Arrange the following species in decreasing order of their nucleophilicity in a polar protic solvent: (I) CH ₃ CH ₂ S ⁻ (II) CH ₃ CH ₂ O ⁻ (III) CH ₃ CO ₂⁻

Step-by-step solution

In a polar protic solvent, nucleophilicity depends on the polarizability and the extent of solvation of the ion. Larger atoms are less solvated by hydrogen bonding and are more polarizable, making them better nucleophiles. Since sulphur is larger than oxygen, CH ₃ CH ₂ S ⁻ (I) is a stronger nucleophile than the oxygen-containing species (II) and (III). Between CH ₃ CH ₂ O ⁻ (II) and CH ₃ CO ₂⁻ (III), the negative charge in CH ₃ CO ₂⁻ is delocalized over two oxygen atoms due to resonance, which decreases its electro