Arrange the folliwing halides in order of increasing `SN^(2)` and `SN^(1)` reactions.
I. `CH_(3)Cl` , II. `CH_(3)Br`
III. `CH_(3)CH_(2)Cl` , IV. `(CH_(3))_(2)CHCl`

Explanation

`(IV) lt (III) lt (I) lt (II)`
`SN^(2) = (CH_(3))_(2) CHCl lt CH_(3) CH_(2)Cl lt CH_(3)Cl lt CH_(3)Br`.
As the size of the alikyl group increases, the `SN^(2)` reactivity decreases. Further the `(C-Cl)` bond is stronger and more diffcult to cleave than the `(C_Br)` bond.
`SN^(2) = (CH_(3))_(2) CHCl lt CH_(3) CH_(2)Cl lt CH_(3)Br lt CH_(3)Cl`.
`(IV) gt (III) gt (II) gt (I)`