Animated Solution for Chemistry - Organic Chemistry: The major product of the following reaction is

Let's start by looking closely at our reactant. We have a benzene ring with a chlorine atom on one side, and an alkyl chain on the other. Notice the alkyl chain has a chlorine atom attached to a secondary carbon. This is our reactive site. The molecule is $1$-chloro-$4$-(2-chloropropyl)benzene.

The reaction conditions are given in two distinct steps. First, we treat the molecule with alcoholic $\text{KOH}$. Second, we subject the resulting product to free radical polymerisation.

Elimination

Now, what happens when we add alcoholic $\text{KOH}$? Alcoholic $\text{KOH}$ is a strong base, and it's famous for causing dehydrohalogenation (an ${\text{E}}_2$ elimination). It will remove a hydrogen atom and a halogen atom to form a double bond.

But wait, there's a catch here. The base has two choices for removing a proton. It can take it from the benzylic ${\text{CH}}_2$ group, or from the terminal ${\text{CH}}_3$ group. Which one will it prefer?

According to Saytzeff's rule, the more substituted and stable alkene is formed. If we remove the benzylic proton, the resulting double bond is in conjugation with the benzene ring. This resonance stabilization makes it thermodynamically much more stable. Therefore, the major product of the first step is the conjugated alkene: $p{\text{-Cl-C}}_6{\text{H}}_4{\text{-CH=CH-CH}}_3$.

Polymerisation

So, let's move forward. The second step is free radical polymerisation. In this process, the $\pi$ bond of our newly formed alkene breaks open, and the molecules start linking together to form a long chain.

Imagine visualizing this chain. The repeating unit will consist of the two carbons that originally shared the double bond. One of these carbons holds a methyl group, and the other holds the $p$-chlorophenyl group. This gives us our final polymer structure, which can be written as ${\left[{\text{CH(CH}}_3\text{)}-{\text{CH(C}}_6{\text{H}}_4\text{Cl)}\right]}_n$.

Comparing our derived structure with the given options, we can clearly see that it perfectly matches Option (b). It's a simple yet beautiful sequence of elimination followed by polymerisation.