Chapter 22 slides

Chapter 22 slides - Chapter 22 Alpha Substitution and...

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Chapter 22 Alpha Substitution and Condensations of Enols and Enolate Ions
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Chapter 22 2 Alpha Substitution Replacement of a hydrogen on the carbon adjacent to the carbonyl, C=O. enolate ion =>
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Chapter 22 3 Condensation with Aldehyde or Ketone Enolate ion attacks a C=O and the alkoxide is protonated. The net result is addition. C O C _ C O C O C C O _ ROH C O C C OH => (Aldol Condensation - Borodin)
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Chapter 22 4 Condensation with Esters Loss of alkoxide ion results in nucleophilic acyl substitution. =>
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Chapter 22 5 Keto-Enol Tautomers • Tautomers are isomers which differ in the placement of a hydrogen. • One may be converted to the other. • In base: =>
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Chapter 22 6 Keto-Enol Tautomers (2) • Tautomerism is also catalyzed by acid. • In acid: =>
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Chapter 22 7 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 8 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 9 Enolate Reaction => As enolate ion reacts with the electrophile, the equilibrium shifts to produce more. Energy Diagram
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Chapter 22 10 Acid-Base Reaction to Form Enolate Very strong base is required for complete reaction. Lithium Diisopropylamide (LDA) NH HC CH 3 H 3 C 3 H 3 C n -BuLi N 3 H 3 C 3 H 3 C Li + _ O H H O - + H LDA + C 3 H 7 ) 2 ( i - p K a = 19 p K a = 40 => Sterically hindered; Good base, poor nucleophile.
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Chapter 22 11 α Halogenation • Base-promoted halogenation of ketone.
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This note was uploaded on 04/08/2008 for the course CHEM 224 taught by Professor Pearson during the Spring '08 term at Case Western.

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Chapter 22 slides - Chapter 22 Alpha Substitution and...

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