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Question:

The increasing order of the reactivity of the following halides for the SN1 reaction is:
CH3C(Cl)HCH2CH3 (I)
CH3CH2CH2Cl (II)
p-H3CO-C6H4-CH2Cl (III)

(II)<(I)<(III)

(I)<(III)<(II)

(III)<(II)<(I)

(II)<(III)<(I)

Solution:

The increasing order of the reactivity of the halides for the SN1 reaction is (II)<(I)<(III).
(II) is a primary alkyl halide.
(I) is a secondary alkyl halide.
Primary alkyl halides are less reactive towards SN1 reactions than secondary alkyl halides.
In (III), the carbocation generated is stabilized due to resonance. Hence, (III) is most reactive towards the SN1 reaction.