Question

In the following compounds, what is the increasing order of their reactivity towards nucleophilic addition reactions?

• A. Benzaldehyde $<$ p-Tolualdehyde $<$ p-Nitrobenzaldehyde $<$ Acetophenone
• B. Acetophenone $<$ Benzaldehyde $<$ p-Tolualdehyde $<$ p-Nitrobenzaldehyde
• C. Benzaldehyde $<$ Acetophenone $<$ p-Tolualdehyde $<$ p-Nitrobenzaldehyde
• D. Acetophenone $<$ p-Tolualdehyde $<$ Benzaldehyde $<$ p-Nitrobenzaldehyde

Answer 1

The Correct Option is D

Nucleophilic addition reactivity in carbonyl compounds is primarily governed by two factors: electron withdrawal and steric hindrance. Electron-withdrawing substituents enhance reactivity by increasing the carbonyl carbon's electrophilicity, whereas steric bulk diminishes it. The following compounds were examined:

Compound	Factors Influencing Reactivity
Acetophenone	The presence of an electron-donating alkyl group reduces the carbonyl carbon's electrophilicity, thus lowering reactivity.
p-Tolualdehyde	A methyl group on the benzene ring exhibits a slight electron-donating effect, resulting in lower reactivity than benzaldehyde but higher than acetophenone.
Benzaldehyde	In the absence of additional electron-donating groups, it is more reactive than acetophenone and p-tolualdehyde, but less reactive than p-nitrobenzaldehyde.
p-Nitrobenzaldehyde	The nitro group acts as a potent electron-withdrawing substituent, leading to the highest increase in the carbonyl carbon's reactivity.

Consequently, the compounds exhibit an increasing order of reactivity towards nucleophilic addition reactions as follows:

Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde.