aldol reactions - Aldol reactions E.V. Blackburn, 2008 The...

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© E.V. Blackburn, 2008 Aldol reactions
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© E.V. Blackburn, 2008 Acidity of the α- hydrogens The α hydrogens are weakly acidic in comparison with the hydrogen of the carboxylic acid group. However they are the most acidic hydrogens of aldehydes and ketones: Why? C H H H K a = 10 -50 C H H K a = 10 -44 C H K a = 10 -25 C R O C H K a = 10 -19 - 10 -20
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© E.V. Blackburn, 2008 Acidity of the α- hydrogens Ionization of an α hydrogen gives a resonance stabilized carbanion: C C O - + HB + C H C O + :B C C O - C C O -
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© E.V. Blackburn, 2008 Keto - enol tautomerism Structural isomers that are formally related only by the shift of a hydrogen and one or more π bonds are called tautomers . OH H O H - - H + C C H H O + H + O H - stronger acid weaker acid OH H C C H H O enol structure keto structure
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© E.V. Blackburn, 2008 Halogenation of ketones C H C O + X 2 OH - or H + C X C O + HX α -haloketone X 2 = Cl 2 , Br 2 , or I 2
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© E.V. Blackburn, 2008 rate = k[acetone][B:] slow fast CH 3 COCH 3 + Br 2 + :B CH 3 COCH
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aldol reactions - Aldol reactions E.V. Blackburn, 2008 The...

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