Question

Aldehydes with strongly electron donating groups, such as p-dimethylaminobenzaldehyde, or with strongly electron withdrawing groups, such as p-nitrobenzaldehyde, fail to undergo self-condensation in the presence of cyanide ion. However such aldehydes will undergo mixed condensations with benzaldehyde. Explain.

Answer 1

Solution :-

The both p-nitrobenzaldehyde and p-dimethylaminobenzaldehyde when reacts with the CN- ion and forms the anion then this anion is very unstable because in the p-dimethylaminobenzaldehyde the nitrogen is already having lone pair of electrons and it shows the electron donating effect so it destabilizes the anion formed after removing the proton

And in p-nitrobenzaldehyde the anion formed after the removal of the proton is undergoes the resonance changes and forms two oxygen anions because nitro group is electron withdrawing group

Therefore these two compounds cannot undergo the self condensation reaction.

But when they react with benzaldehyde then benzaldehyde forms stable anion which can attack the carbonyl groups of these two compounds to form the cross condensed products.

Following image shows the structures of the anions formed.