Ch18: Aldol reaction of RC=OR

Chapter 18: Enols and Enolates

The Aldol Reaction of Ketones Aldol reaction of a ketone

Reaction type : Nucleophilic addition

Summary

Reagents : commonly a base such as NaOH or KOH is added to the ketone.
Although ketone enolates are good nucleophiles, the aldol reaction of ketones is usually 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) (overall = an aldol condensation).

Study Tip:
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

MECHANISM OF THE ALDOL CONDENSATION OF A KETONE
1. MECHANISM OF THE ALDOL REACTION OF A KETONE
Step 1: First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-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 β-hydroxyketone, the aldol product.

2. MECHANISM OF THE DEHYDRATION OF THE ALDOL PRODUCT
Step 1: First, an acid-base reaction. Hydroxide functions as a base and removes an acidic α-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.