Q19P - Organic Chemistry (Mcmurry)

Question

One of the chain-termination steps that sometimes occurs to interrupt polymerization is the following reaction between two radicals. Propose a mechanism for the reaction, using fishhook arrows to indicate electron flow.

Step-by-Step Solution

Answer

One of the chain-termination steps that occur to interrupt polymerization is the following reaction between free radicals:

 

This reaction takes place through the following mechanism:

Step1:Radical addition to alkenes generates polymers

Alkenes are given a radical addition to create polymers. A polymer is a big molecule consisting of monomers, which are repeating units. There are two types of polymerization techniques namely addition polymerization and condensation polymerization. Addition polymerization proceeds through the radical mechanism. Radical additions occur via a 3 step process.

The first step in a radical reaction is the initiation step. Heat is added to the reaction mixture to generate a radical. The radical then attacks a monomer, generating a monomer radical, or activated monomer. Radicals created in the initiation stage are added to other monomers in the second monomer to create a lengthy chain. The polymerization of this process is known. The final step of the mechanism is termination. In this step, all the radicals are quenched.

Step2:The chain-termination

One of the chain-termination steps that occur to interrupt polymerization is the following reaction between to free radicals:

 

The C-H bond splits into two free radicals H (hydrogen radical) and  ${\text{R-CH-CH}}_{\text{2}}$radical. The H attacks on,${\text{R-CH-CH}}_{\text{2}}$ yielding. ${\text{R-CH}}_{\text{2}}{\text{-CH}}_3$While the single electrons in ${\text{R-CH-CH}}_{\text{2}}$ form a pi bond.