Question

(i) Why is $H_{2}CO$ always oxidized in the crossed-Cannizzaro reactions in previous problem?
(ii) Give the products of the Cannizzaro reaction of $PhCOCHO$ (A) and $PhCOC{HCl}_2$ (B) and explain their formation.

Answer 1

1. since $H_{2}O$ is the most reactive aldehyde, it exists in aqueous $O{H}^{-}$ solution mainly as the conjugate base of its hydrate , $H_{2}C\left(OH\right)O^{-}$. there is also a statistical factor, because $H_{2}O$ has two H'S available for transfer the other aldehyde hydrate anion has only one.

2. there are internal crossed-cannizzaro reactions both A and B give $PhCH\left(OH\right)CO{O}^{-}$ .in b, prior alkaline hydrolysis gives compound A. The CHO is oxidized because it has the aldehydic H needed for hydrate transfer Keto groups can only be reduced , not oxidized ,in crossed-cannizzaro reactions.