Chapter 18: Enols and Enolates

Summary

• Reagents : commonly a base such as NaOH or KOH is added to the aldehyde.
• The reaction involves an enolate reacting with another molecule of the aldehyde.
• Remember enolates are good nucleophiles and carbonyl C are electrophiles.
• Since the pKa of an aldehyde is close to that of NaOH, both enolate and aldehyde are present.
• The products of these reactions are b-hydroxyaldehydes or aldehyde-alcohols = aldols.
• The simplest aldol reaction is the condensation of ethanal. This is shown below in 2 different representations (the line diagrams are less cluttered).....

Try to identify the enolate portion and the carbonyl portion in the different representations		Highlight enolate component Nu Highlight carbonyl component E

• These Aldol products can often undergo dehydration (loss of water) to give conjugated systems  (an elimination reaction)

• Students often find this reaction difficult, probably because they are intimidated by it..... break it down into the component steps.
• Think of it as a nucleophile adding to the carbonyl of the aldehyde as we saw in chapter 17 (review ?)
• Try to recognise the two units that came together and identify the nucleophilic (enolate) portion and the electrophilic (carbonyl) portion.
• The nucleophilic enolate will still contain the C=O unit
• The electrophilic carbonyl will have become an alcohol C-OH (or dehydrated to C=C)

• Recognise the products, either the  b-hydroxyaldehydes = the aldol, or the conjugated product, the enal as being from an Aldol reaction.

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

MECHANISM OF THE ALDOL CONDENSATION
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 aldehyde 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-hydroxyaldehydes or 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 aldehyde.