Question

Which of the following arrangements with respect to their reactivity in nucleophilic addition reaction is correct?

• A. benzaldehyde \(<\) acetophenone \(<\) p-nitrobenzaldehyde \(<\) p-tolualdehyde
• B. acetophenone \(<\) benzaldehyde \(<\) p-tolualdehyde \(<\) p-nitrobenzaldehyde
• C. acetophenone \(<\) p-tolualdehyde \(<\) benzaldehyde \(<\) p-nitrobenzaldehyde
• D. p-nitrobenzaldehyde \(<\) benzaldehyde \(<\) p-tolualdehyde \(<\) acetophenone

Hint:

In nucleophilic addition reactions, electron-withdrawing groups increase the reactivity of carbonyl compounds, while electron-donating groups decrease their reactivity. This principle is crucial in determining the reactivity order of different compounds.

Answer 1

The Correct Option is C

In nucleophilic addition reactions, the reactivity of aldehydes and ketones is affected by the electron-withdrawing or electron-donating effects of substituents attached to the aromatic ring or carbonyl group. 
The general trend in reactivity is as follows:
- Electron-withdrawing groups (such as \( -NO_2 \)) attached to the aromatic ring increase the electrophilicity of the carbonyl carbon, thus increasing reactivity towards nucleophiles.
- Electron-donating groups (such as \( -CH_3 \)) decrease the electrophilicity of the carbonyl carbon, decreasing reactivity towards nucleophiles.
Now, let's examine the options:
1. \( \text{Benzaldehyde} \): No electron-donating or electron-withdrawing groups are attached to the benzene ring. It has moderate reactivity.
2. \( \text{Acetophenone} \): The \( -CH_3 \) group is an electron-donating group, decreasing the electrophilicity of the carbonyl carbon, so its reactivity is lower than that of benzaldehyde.
3. \( \text{p-Tolualdehyde} \): The \( -CH_3 \) group (electron-donating) attached to the benzene ring lowers the electrophilicity of the carbonyl group, but less than acetophenone. It is more reactive than acetophenone.
4. \( \text{p-Nitrobenzaldehyde} \): The \( -NO_2 \) group (electron-withdrawing) significantly increases the electrophilicity of the carbonyl carbon, making it highly reactive in nucleophilic addition reactions.
Thus, the reactivity order is: \[ \text{Acetophenone} < \text{p-Tolualdehyde} < \text{Benzaldehyde} < \text{p-Nitrobenzaldehyde} \] Final Answer: The correct answer is (3).