Question

{The correct order of the rate of the reaction for the following reaction with respect to nucleophiles is:} \[ \mathrm{CH_3Br + Nu^- \rightarrow CH_3Nu + Br^-} \]

• A. \( \mathrm{PhO^- > OH^- > CH_3COO^- > ClO_4^-} \)
• B. \( \mathrm{ClO_4^- > CH_3COO^- > OH^- > PhO^-} \)
• C. \( \mathrm{CH_3COO^- > PhO^- > OH^- > ClO_4^-} \)
• D. \( \mathrm{OH^- > PhO^- > CH_3COO^- > ClO_4^-} \)

Hint:

For oxygen nucleophiles: \[ \text{Less resonance stabilization} \Rightarrow \text{Greater nucleophilicity} \] Thus: \[ \mathrm{OH^- > PhO^- > CH_3COO^- > ClO_4^-} \]

Answer 1

The Correct Option is D

Concept: The reaction shown is an \(S_N2\) nucleophilic substitution reaction. Rate depends on nucleophilicity of the attacking nucleophile.

• Greater electron density implies stronger nucleophile.
• Resonance decreases nucleophilicity.
• Stable ions are weaker nucleophiles.

Step 1: Compare \( \mathrm{OH^-} \) Hydroxide ion has high electron density and no resonance stabilization. Thus it is a very strong nucleophile.

Step 2: Compare \( \mathrm{PhO^-} \) Phenoxide ion undergoes resonance stabilization. Hence electron density is delocalized and nucleophilicity decreases compared to \( \mathrm{OH^-} \).

Step 3: Compare \( \mathrm{CH_3COO^-} \) Acetate ion is strongly resonance stabilized. Thus it is weaker than phenoxide ion.

Step 4: Compare \( \mathrm{ClO_4^-} \) Perchlorate ion is highly resonance stabilized and extremely stable. Hence it is the weakest nucleophile. Therefore, the correct order is \[ \mathrm{OH^- > PhO^- > CH_3COO^- > ClO_4^-} \]