Aldol Condensation
Aldehydes and ketones undergo a number of reactions due to the acidic nature of α-hydrogen. Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes or β-hydroxy ketones, respectively. This is known as aldol reaction. The aldol and ketol readily lose water to give α, β-unsaturated carbonyl compounds, which are aldol condensation products; the reaction is called aldol condensation. When aldol condensation is carried out between two different aldehydes and/or ketones, it is called cross aldol condensation. Ethanal and propanal undergo this reaction to form mixture of products. However, benzaldehyde and acetophenone react to give only one product . Moreover, aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction ( disproportionation) reaction on heating with concentrated alkali. The reaction is known as Cannizzaro reaction. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.
Identify the product formed during the aldol condensation reaction of acetone.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 4-Methylpent-3-en-2-one
Ketones with α-hydrogens also undergo aldol condensation. The product of the first step is a ketol (β-hydroxy ketone), which loses water to form an α,β-unsaturated ketone. In acetone, the carbanion formed from one molecule attacks the C=O carbon of a second molecule:
\( \displaystyle \ce{2CH3COCH3} \xrightarrow{\ce{Ba(OH)2}} \ce{(CH3)2C(OH)-CH2-CO-CH3} \)
\( \displaystyle \ce{(CH3)2C(OH)-CH2-CO-CH3} \xrightarrow{\text{heat}} \ce{(CH3)2C=CH-CO-CH3 + H2O} \)
Naming the product \( \ce{(CH3)2C=CH-CO-CH3} \):
The longest chain has five carbon atoms: pentane.
Numbering from the end nearer the C=O group puts the ketone at C-2 and the double bond between C-3 and C-4: pent-3-en-2-one.
The methyl branch is on C-4: 4-methyl.
So the aldol condensation product of acetone is 4-methylpent-3-en-2-one (mesityl oxide).
NCERT (2026-27 edition):
Aldol Condensation
