Numford
Name reaction

Aldol condensation

Aldehydes and ketones with alpha hydrogen, in dilute alkali, form a beta-hydroxy aldehyde or ketone (aldol), which loses water on heating to give an alpha,beta-unsaturated carbonyl compound.

Class 12, Chapter 8 Aldehydes, Ketones and Carboxylic Acids

  1. OH
    EthanalCH₃CHOAldehyde
  2. Step 1

    dilute NaOH

    Aldol condensation
    i

    Aldol addition: the enolate of one ethanal adds to the C=O of another. Needs an alpha hydrogen.

    OHOH
    3-HydroxybutanalCH₃CH(OH)CH₂CHOAldehyde

Reagents: dilute NaOH; heat for condensation

Mechanism

How it works

  1. Base removes an alpha hydrogen to give an enolate.
  2. The enolate attacks the C=O of a second molecule (aldol addition).
  3. On heating, the aldol loses water to give the conjugated unsaturated carbonyl.
Used to prepare

What it makes

3-Hydroxybutanal and but-2-enal from ethanal.

Limitations

Where it fails

At least one alpha hydrogen is needed. Methanal and benzaldehyde cannot form the enolate.

Which chapter

Class 12, Chapter 8 Aldehydes, Ketones and Carboxylic Acids

Questions

Aldol condensation: quick answers

What is the product of two ethanal molecules in dilute NaOH?

3-Hydroxybutanal (aldol); heating gives but-2-enal.

Why does methanal not undergo the aldol reaction?

It has no alpha hydrogen.

What do ketones give?

Propanone gives the ketol 4-hydroxy-4-methylpentan-2-one.

Plan an organic conversion