Chapter 18: Enols and Enolates

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:

• The Aldol product must still have an a-hydrogen between the carbonyl and the -OH group.
• Look for extended conjugated in the product (i.e. increased stability).
• Heating the reaction often favours elimination of water (dehydration).
• Non-aqueous reaction conditions favour the removal of water (equilbrium shifts to dehydration products).

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:

• Only one reactent can form an enolate.
• This means the other reactent lacks a-hydrogens (for examples, see below) and therefore must be the electrophilic carbonyl component.
• One reactent is more electrophilic than the other and functions as the electrophilic carbonyl component.
• Recall that in general, aldehydes are more reactive (electrophilic) than ketones and will usually be the electrophile.

Intramolecular Aldols

• Dicarbonyl compounds can be used to give intramolecular Aldol reactions.
• This means the enolate component and the carbonyl component are parts of a single larger molecule.
• The selectivity is typically controlled by the stability of the cyclic Aldol product.
• This favours the formation of the more stable 5- or 6-membered rings (review)
• To work out the best product, check the permutations that are possible and look for the favourable ring sizes.

1. The aldehyde will be the electrophile and the ketone the enolate (see above)
2. There two possible enolates (at C5 or C7)
3. The more favourable ring size will the 5-membered ring.