Ncert Exercises & Solutions

In the presence of peroxide addition of HBr to propene takes place according to anti Markownikoff's rule but peroxide effect is not seen in the case of HCl and HI. Explain.

\underset{propene}{{CH}_{3} - CH - {CH}_{2}} + HBr \overset{Peroxide}{\to} \underset{n - propyl bromide}{{CH}_{3} - {CH}_{2} {CH}_{2} Br}

The mechanism of the reaction is

Step I

Step II

Peroxide effect is effective only in the case of HBr and not seen in the case of HCl and HI. This is due to the following reasons

(i) H-Cl bond (103 kcal/mol) is stronger than H-Br bond (87 kcal/mol)

H-Cl bond is not decomposed by the peroxide free radical whereas the H-I bond is weaker (71 kcal/mol) form iodine free radicals.

(ii) Iodine free radical

(I °)

formed as H-I bond is weaker but iodine free radicals readily combine with each other to form iodine molecules rather attacking the double bond.

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