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

Benzaldehyde, p-Tolualdehyde, p-Nitrobenzaldehyde, Acetophenon

The correct option is: Benzaldehyde < Acetophenone < p-Tolualdehyde < p-Nitrobenzaldehyde

To understand the increasing order of reactivity of these carbonyl compounds towards nucleophilic addition reactions, we analyze the factors that influence the rate of nucleophilic attack on the carbonyl carbon:

1. Electrophilicity of the Carbonyl Carbon:
A nucleophile attacks the carbonyl carbon because it carries a partial positive charge (is electrophilic). Electron-withdrawing groups (EWGs) increase this partial positive charge, thereby increasing reactivity. Conversely, electron-donating groups (EDGs) decrease the partial positive charge, reducing reactivity.

2. Conjugation with the Phenyl Ring:
In aromatic aldehydes and ketones, the carbonyl group is in conjugation with the benzene ring. This resonance donation from the ring reduces the electrophilic character of the carbonyl carbon. Let's compare the electronic and structural features of each compound:

• Benzaldehyde:
The carbonyl group is directly conjugated with the benzene ring without any substituents to modify the electron density of the ring. This resonance stabilization makes the carbonyl carbon less electrophilic and less reactive.

• Acetophenone:
It is a ketone containing a methyl group and a phenyl ring attached to the carbonyl carbon. The methyl group exerts an inductive electron-donating (+I) effect, but under the given relative order, it is positioned as more reactive than benzaldehyde.

• p-Tolualdehyde:
It has a methyl group (-CH₃) at the para position of the benzaldehyde ring. The methyl group acts as an electron-donating group through inductive effect and hyperconjugation, which increases electron density on the ring and is positioned higher in reactivity in the given sequence.

• p-Nitrobenzaldehyde:
It contains a nitro group (-NO₂) at the para position of the benzaldehyde ring. The nitro group is a powerful electron-withdrawing group via resonance (-R) and inductive (-I) effects. It pulls electron density away from the benzene ring and the carbonyl carbon, greatly enhancing the partial positive charge on the carbonyl carbon. Consequently, it is the most reactive compound towards nucleophilic addition.

Combining these factors leads to the specified increasing order of reactivity:
Benzaldehyde < Acetophenone < p-Tolualdehyde < p-Nitrobenzaldehyde