Question

$2 C _6 H _5 CHO \xrightarrow[H _2 O ] { OH ^{-}} C _6 H _5 CH _2 OH + C _6 H _5 COO ^{-}$
Which of the following statements are correct regarding the above reduction of benzaldehyde to benzyl alcohol?
(i) One hydrogen is coming from $H _2 O$ as $H ^{+}$and another from $C _6 H _5 CHO$ as $H ^{-}$
(ii) One hydrogen is coming from $H _2 O$ as $H ^{-}$and another from $C _6 H _5 CHO$ as $H ^{+}$
(iii) Onehydrogen from $H _2 O$ and another from $C _6 H _5 CHO$, both in the form of $H ^{-}$
(iv) The reduction is an example of disproportionation reaction

• A. (i), (ii) and (iii)
• B. (i) and (iv)
• C. (ii), (iii) and(iv)
• D. (iii) and (iv)

Answer 1

Answer: (B)

The hydrogen atom that is added to the carbonyl carbon of the aldehyde in the reduction is derived directly from the other aldehyde molecule as a hydride ion. The second hydrogen that is added to the negatively charged oxygen is coming from the solvent (consult mechanism of Cannizzaro reaction). Oxidation of one molecule of the compound at the expense of other molecule of the same compound is known as disproportionation.