Question

The double bond in aldehydes and ketones is reactive towards nucleophilic reagents like CN - whereas that in an alkene is not.

Answer 1

The carbonyl group in aldehydes and ketones add on $C{N}^{-}$ resulting in the formation of an anion where the negative charge resides on oxygen. However if a nucleophile adds on to an alkene the negative charge resides on carbon. Since carbon is much less strongly electron attracting than oxygen, this species is less stable and hence not readily formed.