question_answer 1)

  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.

Answer:

  \[\underset{\text{Propene}}{\mathop{{{H}_{3}}C-CH=C{{H}_{2}}+HBr}}\,\xrightarrow{Peroxide}\underset{\text{n-Propyl}\,\text{bromide}}{\mathop{{{H}_{3}}C-C{{H}_{2}}C{{H}_{2}}Br}}\,\] 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.