Ch 19 - Formation of an Enolate Anion Enolate anions are...

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Formation of an Enolate Anion Enolate anions are formed by treating an aldehyde or ketone with base most of the negative charge in an enolate anion is on oxygen CH 3 -C-H O NaOH H C C-H O H H C C-H - Na + H H 2 O + + An enolate anion 1
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Enolate Anions • Enolate anions are nucleophiles in S N 2 reactions and carbonyl addition reactions An enolate anion nucleophilic substitution A 1° haloalkane or sulfonate R R R O + R' Br O R R R S N 2 R' + Br An enolate anion nucleophilic    addition A ketone A tetrahedral carbonyl  addition intermediate R R R O + R' R' O O R R R O R' R' 2
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The Aldol Reaction The product of an aldol reaction is a β -hydroxyaldehyde or a β -hydroxyketone CH 3 -C-H O CH 2 -C-H H O NaOH β CH 3 -CH-CH 2 -C-H OH O α + Acetaldehyde Acetaldehyde 3-Hydroxybutanal (a  β -hydroxyaldehyde;  racemic) O CH 3 -C-CH 3 CH 2 -C-CH 3 O H Ba(OH) 2 β α OH CH 3 CH 3 -C-CH 2 -C-CH 3 O Acetone + 4-Hydroxy-4-methyl-2-pentanone (a  β -hydroxyketone) Acetone 3
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Base-Catalyzed Aldol Reaction Step 1: formation of a resonance-stabilized enolate anion Step 2: carbonyl addition gives a TCAI Step 3: proton transfer to O - completes the aldol reaction CH 2 =C-H O  - CH 2 -C-H O H-O-H H-CH 2 -C-H O H-O  - + An enolate anion + p K a  20 (weaker acid) p K a  15.7 (stronger acid) CH 3 -C-H O CH 2 -C-H O CH 3 -CH-CH 2 -C-H O O  - A tetrahedal carbonyl addition intermediate + 4
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Acid-Catalyzed Aldol Reaction Step 1: acid-catalyzed equilibration of keto and enol forms Step 2: proton transfer from HA to the carbonyl group of a second molecule of aldehyde or ketone O OH CH 3 - C-H CH 2 = C- H HA CH 3 -C-H O H A O CH 3 -C-H H A + + 5
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Acid-Catalyzed Aldol Reaction Cont Step 3: attack of the enol of one molecule on the protonated carbonyl group of another molecule Step 4: proton transfer to A - completes the reaction O CH 3 -C-H H CH 2 =C-H H O :A - CH 3 -CH-CH 2 -C-H OH O H-A + + + (racemic) 6
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Aldol Final Products - Loss of H 2 O aldol products are very easily dehydrated to α , β -unsaturated aldehydes or ketones aldol reactions are reversible and often little aldol is present at equilibrium K eq for dehydration is generally large if reaction conditions bring about dehydration, good yields of product ( α , β -unsaturated An  α,β -unsaturated aldehyde β α + O OH O CH 3 CHCH 2 CH CH 3 CH= CHCH H 2 O warm in either acid or base 7
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Crossed or Mixed Aldol Reactions In a crossed aldol reaction, one type of molecule provides the enolate anion and another type provides the carbonyl group CH 3 CCH 3 O HCH O NaOH CH 3 CCH 2 CH 2 OH O 4-Hydroxy-2-butanone + Crossed aldol reactions are most successful if 1) one of the reactants has no α -hydrogen and, therefore, cannot form an enolate anion and 2) the other reactant has a more reactive carbonyl group, namely an aldehyde 8
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Crossed Aldol Reactions Aldol reactions can also be carried out using a nitroalkane (vs aldehyde or ketone)
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This note was uploaded on 03/31/2008 for the course CHEM 262L taught by Professor Mbadugua during the Spring '08 term at UNC.

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Ch 19 - Formation of an Enolate Anion Enolate anions are...

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