Aldol reactions E V Blackburn 2008 Acidity of
Aldol reactions © E. V. Blackburn, 2008
Acidity of the hydrogens The 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? © E. V. Blackburn, 2008
Acidity of the hydrogens Ionization of an hydrogen gives a resonance stabilized carbanion: © E. V. Blackburn, 2008
Keto - enol tautomerism Structural isomers that are formally related only by the shift of a hydrogen and one or more bonds are called tautomers. © E. V. Blackburn, 2008
Halogenation of ketones © E. V. Blackburn, 2008
Base catalyzed halogenation of ketones rate = k[acetone][B: ] slow fast © E. V. Blackburn, 2008
Acid catalyzed halogenation of ketones rate= k[HB][acetone] © E. V. Blackburn, 2008
Acid catalyzed halogenation of ketones © E. V. Blackburn, 2008
Halogenation of ketones the haloform reaction © E. V. Blackburn, 2008
Carbanions The carbanions formed, conjugate bases of very weak acids, are very strong bases. They are nucleophiles! © E. V. Blackburn, 2008
Base-promoted aldol condensation © E. V. Blackburn, 2008
Base-promoted aldol condensation © E. V. Blackburn, 2008
Base-promoted aldol condensation crotonaldehyde © E. V. Blackburn, 2008
Acid-catalyzed aldol condensation © E. V. Blackburn, 2008
Dehydration of aldol products The ease and orientation of the elimination reaction depends on the particular stability of the alkene formed. This stability is due to the fact that the carbon - carbon double bond is conjugated with the carbon - oxygen double bond - remember 1, 3 butadiene? © E. V. Blackburn, 2008
Dehydration of aldol products © E. V. Blackburn, 2008
Crossed aldol reactions? This can work! How? © E. V. Blackburn, 2008
Crossed aldol condensations Good yields can be obtained under certain conditions: • Mix the carbonyl compound having no hydrogen with the base. • Slowly add the carbonyl compound having an hydrogen to this mixture. Why? © E. V. Blackburn, 2008
Crossed aldol condensations © E. V. Blackburn, 2008
Cyclization? © E. V. Blackburn, 2008
Problems Try problems 17. 28, 17. 31, 17. 32, 17. 33, 17. 34, 17. 35, 17. 37, 17. 38, 17. 39, and 17. 41. © E. V. Blackburn, 2008
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