Question

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)

Answer 1

Answer:

Explanation has been given below

Explanation:

• Rate law = $k[benzaldehyde][CN^{-}]$,  where k is rate constant and species inside third bracket represents concentrations.
• Clearly, rate depends upon concentration of free $CN^{-}$

• In acidic medium (pH = 5.6), concentration of free $CN^{-}$ 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
• Full reaction mechanism has been shown below.