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

The question asks about the relative reactivity of different carbonyl compounds in a nucleophilic addition reaction. Understanding the electronic effects and steric factors that influence the reactivity is essential in predicting the outcome. 

The general order of reactivity of carbonyl compounds towards nucleophilic addition is influenced by two main factors:

• Electronic effects: Electron-withdrawing groups enhance the electrophilicity of the carbonyl carbon, thereby increasing its reactivity.
• Steric effects: Smaller groups on the carbonyl carbon make it more accessible to nucleophiles, enhancing reactivity.

Let's analyze each compound:

1. Acetophenone: Contains a phenyl group which is electron-donating via resonance, reducing the electrophilicity of the carbonyl carbon. Moreover, the bulkiness of the phenyl group can cause steric hindrance, further reducing reactivity.
2. p-Tolualdehyde: Has a methyl group which is slightly electron-donating, but since the carbonyl group is still more accessible than in acetophenone, it is more reactive.
3. Benzaldehyde: Lacks additional bulky substitutions and electron-donating groups compared to acetophenone, thus increasing its reactivity compared to both acetophenone and p-tolualdehyde.
4. p-Nitrobenzaldehyde: Contains a nitro group, which is a strong electron-withdrawing group. This significantly enhances the electrophilicity of the carbonyl carbon, making it the most reactive among the given compounds.

Therefore, considering these effects, the correct order of reactivity for nucleophilic addition reactions is:

\(\text{acetophenone} < \text{p-tolualdehyde} < \text{benzaldehyde} < \text{p-nitrobenzaldehyde}\)

This matches with the given correct option:

acetophenone \(<\) p-tolualdehyde \(<\) benzaldehyde \(<\) p-nitrobenzaldehyde 
 

Answer 2

Given: The reactivity of aldehydes and ketones in nucleophilic addition reactions is influenced by the electron-withdrawing or electron-donating effects of substituents attached to the aromatic ring or carbonyl group.

General Trend in Reactivity

- 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.

Analysis of the Options

1. Benzaldehyde

- Benzaldehyde has no electron-donating or electron-withdrawing groups attached to the benzene ring. - It has moderate reactivity towards nucleophiles.

2. Acetophenone

- The \( -CH_3 \) group is an electron-donating group, which decreases the electrophilicity of the carbonyl carbon. - Therefore, acetophenone has lower reactivity compared to benzaldehyde.

3. p-Tolualdehyde

- The \( -CH_3 \) group is an electron-donating group, but less so than in acetophenone. - It lowers the electrophilicity of the carbonyl group but is more reactive than acetophenone.

4. p-Nitrobenzaldehyde

- The \( -NO_2 \) group is an electron-withdrawing group, which significantly increases the electrophilicity of the carbonyl carbon. - This makes p-nitrobenzaldehyde highly reactive towards nucleophiles.

Reactivity Order

Based on the electron-donating and electron-withdrawing effects of the substituents, the reactivity order is: \[ \text{Acetophenone} < \text{p-Tolualdehyde} < \text{Benzaldehyde} < \text{p-Nitrobenzaldehyde}. \]

Final Answer:

The correct answer is \( \boxed{(3)} \), which corresponds to **p-Tolualdehyde**.