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 Chapter 18: Enols and Enolates 
Ch 18 contents
The Aldol Reaction of Aldehydes
Aldol reaction of an aldehdye
Reaction type : Nucleophilic addition

Summary

Aldol reaction of ethanal  
Try to identify the enolate portion and the carbonyl portion in the different representations  


QUESTION
Why isn't the simplest example of an aldol the condensation of methanal ? ANSWER

loss of water gives a C=C in a conjugated aldehyde
STUDY TIPS:
  • 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)
A simple Aldol condensation reaction
  • Recognise the products, either the β-hydroxyaldehydes = the aldol, or the conjugated product, the enal as being from an Aldol reaction.
taking an Aldol reaction apart to find the pieces

Related Reactions

MECHANISM OF THE ALDOL CONDENSATION


1. MECHANISM OF THE ALDOL REACTION

Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the reactive enolate.
mechanism of the Aldol condensation
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 β-hydroxyaldehydes or 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.

mechanism of the dehydration of an Aldol product
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.

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