The aldehydes and ketones undergo many nucleophilic reactions. Aldehydes are generally more reactive than Ketones in nucleophilic addition reactions due to steric and electronic reasons. The aldehydes and ketones undergo a number of reactions due to acidic nature of α-Hydrogen also. Aldol and cross aldol reactions occur in carbonyl compounds which contain α-Hydrogen. Aldehydes and ketones having at least one α-Hydrogen undergo reaction in dil. alkali to form β-hydroxy aldehyde or β-hydroxy ketone. This is aldol reaction. Aldol or ketol lose water to form α,β unsaturated carbonyl compounds which is called aldol condensation. Carbonyl compounds react with ammonia derivatives such as NH2OH, NH2NHCONH2, 2,4 DNP to give products useful in their identification.
Identify the name of the reaction which is used to convert Propanone to 4-Methylpent-3-en-2-one.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Aldol condensation
Aldehydes and ketones with at least one α-hydrogen react in dilute alkali to give a β-hydroxy carbonyl compound (the aldol reaction). On heating, this loses water to give an α,β-unsaturated carbonyl compound; the whole change is the aldol condensation.
Two molecules of propanone (a ketone with six α-hydrogens) give 4-hydroxy-4-methylpentan-2-one (the ketol), which loses water to form 4-methylpent-3-en-2-one:
\( \displaystyle \ce{2CH3COCH3} \xrightarrow{\ce{Ba(OH)2}} \ce{(CH3)2C(OH)CH2COCH3} \)
\( \displaystyle \ce{(CH3)2C(OH)CH2COCH3} \xrightarrow{\Delta} \ce{(CH3)2C=CHCOCH3 + H2O} \)
Why the other options are incorrect
Option (1) Aldol reaction — Incorrect. It stops at the β-hydroxy ketone, 4-hydroxy-4-methylpentan-2-one; the product asked for has lost water and contains a C=C bond.
Option (2) Cannizzaro reaction — Incorrect. It is given by aldehydes with no α-hydrogen, in concentrated alkali, and produces an alcohol and a carboxylic acid salt.
Option (4) Cross aldol condensation — Incorrect. It occurs between two different carbonyl compounds; here both molecules are propanone.
NCERT (2026-27 edition):
Aldol Condensation – “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 (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction.
The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products. The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation.”