Q 2.1 Benz aldehyde and hydrogen cyanide fail to react at pH=5.6 and slowly at pH=7. However, the reaction occurs spontaneously at pH>7. Discuss these observations with the aid of a detailed mechanism. (6)
Rate law = [tex]k[benzaldehyde][CN^{-}][/tex], where k is rate constant and species inside third bracket represents concentrations.
Clearly, rate depends upon concentration of free [tex]CN^{-}[/tex]
In acidic medium (pH = 5.6), concentration of free [tex]CN^{-}[/tex] is negligible as HCN is a weak acid. So, reaction between benzaldehyde and HCN does not occur at all.
In neutral medium (pH = 7), concentration of cyanide ion somewhat increases. Therefore reaction is slow in neutral pH
In basic medium (pH > 7), concentration of cyanide ions become sufficient to make the reaction spontaneous. In basic condition, concentration of cyanide increases due to increase in ionization of HCN
