Which of the following reaction is shown by aldehydes that do not have alpha H-atom?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Cannizzaro reaction
Aldehydes that have no α-hydrogen atom (such as HCHO, \( \ce{C6H5CHO} \), \( \ce{(CH3)3CCHO} \)) cannot form the carbanion needed for aldol condensation. On heating with concentrated alkali they undergo self oxidation and reduction: one molecule is reduced to the alcohol and another is oxidised to the salt of the acid. This is the Cannizzaro reaction:
\( \displaystyle \ce{2C6H5CHO + NaOH} \xrightarrow[\Delta]{\text{conc.}} \ce{C6H5CH2OH + C6H5COONa} \)
Why the other options are incorrect
Option (1) Aldol condensation — Incorrect. It needs at least one α-hydrogen, which dilute alkali removes to start the reaction.
\( \displaystyle \ce{2CH3CHO} \xrightarrow{\text{dil. NaOH}} \ce{CH3-CH(OH)-CH2-CHO} \)
Option (2) Cross aldol condensation — Incorrect. It is an aldol condensation between two different carbonyl compounds; at least one of them must have an α-hydrogen.
Option (4) Kolbe’s reaction — Incorrect. It is the reaction of sodium phenoxide with \( \ce{CO2} \) that gives salicylic acid (or the electrolysis of carboxylate salts); it is not a reaction of aldehydes.
\( \displaystyle \ce{C6H5ONa + CO2} \xrightarrow[\text{(ii) }\ce{H+}]{\text{(i) 400 K, 4-7 atm}} \text{salicylic acid} \)
NCERT (2026-27 edition):
Other Reactions
