Aldehydes And Ketones - Cannizzaro Reaction

The Cannizzaro reaction is a base-induced disproportionation restricted to aldehydes that have no alpha-hydrogen, in which one molecule is oxidised to a carboxylate while another is reduced to an alcohol. Benzaldehyde has its carbonyl directly on the ring and possesses no alpha-hydrogen, so with concentrated alkali it gives benzoate ion and benzyl alcohol, making it the correct choice. Ethanal is excluded because it carries three alpha-hydrogens and therefore prefers aldol condensation. Propanone is a ketone and does not undergo Cannizzaro at all, and although it lacks the relevant reactive feature it also is not an aldehyde. Phenylethanal has a CH2 group bearing alpha-hydrogens adjacent to the carbonyl, so it too undergoes aldol rather than Cannizzaro. This selectivity, driven by the absence of alpha-hydrogen, is emphasised in NCERT and is a standard JEE Advanced discrimination question. Mechanistically the strongly basic hydroxide adds to one aldehyde to form a tetrahedral alkoxide that then transfers a hydride ion to a second aldehyde molecule, which is the actual reduction step, while the first molecule becomes the carboxylate. Other examples that obey this rule include formaldehyde and 2,2-dimethylpropanal, both lacking alpha-hydrogens. As a plausibility check, the defining requirement of no alpha-hydrogen is satisfied only by benzaldehyde among the four, confirming the answer.