What is the product of the cross-Cannizzaro reaction between benzaldehyde and an excess of formaldehyde in the presence of concentrated ?
Answer and explanation
Correct answer: B. Benzyl alcohol and sodium formate
- Step 1: In a cross-Cannizzaro reaction between two different non-enolizable aldehydes, nucleophilic addition of hydroxide occurs preferentially at the more electrophilic carbonyl carbon.
- Step 2: Formaldehyde () is far more electrophilic than benzaldehyde () due to the absence of phenyl resonance and steric crowding.
- Step 3: Hydroxide attacks formaldehyde to form a tetrahedral dianion, which then transfers a hydride ion () to benzaldehyde, reducing benzaldehyde to benzyl alcohol while formaldehyde is oxidized to sodium formate.
Answer
Benzyl alcohol and sodium formate.
Why not the others: Benzoic acid and methanol inverts the redox roles of the two aldehydes; Benzyl alcohol and methanol would require reduction of both species; Sodium benzoate and sodium formate would require oxidation of both species without a hydride recipient.
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