For (i) \(I^-\) (ii) \(Cl^-\) (iii) \(Br^-\), the usual increasing order of nucleophilicities would be
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(Cl^- \lt Br^- \lt I^-\)
Nucleophilicity is the rate at which a species attacks an electron-poor carbon. For halide ions in the usual protic solvents (water, alcohols) it increases down the group:
A larger ion holds its outer electrons less tightly; its electron cloud is more polarisable and can begin bonding to carbon from farther away.
A small ion such as \( \ce{Cl^{-}} \) has a high charge density and is strongly solvated (hydrogen bonded) by protic solvent molecules, which must be stripped off before it can attack; the large \( \ce{I^{-}} \) is weakly solvated.
Size: \( \ce{Cl^{-}} \) < \( \ce{Br^{-}} \) < \( \ce{I^{-}} \), so the usual increasing order of nucleophilicity is Cl− < Br− < I−.
Note: The question is from beyond the NCERT textbook.