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 condensation

12. Base-promoted aldol condensation

13. Base-promoted aldol condensation
crotonaldehyde

14. Acid-catalyzed aldol condensation

15. 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?

16. Dehydration of aldol products

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

18. 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?

19. Crossed aldol condensations

20. Cyclization?

21. Problems
Try problems 17.28, 17.31, 17.32, 17.33, 17.34, 17.35, 17.37, 17.38, 17.39, and