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Chapter 19-2

Chapter 19-2 - 11 CHEM 262 Wei You Chapter 19 Enolate...

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Unformatted text preview: 11 CHEM 262 Wei You Chapter 19. Enolate Anions (2) Click to edit Master subtitle style Orgo II is All About…. 22 33 Two Types of Reactions for Enolate Anions Enolate anions are carbon ( not Oxygen! ) nucleophiles in SN2 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' Type 1: SN 2 Type 2: carbonyl addition Click to edit Master subtitle style • SN2 reaction:, the leaving group X can be chloride, bromide, iodide, or tosylate • R should be primary or methyl and preferably should be allylic or benzylic • Secondary halides react poorly, and tertiary halides don't react at all because of competing elimination Type I: SN 2 – Alkylation of Enolate Ion 44 55 Type II: Aldol Reaction • The most important reaction of enolate anions is nucleophilic addition to the carbonyl group of another molecule of the same or different compound – although these reactions may be catalyzed by either acid or base, base catalysis is more common 66 The Aldol Reaction • The product of an aldol reaction is – a β-hydroxy aldehyde – or a β-hydroxy ketone 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 3-C-CH 3 O CH 2-C-CH 3 O H Ba(OH) 2 Ba(OH) 2 β α OH CH 3 CH 3-C-CH 2-C-CH 3 CH 3 CH 3-C-CH 2-C-CH 3 O Acetone + 4-Hydroxy-4-methyl-2-pentanone (a β-hydroxyketone) + Acetone 77 Base Catalyzed Aldol Reaction • 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 + 88 Acid Catalyzed Aldol Reaction • 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 + + 99 Acid Catalyzed Aldol – Step 3: attack of the enol of one molecule on the protonated carbonyl...
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Chapter 19-2 - 11 CHEM 262 Wei You Chapter 19 Enolate...

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