Chapter 18: Enols and Enolates

Summary

• Reagents : commonly a base such as NaOH or KOH is added to the ketone.
• Although ketone enolates are good nucleophiles, the aldol condensation of ketones is not particularly successful.
• Recall that ketones are less reactive than aldehydes towards nucleophilic addition due to steric and electronic effects.
• These Aldol products can often undergo dehydration (loss of water) to give conjugated systems  (an elimination reaction)

• Try to recognise the two units that came together and identify the nucleophilic (enolate) portion and the electrophilic (carbonyl) portion.
• Recognise the enone that arises from dehydration.

Related Reactions

• Aldol condensation of Aldehydes
• Other reactions of Enolates
• Claisen Condensation of Esters

MECHANISM OF THE ALDOL CONDENSATION OF A KETONE
Step 1: First, an acid-base reaction. Hydroxide functions as a base and removes the acidic a-hydrogen giving the reactive enolate.
Step 2: The nucleophilic enolate attacks the ketone at the electrophilic carbonyl C in a nucleophilic addition type process giving an intermediate alkoxide.
Step 3: An acid-base reaction. The alkoxide deprotonates a water molecule creating hydroxide and the b-hydroxyketone, the aldol product.

MECHANISM OF THE DEHYDRATION OF THE ALDOL PRODUCT
Step 1: First, an acid-base reaction. Hydroxide functions as a base and removes an acidic a-hydrogen giving the reactive enolate.
Step 2: The electrons associated with the negative charge of the enolate are used to form the C=C and displace the leaving group, regenerating hydroxide giving the conjugated ketone.