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Aldol reactions

Aldol reactions. 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?. Acidity of the  hydrogens.

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Aldol reactions

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  1. Aldol reactions

  2. 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?

  3. Acidity of the hydrogens Ionization of an  hydrogen gives a resonance stabilized carbanion:

  4. Keto - enol tautomerism Structural isomers that are formally related only by the shift of a hydrogen and one or more  bonds are called tautomers.

  5. Halogenation of ketones

  6. Base catalyzed halogenation of ketones rate = k[acetone][B:] slow fast

  7. Acid catalyzed halogenation of ketones rate= k[HB][acetone]

  8. Acid catalyzed halogenation of ketones

  9. Halogenation of ketones the haloform reaction

  10. Carbanions The carbanions formed, conjugate bases of very weak acids, are very strong bases. They are nucleophiles!

  11. Base-promoted aldol addition

  12. Base-promoted aldol condensation

  13. Base-promoted aldol condensation crotonaldehyde

  14. Acid-catalyzed aldol addition

  15. Dehydration of aldol products

  16. 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?

  17. Dehydration of aldol products

  18. Crossed aldol reactions? This can work! How?

  19. 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?

  20. Crossed aldol condensations

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