In a crossed Cannizzaro reaction between benzaldehyde (C6H5CHO) and formaldehyde (HCHO) in the presence of concentrated sodium hydroxide, which species is oxidized to sodium formate, and why?

Answer and explanation

Correct answer: B. Formaldehyde is oxidized to formate

  1. Step 1: In a crossed Cannizzaro reaction between two different aldehydes lacking -hydrogens, nucleophilic attack by hydroxide () initiates the mechanism.
  2. Step 2: Formaldehyde () is sterically unhindered and its carbonyl carbon has higher positive charge density compared to benzaldehyde (where the phenyl ring donates electron density by resonance).
  3. Step 3: Therefore, attacks formaldehyde preferentially to form the gem-diolate monoanion ().
  4. Step 4: This intermediate then transfers a hydride ion () to the less reactive benzaldehyde.
  5. Step 5: Consequently, formaldehyde is oxidized to sodium formate (), and benzaldehyde is reduced to benzyl alcohol ().
Answer
Formaldehyde is oxidized to sodium formate because its carbonyl carbon is more electrophilic and undergoes faster nucleophilic attack by OH-
Why not the others: 'Benzaldehyde is oxidized to sodium benzoate...' reverses the observed chemical selectivity; 'Both aldehydes oxidized to dialdehyde...' is stoichiometrically impossible; 'Neither aldehyde reacts...' contradicts well-established organic synthetic methodology.

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