JEE Chemistry 2026 Question with Solution

Given: The reaction is

and we need the rate order with respect to the nucleophiles.

Find: The correct order of nucleophilicity for the given oxygen-based nucleophiles.

This is an $S_N2$ substitution on a methyl halide. In an $S_N2$ reaction, the rate depends directly on the nucleophilicity of the attacking species.

For nucleophiles attacking through the same atom, here oxygen, nucleophilicity follows the same general trend as basicity. A stronger base is a better nucleophile.

The stability of the anion controls its basicity. More stable anions are weaker bases and hence poorer nucleophiles.

1. $ClO_4^\ominus$ is extremely stable because of extensive resonance over four oxygen atoms. Its conjugate acid $HClO_4$ is a superacid, so $ClO_4^\ominus$ is a very poor nucleophile.
2. $CH_3COO^\ominus$ is resonance-stabilized, so it is less basic and less nucleophilic than hydroxide and phenoxide.
3. $PhO^\ominus$ is also resonance-stabilized, but its conjugate acid phenol has higher $pK_a$ than acetic acid, so phenoxide is a stronger base and better nucleophile than acetate.
4. ${}^\ominus OH$ has conjugate acid water with $pK_a \approx 14$, making it the strongest base among these and therefore the best nucleophile.

Hence, the nucleophilicity order is

Comparing the derived order

with the given options, it matches Option C.

Therefore, the correct option is C.