Give the decreasing order of the reaction rates of the following benzyl with HBr.

I > I I > I I I > I V

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I V > I I I > I I > I

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I I > I > I I I > I V

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I V > I I I > I > I I

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Solution

The correct option is A $I > I I > I I I > I V$
The positive charge that develops on benzylic C in this $S N^{1}$ reaction is most effectively delocalised by $(O M e)$ group ( $+ R$ and $- I$ ). The $(p - N O_{2})$ group withdraws $¯ e$ density from the ring by $- I$ and $- R .$ $(p - C l)$ group withdraws $¯ e$ ' s from the ring by $- I$ only.
More EDG (e.g. $- O M e$ ) stabilised positive charge on the benzyl $C$ atom, whereas EWG (e.g., $- N O_{2},$ -CI) destabilises the positive charge. Hence, the reactivity order is: $I > I I > I I I > I V .$

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