Which of the following compounds does not undergo Cannizzaro reaction?

The correct answer is Option (4) → Acetaldehyde

The Cannizzaro reaction is given only by aldehydes that have no α-hydrogen (no H on the carbon next to the –CHO group). On heating with concentrated alkali, one molecule of the aldehyde is reduced to an alcohol and another is oxidised to the salt of a carboxylic acid (disproportionation):

    \( \displaystyle \ce{2HCHO} \xrightarrow{\text{conc. }\ce{KOH},\ \Delta} \ce{CH3OH + HCOOK} \)

Aldehydes that have an α-hydrogen react in another way. The α-hydrogen is acidic, so the base removes it, and the aldehyde gives the aldol reaction instead.

Acetaldehyde, \( \ce{CH3-CHO} \), has three α-hydrogens on its methyl group. With dilute alkali it gives aldol (3-hydroxybutanal), not the Cannizzaro products:

    \( \displaystyle \ce{2CH3CHO} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{CH3-CH(OH)-CH2-CHO} \)

• Option (1) Formaldehyde — Undergoes the Cannizzaro reaction. HCHO has no α-carbon at all, so it has no α-hydrogen. It gives methanol and potassium formate, as shown above.

• Option (2) 2,2-Dimethylbutanal — Undergoes the Cannizzaro reaction. Its structure is \( \ce{CH3CH2-C(CH3)2-CHO} \). The α-carbon carries two methyl groups and an ethyl group, but no hydrogen, so the aldehyde gives 2,2-dimethylbutan-1-ol and the salt of 2,2-dimethylbutanoic acid.

• Option (3) Benzaldehyde — Undergoes the Cannizzaro reaction. The –CHO group is attached to a ring carbon that carries no hydrogen, so it gives benzyl alcohol and potassium benzoate:

      \( \displaystyle \ce{2C6H5CHO} \xrightarrow{\text{conc. }\ce{KOH},\ \Delta} \ce{C6H5CH2OH + C6H5COOK} \)

• Option (4) Acetaldehyde — Does not undergo the Cannizzaro reaction. It has three α-hydrogens, so it gives the aldol reaction, as shown above.

NCERT (2026-27 edition):
Chemical Reactions – “5. Other reactions
(i) Cannizzaro reaction: Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.”

Reactions due to α-hydrogen – “(i) 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.”