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(B) The rate of $S_N1$ reaction depends on the stability of the carbocation intermediate formed after the departure of the leaving group $(Br^-)$.In a

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$iv > iii > ii > i$

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(B) The rate of $S_N1$ reaction depends on the stability of the carbocation intermediate formed after the departure of the leaving group $(Br^-)$.In all these cases,a benzylic carbocation is formed: $Ar-CH_2^+$.The stability of the benzylic carbocation is increased by electron-donating groups $(EDG)$ at the para position due to the $+I$ effect and hyperconjugation.The order of electron-donating ability of the alkyl groups is: isopropyl $(>CH(CH_3)_2)$ > ethyl $(-CH_2CH_3)$ > methyl $(-CH_3)$ > hydrogen $(-H)$.- Compound (iv) has an isopropyl group,which provides the strongest $+I$ effect and hyperconjugation,making the carbocation most stable.- Compound (iii) has an ethyl group,which is the next most effective electron donor.- Compound (ii) has a methyl group,which is the least effective electron donor among the alkyl groups.- Compound $(i)$ has no electron-donating group.Therefore,the stability of the carbocations and the rate of $S_N1$ reaction follow the order: $(iv) > (iii) > (ii) > (i)$.

Arrange the following compounds in order of decreasing rate of hydrolysis for $S_N1$ reaction:
$(i)$ $C_6H_5CH_2Br$
(ii) $p-CH_3-C_6H_4-CH_2Br$
(iii) $p-CH_3CH_2-C_6H_4-CH_2Br$
(iv) $p-(CH_3)_2CH-C_6H_4-CH_2Br$

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Arrange the following compounds in order of decreasing rate of hydrolysis for $S_N1$ reaction:$(i)$ $C_6H_5CH_2Br$(ii) $p-CH_3-C_6H_4-CH_2Br$(iii) $p-CH_3CH_2-C_6H_4-CH_2Br$(iv) $p-(CH_3)_2CH-C_6H_4-CH_2Br$