Ncert Exercises & Solutions

Which option shows 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

{S_{N}}_{N}^{1}

${S_{N}^{}}^{1}$ mechanism.

{S_{N}}_{N}^{1}

${S_{N}}^{1}$ 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, $- {NO}_{2}$ $- {NO}_{2}$ group is a stronger EWG than —Cl. NO₂-C₆H₅ - $CH_{2}^{+}$
$_{}_{}$

carbocation will be less stable than Cl— $+$
$C_{6} H_{5} - {CH}_{2}$ carbocation.
Thus, the order of stability of carbocation is ncert-exemplar-problems-class-12-chemistry-alcohols-phenols-ethers-16

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

Hint: It follows

$\mathrm{S}_{\mathrm{N}}{ }^1$ mechanism.

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