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

Step 1: Understanding the Concept:
The given equation describes a classical bimolecular nucleophilic substitution ($\text{S}_{\text{N}}2$) pathway on a primary alkyl halide ($\text{CH}_3\text{Br}$). The rate of an $\text{S}_{\text{N}}2$ reaction depends heavily on the nucleophilicity of the attacking species ($\text{Nu}^-$). When the attacking atom is identical (oxygen, in this case), nucleophilicity correlates inversely with the stability of the negative charge (its conjugate basicity). A stronger, less stable base is a more effective nucleophile.
Step 2: Key Formula or Approach:
1. Identify the corresponding conjugate acids for each nucleophilic anion: $\text{OH}^- \rightarrow \text{H}_2\text{O}$, $\text{PhO}^- \rightarrow \text{PhOH}$ (phenol), $\text{CH}_3\text{COO}^- \rightarrow \text{CH}_3\text{COOH}$ (acetic acid), and $\text{ClO}_4^- \rightarrow \text{HClO}_4$ (perchloric acid). 2. Rank the acidic strengths of these conjugate acids. 3. Use the inverse relationship rule: $\text{Stronger Acid} \rightarrow \text{Weaker Conjugate Base} \rightarrow \text{Poorer Nucleophile}$.
Step 3: Detailed Explanation:
Let's analyze the stability of the negative charge on each oxygen atom by examining their conjugate acids and resonance behaviors: - $\text{HClO}_4$ (perchloric acid) is a renowned mineral superacid. The negative charge in its conjugate base ($\text{ClO}_4^-$) is extensively delocalized via resonance across four highly electronegative oxygen atoms, making it incredibly stable and exceptionally weak as a nucleophile. - $\text{CH}_3\text{COOH}$ (acetic acid) is a moderately weak organic acid. Its conjugate base ($\text{CH}_3\text{COO}^-$) stabilizes its negative charge by resonance over two equivalent oxygen atoms. - $\text{PhOH}$ (phenol) is a weaker acid than acetic acid. The negative charge on the phenoxide ion ($\text{PhO}^-$) is delocalized into the aromatic carbon ring. Because carbon is less electronegative than oxygen, this resonance stabilization is less effective than that in acetate, making $\text{PhO}^-$ a stronger base and a superior nucleophile compared to $\text{CH}_3\text{COO}^-$. - $\text{H}_2\text{O}$ (water) is the weakest acid in this sequence. The negative charge on the hydroxide ion ($\text{OH}^-$) is completely localized on a single oxygen atom with no resonance stabilization, making it the strongest base and the most reactive nucleophile. Arranging the conjugate acids by increasing acidity gives: \[ \text{H}_2\text{O}<\text{PhOH}<\text{CH}_3\text{COOH}<\text{HClO}_4 \] Inverting this sequence gives the correct order of basicity and nucleophilic reactivity: \[ \text{OH}^->\text{PhO}^->\text{CH}_3\text{COO}^->\text{ClO}_4^- \]
Step 4: Final Answer:
The correct order of the rate of reaction is $\text{OH}^->\text{PhO}^->\text{CH}_3\text{COO}^->\text{ClO}_4^-$.