Chapter22

Chapter22 - Chapter 22 Alpha Substitution and Condensations...

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Chapter 22 Alpha Substitution and Condensations of Enols and Enolate Ions
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Chapter 22 2 Keto-Enol Tautomers Tautomers are isomers which differ in the placement of a double bond and a hydrogen. One may be converted to the other. In base: =>
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Chapter 22 3 Keto-Enol Tautomers (2) Tautomerism is also catalyzed by acid. In acid: =>
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Chapter 22 4 Equilibrium Amounts For aldehydes and ketones, the keto form is greatly favored at equilibrium. An enantiomer with an enolizable hydrogen can form a racemic mixture. =>
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Chapter 22 5 Acidity of α -Hydrogens p K a for α -H of aldehyde or ketone ~20. Much more acidic than alkane or alkene (p K a > 40) or alkyne (p K a = 25). Less acidic than water (p K a = 15.7) or alcohol (p K a = 16-19). In the presence of hydroxide or alkoxide ions, only a small amount of enolate ion is present at equilibrium. =>
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Chapter 22 6 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
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Chapter 22 7 Enolate Reaction => As enolate ion reacts with the electrophile, the equilibrium shifts to produce more.
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Chapter 22 8 Acid-Base Reaction to Form Enolate Very strong base is required for complete reaction. Example: =>
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Chapter 22 9 α Halogenation Base-promoted halogenation of ketone. Base is consumed. Other products are water and chloride ion. OH _ O H H O H _ O H Cl Cl Cl =>
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Chapter 22 10 Multiple Halogenations The α -halo ketone produced is more reactive than ketone. Enolate ion stabilized by e - -withdrawing halogen. O H Cl Cl 2 OH , H 2 O _ O Cl Cl O Cl Cl Cl O Cl Cl Cl Cl =>
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Chapter 22 11 Acid Catalyzed Halogenation of Ketones Can halogenate only one or two α -H’s. Use acetic acid as solvent and catalyst. =>
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Chapter 22 12 Alkylation Enolate ion can be a nucleophile. Reacts with unhindered halide or tosylate via S N 2 mechanism. O H H O H ( i -Pr) 2 N - Li + CH 3 Br O H CH 3 =>
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Chapter 22 13 The 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
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Chapter 22 14 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
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Chapter22 - Chapter 22 Alpha Substitution and Condensations...

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