Question

Please show the mechanism of RCHO or a ketone reacting with a nucleophile.

Answer 1

The Aldol Reaction of Aldehydes

Reaction type : Nucleophilic addition

Summary

• Reagents : commonly a base such as NaOH or KOH is added to the aldehyde.
• The reaction involves an aldehyde enolate reacting with another molecule of the aldehyde.
• Remember enolates are good nucleophiles and carbonyl C are good electrophiles.
• Since the pK_a of an aldehyde is close to that of NaOH, both enolate and aldehyde are present simultaneously.
• The products of these reactions are ?-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

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

• These Aldol products can often then undergo dehydration (loss of water) to give conjugated systems (an elimination reaction). This is then often called an Aldol Condensation.

• 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 ?-hydroxyaldehydes = the aldol, or the conjugated product, the enal as being from an Aldol reaction.