Question: Arrange the following in the increasing order of their reactivity towards nucleophilic addition reaction: $\text{CH}_3\text{COCH}_3, \text{CH}_3\text{CHO}, \text{HCHO}, \text{C}_6\text{H}_5\text{COCH}_3$

Step 1 (Governing Principle): [0.25 Mark]
The reactivity of carbonyl compounds towards nucleophilic addition is governed by two factors:

• Steric Hindrance: As the size of alkyl/aryl groups attached to the carbonyl carbon increases, the approach of the nucleophile is hindered.
• Electronic Effect: Alkyl groups exert a $+I$ (inductive) effect, which decreases the electrophilicity of the carbonyl carbon. Aryl groups ($\text{C}_6\text{H}_5-$) provide resonance stabilization to the carbonyl group, further reducing its reactivity.

Step 2 (Step-by-Step Analysis): [0.5 Mark]
Comparing the structures:

• $\text{HCHO}$ (Formaldehyde): No alkyl groups attached; least steric hindrance and highest electrophilicity.
• $\text{CH}_3\text{CHO}$ (Acetaldehyde): One methyl group attached; moderate steric hindrance and $+I$ effect.
• $\text{CH}_3\text{COCH}_3$ (Acetone): Two methyl groups attached; higher steric hindrance and $+I$ effect.
• $\text{C}_6\text{H}_5\text{COCH}_3$ (Acetophenone): One bulky phenyl group attached; maximum steric hindrance and resonance stabilization of the carbonyl carbon.

Step 3 (Final Concluded Answer): [0.25 Mark]
Arranging in increasing order of reactivity: $$\text{C}_6\text{H}_5\text{COCH}_3 < \text{CH}_3\text{COCH}_3 < \text{CH}_3\text{CHO} < \text{HCHO}$$

💡 EL10 Concept Insight: Think of the carbonyl carbon as a target for a nucleophile; if the target is surrounded by big, bulky groups (like phenyl or methyl), the nucleophile can't get close enough to hit it, making the molecule less reactive.