Why does pure `HCN` fail to react with aldehydes and ketones ?

Explanation

`HCN` is mainly a covalent compound and can not provides `CN^(-)` ion readily for the nucleophilic attack on the carbonyl compound. The reaction is carried in the presence of base which helps in the ionisation of the acid to liberate `CN^(-)` ion.
`HCN + OH^(-) rarr H_(2)O + CN^(-)`