Which of the following compounds will not give Cannizzaro reaction?

The correct answer is Option (3) → phenylacetaldehyde, C₆H₅CH₂CHO (structure 3)

The Cannizzaro reaction (disproportionation with concentrated alkali) is given only by aldehydes that have no α-hydrogen. An aldehyde with α-hydrogens forms an enolate with alkali and undergoes aldol condensation instead.

• Option (1) benzaldehyde — Gives the Cannizzaro reaction. The CHO group is on a ring carbon with no hydrogen, so there is no α-hydrogen.

• Option (2) 3-(dimethylamino)benzaldehyde — Gives the Cannizzaro reaction. The CHO group is again on a ring carbon; the –\( \ce{N(CH3)2} \) group elsewhere on the ring adds no α-hydrogen.

• Option (3) phenylacetaldehyde — Does not give the Cannizzaro reaction. In \( \ce{C6H5-CH2-CHO} \) the carbon next to CHO is a \( \ce{CH2} \) group with two α-hydrogens, so it gives the aldol reaction:

      \( \displaystyle \ce{2C6H5CH2CHO} \xrightarrow{\text{dil. NaOH}} \ce{C6H5CH2-CH(OH)-CH(C6H5)-CHO} \)

• Option (4) pyridine-4-carbaldehyde — Gives the Cannizzaro reaction. The CHO group is on a ring carbon of pyridine, so it has no α-hydrogen.

NCERT (2026-27 edition):
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