Mark the correct increasing order of reactivity of the following compounds with HBr / HCl.

(a) I < II < III
(b) II < I < III
(c) II < III < I
(d) III < II < I

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Solution

Reaction of the given compounds with HBr/HCI is a nucleophilic substitution reaction. It follows $S_{N}^{1}$ mechanism. $S_{N}^{1}$ mechanism depends upon the stability of carbocation. Presence of electron withdrawing group decreases the stability of carbocation. In compound (II) and (III) EWG is present at para position.
Since, - $N O_{2}$ group is a stronger EWG than - CI. So, $N O_{2} - C_{6} H_{5} - + C H_{2}$ carbocation will be less stable than $C l - C_{6} H_{5} - + C H_{2}$ carbocation.
Thus, the order of stability of carbocation is

Therefore, compound (II) is least reactive and correct option is(c).

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