Name: Mechanism of Addition Reactions of Alkenes
Uploaded: 2019-07-19
Duration: 11 min 16 s
Description: The most common reaction for an alkene is the addition reaction to a C=C double bond. Watch the video to learn more about the addition reactions of alkenes.

Question

In the presence of peroxide addition of

HBr

to propene takes place according to anti-Markovnikov’s rule but peroxide effect is not seen in the case of

HCI

and

HI

. Explain.

Accepted Answer

In the presence of peroxide, addition of $HBr$ to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. This reaction is known as peroxide or Kharash effect or addition reaction anti to Markovnikov rule.

${CH}_{3} - CH = {CH}_{2} + HBr \overset{{(C_{6} H_{5} CO)}_{2} O_{2}}{\to} \underset{1 - Bromopropane}{{CH}_{3} - {CH}_{2} - {CH}_{2} Br}$

Peroxide effect proceeds via free radical chain mechanism as given below:

$C_{6} H_{5} - \underset{\underset{O}{∥}}{C} - O - O - \underset{\underset{O}{∥}}{C} - C_{6} H_{5} \overset{Homolysis}{\to} 2 C_{6} H_{5} - \underset{\underset{O}{∥}}{C} - O^{\cdot} \to 2 C_{6} H_{5} + 2 {CO}_{2}$

${\dot{C}}_{6} H_{5} + H - Br \overset{Homolysis}{\to} C_{6} H_{6} + \dot{B} r$

${CH}_{3} - \dot{C} H - {CH}_{2} Br + H - Br \overset{Homolysis}{\to} {CH}_{3} - {CH}_{2} - {CH}_{2} Br + \dot{B} r$

Peroxide effect is observed only in the case of $HBr$ and not seen in case of $HI$ and $HCl$ . This is due to the fact that the $H - Cl$ bond is stronger than $H - Br$ bond. Also, bond energy of $H - Cl$ bond is higher than $H - Br$ . $H - Cl$ bond is not cleaved by the free radical whereas the $H - I$ bond is weaker and iodine free radicals prefer to combine with each other to form iodine molecules.

In the presence of peroxide addition of $HBr$ to propene takes place according to anti-Markovnikov’s rule but peroxide effect is not seen in the case of $HCI$ and $HI$ . Explain.

Important Questions on Hydrocarbons

Learn and Prepare for any exam you want !

In the presence of peroxide addition of HBr to propene takes place according to anti-Markovnikov’s rule but peroxide effect is not seen in the case of HCI and HI. Explain.

Important Questions on Hydrocarbons

Learn and Prepare for any exam you want !

In the presence of peroxide addition of $HBr$ to propene takes place according to anti-Markovnikov’s rule but peroxide effect is not seen in the case of $HCI$ and $HI$ . Explain.