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

1.  I < II < III 

2.  II <I < III

3.  II < III < I 

4.  III < II < I 

Reaction of the given compounds with HBr/HCl is a nucleophilic substitution reaction. It follows SN1mechanism. SN1 mechanism depends upon the stability of carbocation.

Presence of electron withdrawing group decreases the stability of carbocation. In compound (Il) and (Ill) EWG is present at para position

Since, -NO2 group is a stronger EWG than —Cl.  NO2-C6H5 - \(CH_{2}^{+}\)

carbocation will be less stable than Cl—              +
C6H5-CH2 carbocation.
Thus, the order of stability of carbocation isncert-exemplar-problems-class-12-chemistry-alcohols-phenols-ethers-16

Therefore, compound (Il) is least reactive and correct option is (III).