Following is the list of four halides. Select correct sequence of decreasing order of reactivity for S_N1 reaction using the codes given below

I)         II) ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}-{\text{CH}}_2-\text{Br}$         III)

           

 IV) ${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}-{\text{CH}}_2-\text{I}$

The benzylic carbocation is most stable due to more resonating structures and allylic carbocations are more stable than aliphatic carbocation due to resonance so the order of stability of carbocations is benzyl > allyl > 3⁰>2⁰>1⁰.

3⁰ carbocation are more stable than others or stability order of carbocations is 3⁰>2⁰>1⁰ so order of alkyl halides towards SN¹ reaction is 3⁰>2⁰>1⁰.)

So order of reactivity for SN¹ mechanism is 
benzyl > allyl > 3⁰>2⁰>1⁰.