What is the order of reactivity of (I)HCHO (II) CH₃CHO (III) C₆H₅CHO towards nucleophile? 

I > II > III

The correct answer is option 4. I > II > III.

The reactivity of aldehydes towards nucleophiles is influenced by the nature of the substituents attached to the carbonyl carbon. Let's break down the reactivity of each aldehyde provided:

1. HCHO (Formaldehyde):

Formaldehyde has no substituents attached to the carbonyl carbon, meaning it has only hydrogen atoms. These hydrogen atoms are weakly electron-donating. In the absence of electron-withdrawing groups, the carbonyl carbon in formaldehyde has relatively high electron density, making it highly reactive towards nucleophiles. The absence of bulky substituents also allows for easy access of nucleophiles to the carbonyl carbon, further enhancing its reactivity.

2. \(CH_3CHO\) (Acetaldehyde):

Acetaldehyde has a methyl group \((–CH_3)\) attached to the carbonyl carbon. The methyl group is weakly electron-donating due to its +I effect. Although the methyl group donates some electron density to the carbonyl carbon, it is less electron-donating than hydrogen atoms. As a result, the carbonyl carbon in acetaldehyde has slightly lower electron density compared to formaldehyde, resulting in lower reactivity towards nucleophiles.

3. \(C_6H_5CHO\) (Benzaldehyde):

Benzaldehyde has a phenyl group \((–C_6H_5)\) attached to the carbonyl carbon. The phenyl group is electron-withdrawing due to resonance effects. The π-electrons in the benzene ring can delocalize into the carbonyl group, reducing the electron density at the carbonyl carbon. This reduction in electron density decreases the reactivity of the carbonyl carbon towards nucleophiles compared to formaldehyde and acetaldehyde. Additionally, the bulkiness of the phenyl group may hinder the approach of nucleophiles, further reducing the reactivity.

Formaldehyde \((HCHO)\) is the most reactive towards nucleophiles due to the absence of electron-withdrawing substituents and the presence of electron-donating hydrogen atoms.

Acetaldehyde \((CH_3CHO)\) is less reactive than formaldehyde due to the presence of a weakly electron-donating methyl group.

Benzaldehyde \((C_6H_5CHO)\) is the least reactive towards nucleophiles among the options provided due to the electron-withdrawing nature of the phenyl group attached to the carbonyl carbon.

Therefore, the correct order of reactivity towards nucleophiles is:
\(\text{HCHO} (I) > \text{CH}_3\text{CHO} (II) > \text{C}_6\text{H}_5\text{CHO} (III)\)