Question

The correct order of reactivity of CH$_3$Br in methanol with the following nucleophiles is
$ \mathrm{F^- ,\ I^- ,\ C_2H_5O^- \ and\ C_6H_5O^- }$

• A. $I^->C_2H_5O^->F^->C_6H_5O^-$
• B. $I^->C_6H_5O^->F^->C_2H_5O^-$
• C. $I^->F^->C_6H_5O^->C_2H_5O^-$
• D. $I^->C_2H_5O^->C_6H_5O^->F^-$

Hint:

In polar protic solvents, nucleophilicity of anions depends more on solvation than basicity.

Answer 1

The Correct Option is C

To determine the order of reactivity of CH₃Br with different nucleophiles in methanol, we need to consider the nature and strength of each nucleophile. The given nucleophiles are F^-, I^-, C₂H₅O^-, and C₆H₅O^-.

1. Nucleophilic strength: Nucleophilicity is generally influenced by the charge, electronegativity, steric effects, and the solvent used. In polar protic solvents like methanol, larger ions are more nucleophilic because they are less solvated than smaller ones.
2. Analyzing the nucleophiles:
   • \(I^-\): This is a large ion with low solvation energy, making it a strong nucleophile in methanol.
   • \(F^-\): It is a small ion and highly solvated, making it a weaker nucleophile in polar protic solvents.
   • \(C_2H_5O^-\): Ethoxide ion is a strong nucleophile in most conditions, but its reactivity can be affected by steric hindrance and solvation.
   • \(C_6H_5O^-\): Phenoxide ion has resonance stabilization with the phenyl ring, making it less nucleophilic compared to aliphatic alkoxide like ethoxide.
3. Reactivity Order: Based on the above analysis, we consider the nucleophiles' size and basicity, concluding the order to be: 
   \(I^- > F^- > C_6H_5O^- > C_2H_5O^-\)
   This order reflects how I^-, being the largest and least solvated, is the strongest nucleophile, followed by F^-, despite being highly solvated, and finally contrasting the resonance effects in C₆H₅O^- with steric hindrance in C₂H₅O^-.

Thus, the correct answer is: \(I^- > F^- > C_6H_5O^- > C_2H_5O^-\).