Cross aldol condensation
Two different aldehydes or ketones, each with alpha hydrogen, react under aldol conditions to give a mixture of four products.
PropanalCH₃CH₂CHOAldehyde - Step 1
CH₃CHO (ethanal), dilute NaOH, then heat
Pent-2-enalCH₃CH₂CH=CHCHOAldehyde
How it works
- Each carbonyl compound can form an enolate and each can be attacked.
- Four aldol products form (two self and two crossed).
- Heating gives the four unsaturated aldehydes.
What it makes
Mixtures such as pent-2-enal, 2-methylbut-2-enal, but-2-enal and 2-methylpent-2-enal from ethanal and propanal.
Where it fails
A mixture forms unless one partner has no alpha hydrogen.
Class 12, Chapter 8 Aldehydes, Ketones and Carboxylic Acids
Cross aldol condensation: quick answers
How many products form from ethanal and propanal?
Four aldol condensation products.
How can the mixture be avoided?
Use one partner with no alpha hydrogen, such as benzaldehyde.
Which condition drives dehydration?
Heating the aldol.