the carbonyl substrate. On Chalk Board Further halogenation is slowed because the electron-withdrawing halogen substituent makes enolization more difficult than in the original substrate. Singly halogenated product molecules are not attacked until all of the starting aldehyde or ketone has been used up.
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1/17/12 7 Base catalyzed halogenation proceeds by formation of an enolate ion, which then attacks the halogen. The electron withdrawing power of the halogen substituent makes the remaining α -hydrogens more acidic and complete halogenation of the α -carbon occurs, leaving unreacted starting material (when insufficient halogen is employed). On Chalk Board
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Chap18 - 1/17/12 CHAPTER 18 Enols, Enolates, and the Aldol...

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