Which of the following reactions can be used to convert acetone into propane?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Wolff-Kishner reduction
Acetone (\( \ce{CH3COCH3} \)) and propane (\( \ce{CH3CH2CH3} \)) have the same carbon chain; only the C=O group must become a CH2 group. The Wolff-Kishner reduction does this: the ketone is treated with hydrazine, and the hydrazone formed is heated with KOH in a high-boiling solvent such as ethylene glycol.
\( \displaystyle \ce{(CH3)2C=O + NH2NH2 -> (CH3)2C=NNH2 + H2O} \)
\( \displaystyle \ce{(CH3)2C=NNH2} \xrightarrow[\text{ethylene glycol, heat}]{\ce{KOH}} \ce{CH3CH2CH3 + N2} \)
Why the other options are incorrect
Option (1) Mannich reaction — Incorrect. It is not a reduction of the carbonyl group to a methylene group; that is done by the Clemmensen or Wolff-Kishner reduction.
Option (2) Wittig reaction — Incorrect. It turns a C=O group into a C=C double bond, so it gives an alkene, not an alkane.
Option (4) Cannizzaro reaction — Incorrect. It is given by aldehydes with no α-hydrogen in concentrated alkali and gives an alcohol and a carboxylic acid salt; acetone is a ketone with α-hydrogens.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “(ii) Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc-amalgam and concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).”