Give the most reactive substrate in each of the following pair with `overset (Ө) O H` ions under `SN^2` conditions.
(a) `C_2H_5 I` and `C_2H_5 Cl`
(b) `p-Br-C_6H_4CH_3` and `PhCH_2Br`
(c) `CH_3Cl` and `CH_2Cl_2`
(d)

Explanation

(a) `C_2H_5 I` (`I^(Ө)` is a good leaving group)
(b) `PhCH_2Br` (`Br^(Ө)` is lost easily, not attched to the ring)
But in the `p-Br-C_6H_4CH_3,Br^(Ө)` is not removed easily due to resonance stabilisation (`Br` is attached to the ring).
(c) `CH_(3) to Cl` (I+ effect makes more polarisation and Cl can be removed easily). but in `Cl larr CH_(2) -Cl`, due to -I effect of Cl, other Cl is not removed easily
(d) (`Cl` can be removed easily, no resonance stabilisation). But in Cl is difficult to remove due to the steric hindrance of two methyl groups.