CH₃CHO is more reactive than CH₃COCH₃ towards reaction with HCN.Why ?

• The reaction between compounds having –C=O group and HCN proceeds faster or is more reactive when the groups associated with the carbonyl carbon tend to be less bulkier or the steric hindrance provided to the carbonyl carbon is lesser.

• Presence of more electron pushing groups (like –CH₃) with the carbonyl carbon, decreases the delta positive charge on the carbon atom.

• Due to less positive charge on the carbon atom, the association with the CN^- ion (of HCN) is slower, so the nucleophilic addition reaction happens slowly.

• In CH₃CHO, only one –CH₃ group is present (electron pushing group), whereas in CH₃COCH_3, two –CH₃ groups are present, and in this way due to lesser steric hindrance and less number of electron pushing groups, the reaction of CH₃CHO happens faster with HCN.