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  4. Give the decreasing order of reactivity of the following compounds with HBr. (I) <img class=img-fluid question-image alt=Question src=https://cdn.tardigrade.in/q/nta/c-mzdb9agtgfbp.png /> (II) <img class=img-fluid question-image alt=Question src=https://cdn.tardigrade.in/q/nta/c-mzdb9agtdwnu.png /> (III) <img class=img-fluid question-image alt=Question src=https://cdn.tardigrade.in/q/nta/c-mzdb9agtczoy.png /> (IV) <img class=img-fluid question-image alt=Question src=https://cdn.tardigrade.in/q/nta/c-mzdb9agtgcs9.png />

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Q. Give the decreasing order of reactivity of the following compounds with HBr.

(I) Question

(II) Question

(III) Question

(IV) Question

NTA AbhyasNTA Abhyas 2020Organic Chemistry – Some Basic Principles and Techniques

Solution:

Electron Releasing Group favours $\text{S}_{\text{N}} \text{1}$ reactivity with $\text{HBr}$ . $\text{EWG}$ retards $\text{S}_{\text{N}}\text{1}$ reactivity with $\text{HBr}$ .

$\text{ERG}$ effect of $\left(- O M e\right)\left(\right.+R,-I\left.\right)$ is greater than $\left(\right.Me\left.\right)$ group $\left(+ I \, a n d \, H . C .\right).$ So, reactivity of $III>II.$

In $\left(I V\right),\left(- O M e\right)$ group is at m-position and exerts only $-I$ effect and so reactivity of $\left(I V\right)$ is less than $\left(I\right).$

So the reactivity order is: $III>II>I>IV$