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The aldol condensation takes its name from aldol (3-hydroxybutanal) a name introduced by Wurtz who…
The aldol condensation takes its name from aldol (3-hydroxybutanal) a name introduced by Wurtz who first prepared the beta-hydroxy aldehyde from acetaldehyde in 1872. It is a base catalysed reaction, and the product of aldol condensation undergoes dehydration of intermediate beta-aldols or beta-ketals in presence of a dilute acid to form $\alpha, \beta$ unsaturated carbonyl compound.
Source: https://www.researchgate.net/publication/229950129_The_Aldol_Condensation
(a)[2.0]
Write the mechanism of aldol condensation starting with acetaldehyde.
(b)[1.0]
Name the base used highlighting the role of the base catalyst in facilitating the reaction.
(c)[2.0]
What is crossed aldol condensation? Name the product formed when benzaldehyde undergoes crossed aldol condensation with acetaldehyde.
Answer
Answer (a)
Official answer keyMechanism (base-catalysed):
Step 1: $\mathrm{OH^-}$ removes an $\alpha$-hydrogen of $\mathrm{CH_3CHO}$: $\mathrm{CH_3CHO + OH^- \rightleftharpoons {}^-CH_2CHO + H_2O}$ (enolate ion).
Step 2: The enolate attacks the carbonyl carbon of a second molecule of $\mathrm{CH_3CHO}$: $\mathrm{CH_3CHO + {}^-CH_2CHO \rightleftharpoons CH_3CH(O^-)CH_2CHO}$.
Step 3: The alkoxide ion takes a proton from water: $\mathrm{CH_3CH(O^-)CH_2CHO + H_2O \rightleftharpoons CH_3CH(OH)CH_2CHO + OH^-}$ (aldol, 3-hydroxybutanal).
Step 4: Dehydration through the $\alpha$-hydrogen: $\mathrm{CH_3CH(OH)CH_2CHO \rightleftharpoons CH_3CH=CHCHO + H_2O}$.
Answer (b)
Official answer keyDil. NaOH/Ba(OH)$_2$ can be used as catalyst. $\mathrm{OH^-}$ from the dilute alkali attacks on the first molecule of aldehyde and removes the $\alpha$ hydrogen present in it to form an enolate ion which is resonance stabilised.
Answer (c)
Official answer keyWhen aldol condensation takes place between molecules of different aldehydes or different ketones or one aldehyde and one ketone.
Cinnamaldehyde/ 3-phenylprop-2-enal.
From ISC 2025 Practice Chemistry, question 95.