In the following pairs of halogen compounds which is faster undergoing `S_(N)2` reaction?

Explanation

(i) is a primary alkyl halide while is a secondary alkyl halide, therefore, reacts faster than in a `S_(N)2` reaction.
(iii) Since `I^(-)` ion because of its bigger size and lower electronegativity is a better leaving group than `Cl^(-)` ion, therefore, reacts faster than in a `S_(N)2` reaction.