Question

Explain Aldol condensation with a chemical reaction.

Hint:

Two carbonyl molecules that have an \(\alpha\)-hydrogen combine in dilute base to give a \(\beta\)-hydroxy carbonyl (aldol), which on heating loses water. Try acetaldehyde.

Answer 1

Step 1: Definition. Aldol condensation is a reaction in which two molecules of an aldehyde or ketone that contain at least one \(\alpha\)-hydrogen atom combine in the presence of a dilute base (dilute NaOH or KOH) to form a \(\beta\)-hydroxy aldehyde (called an aldol) or a \(\beta\)-hydroxy ketone (called a ketol).
Step 2: Why an \(\alpha\)-hydrogen is needed. The base removes an acidic \(\alpha\)-hydrogen to form a carbanion (enolate ion). This carbanion acts as the nucleophile and attacks the carbonyl carbon of a second molecule.
Step 3: Aldol formation (example with acetaldehyde).
\[2\,CH_3CHO \xrightarrow{dil.\ NaOH} CH_3CH(OH)CH_2CHO\]
The product is 3-hydroxybutanal (the aldol).
Step 4: Condensation (loss of water). On warming, the aldol loses a water molecule to give an \(\alpha,\beta\)-unsaturated aldehyde.
\[CH_3CH(OH)CH_2CHO \xrightarrow{\Delta,\ -H_2O} CH_3CH=CHCHO\]
The final product is but-2-enal (crotonaldehyde).
Result: Acetaldehyde undergoes aldol condensation to finally give but-2-enal.