Write a note on the addition of hydrogen cyanide to aldehydes and ketones.

(N/A) Aldehydes and ketones react with hydrogen cyanide $(HCN)$ to yield cyanohydrins.
$(i)$ This reaction occurs very slowly with pure $HCN$.
$(ii)$ Therefore,it is catalysed by a base. The generated cyanide ion $(CN^-)$,being a stronger nucleophile,readily adds to the carbonyl carbon of the aldehyde or ketone to yield the corresponding cyanohydrin.
$(iii)$ The reaction mechanism is as follows:
$HCN + OH^- \rightleftharpoons :CN^- + H_2O$
$R_2C=O + :CN^- \rightleftharpoons R_2C(O^-)(CN)$ (Tetrahedral intermediate)
$R_2C(O^-)(CN) + H_2O \rightleftharpoons R_2C(OH)(CN) + OH^-$
$(iv)$ Note: The carbonyl carbon in the reactant is $sp^2$ hybridized,while in the cyanohydrin product,it becomes $sp^3$ hybridized. This is a nucleophilic addition reaction.