• # question_answer 89)   In the presence of peroxide addition of $HBr$to propene takes place according to anti Markownikoffs rule but peroxide effect is not seen in the case of $HCl$ and $HI$. Explain.

$\underset{\text{Propene}}{\mathop{{{H}_{3}}C-CH=C{{H}_{2}}+HBr}}\,\xrightarrow{Peroxide}\underset{\text{n-Propyl}\,\text{bromide}}{\mathop{{{H}_{3}}C-C{{H}_{2}}C{{H}_{2}}Br}}\,$ 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). (ii) Iodine free radical $({{I}^{^{\bullet }}})$  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.