Arrange the following in increasing order of their reactivitty in nucleophilic addition reactions : (i) Ethananl, Propanal, Propanone, Butanone. (ii) Benzaldehyde, p - Tolualdehyde, p - Nitrobenzaldehyde, Acetophenone. [Hint : Consider steric effect and electronic effect.]

Arrange the following in increasing order of their reactivitty in nucl

1.	Arrange the following in increasing order of their reactivitty in nucleophilic addition reactions : (i) Ethananl, Propanal, Propanone, Butanone. (ii) Benzaldehyde, p - Tolualdehyde, p - Nitrobenzaldehyde, Acetophenone. [Hint : Consider steric effect and electronic effect.]
Answer» Solution : (i) As we move from `"ethanal " to "propanal " to "propanone " to " butanone"`, the inductive effect of the alkyl GROUP increases. As a result, electron density on the carbon atom of the carbonyl group progressively decreases and hence attack by the nucleophile BECOMES weaker and weaker. Thus, the reactivity increases in the order : butanone (ii) Acetophenone is a ketone, while all others are aldehydes. Therefore, acetophenone is the least reactive. In p-tolualdehyde, there is a `CH_3` group at the p-position w.r.t. to the carbonyl group, which increases the electron density on the carbon of the carbonyl group by hyperconjugation effect thereby making it less reactive than BENZALDEHYDE. On the other hand, in p- nitrobenzaldehyde, the `NO_2` group is a powerful electron-withdrawing group. It withdraws electrons, by RESONANCE thereby decreasing the electron-density on the carbon atom of the carbonyl group. This facilitates the attack of the nucleophile and hence makes it more reactive than benzaldehyde. Therefore, the reactivity of the given compounds increases in the order: Acetophenone < p-tolualdehyde < benzaldehyde < p-nitrobenzaldehyde.