Arrange the following carboxyl compounds in increasing order of their reactivity in nucleophilic addition reactions. Explain with proper reasoning : Benzaldehyde. p-tolualdeyde, p-nitrobenzaldehyde, Acetophenone.
1 Answer
Answer :
Ans. Acetophenone is a ketone while all others are aldehydes, therefore it is least reactive. In p-tolualdehyde, there is methyl group (CH3 ) at para position w.r.t. to the carboxyl gp, which increases electron density on the carbon of the carboxyl gp by hyperconjugation effect thereby making it less reactive than benzaldehyde.
On the other hand, in p-nitrobenzaldehyde, the NO2 gp is a powerfuil electronwithdrawing gp. It withdraws electrons both by inductive and resonance effect thereby decreasing the electron density on the carbon atom of carboxyl gp. This facilitates the attack of the nucleophile and hence makes it more reactive than benzaldehyde.
Therefore, the overall order of increasing reactivity : acetophenone < p-tolualdehyde < benzaldehyde < p-nitrobenzaldehyde.