Which of the following does not undergo the Cannizzaro reaction?

The correct answer is Option (4) → Acetaldehyde

The Cannizzaro reaction is shown 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 is reduced to the alcohol and another is oxidised to the carboxylic acid salt.

Option (1) 1-Methylcyclohexanecarbaldehyde — Undergoes the Cannizzaro reaction. The –CHO group is on ring carbon 1, which also carries the methyl group and two ring bonds, so this α-carbon has no hydrogen.

Option (2) Benzaldehyde — Undergoes the Cannizzaro reaction. The α-carbon is a ring carbon of benzene with no hydrogen.

    \( \displaystyle \ce{2C6H5CHO + KOH} \xrightarrow{\Delta} \ce{C6H5CH2OH + C6H5COOK} \)

Option (3) Formaldehyde — Undergoes the Cannizzaro reaction. \( \ce{HCHO} \) has no α-carbon at all, so no α-hydrogen.

    \( \displaystyle \ce{2HCHO + KOH} \xrightarrow{\Delta} \ce{CH3OH + HCOOK} \)

Option (4) Acetaldehyde — Does not undergo the Cannizzaro reaction. In \( \ce{CH3CHO} \) the methyl group carries three α-hydrogens; with dilute alkali it gives the aldol reaction instead.

NCERT (2026-27 edition):
8.4 Chemical 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.”