Arrange the halide ions in order of increasing reactivity towards `CH_(3)Br(S_(N)2)` in gaseous medium and explain the order.

Explanation

Since solvation effect is absent in gaseous medium, the smaller anion (in which the negative charge is concentrated in small volume) is more reactive and acts as a stronger nucleophile than the larger anion. Therefore, the reactivity of halide ions towards `CH_(3)Br` in gaseous medium decreases in the order: `F^(Theta) < Cl^(Theta) < Br^(Theta) < I^(Theta)` (their size increases in the order: `F^(Theta) < Cl^(Theta) < Br^(Theta) < I^(Theta)`).