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 Chapter 18: Enols and Enolates 
Ch 18 contents
Advanced Aldol Condensations

The more advanced topics are : Dehydration, Mixed and Intramolecular Aldol reactions.

Dehydration of Aldol products

The dehydration of the initial Aldol products can occur, it is often favoured by the following factors:

note the conjugation in the product

Mixed Aldols

A mixed Aldol reaction means that the enolate and the carbonyl are not from the same aldehyde or ketone.

The most useful mixed Aldols are those where:

Otherwise there are selectivity problems that give rise to complex mixtures of products.
 
examples of aldehydes that have no alpha hydrogens   Neither of these aldehydes has α-hydrogens and therefore they cannot form an enolate.
a mixed aldol reaction   Only one of this pair of aldehydes has an α-hydrogen

QUESTIONS

 
  • In the above example, which aldehyde forms the enolate ?
  • ANSWER
     
  • Why can't the aldol product dehydrate ?
  • ANSWER

    Intramolecular Aldols

    an intramolecular Aldol
    How can you deduce this product ?
    1. Since aldehydes are more electrophilic than ketones, the aldehyde will be the electrophile and the ketone the nucleohile (i.e. the enolate) (see above)
    2. There two possible enolates (at C5 or C7)
    3. The more favourable ring size will the 5-membered ring.

    Study Tip:
    When doing intramolecular reactions, it is a good idea to count carbons to avoid silly errors.

    If you start numbering at the electrophilic C=O C atom, then it will "show you" the rings sizes...


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    © Dr. Ian Hunt, Department of Chemistry, University of Calgary