Animated Solution for Chemistry - Organic Chemistry: On complete hydrogenation, natural rubber produces

Welcome, future engineers and scientists! Today, we are going to unravel a fascinating molecular puzzle from the JEE Advanced 2016 paper. This question beautifully bridges the gap between natural polymers and synthetic chemistry. It asks us a simple yet profound question: What happens when we completely hydrogenate natural rubber?

To tackle this, we first need to understand what natural rubber actually is. Imagine a rubber tree weeping its milky latex. At a molecular level, this latex is composed of a polymer called cis-1,4-polyisoprene.

The monomer here is isoprene, systematically known as 2-methyl-1,3-butadiene. When thousands of these isoprene molecules join hands in a head-to-tail fashion, they form a long, tangled chain. The crucial feature of this chain is the presence of a double bond in every repeating unit.

The structure of the repeating unit can be written as:

Notice that double bond? That is the site of chemical reactivity. It's what makes natural rubber susceptible to oxidation, vulcanization, and, in our case, hydrogenation.

The problem states that we are performing complete hydrogenation. Hydrogenation is a classic addition reaction where hydrogen gas ($H_2$) is added across a carbon-carbon double bond, typically in the presence of a metal catalyst like Nickel ($Ni$), Palladium ($Pd$), or Platinum ($Pt$).

When we subject natural rubber to these conditions, the hydrogen molecules attack every single double bond in the polymer backbone. The pi bond breaks, and each carbon atom involved in the double bond receives a new hydrogen atom.

Let's write down the chemical transformation:

What have we created? We have transformed an unsaturated polymer into a completely saturated one. The backbone is now a continuous chain of single bonds, with a methyl group branching off every fourth carbon atom.

Now comes the detective work. We need to look at our options and see which one matches this newly formed saturated structure.

Let's evaluate Option A: ethylene-propylene copolymer. A copolymer is formed when two different monomers polymerize together. Here, the monomers are ethylene and propylene.

The structure of ethylene is:

The structure of propylene is:

Imagine these two monomers linking up in an alternating fashion. The double bonds open up to form single bonds connecting the units.

Take a very close look at this resulting copolymer structure. It has a four-carbon repeating backbone. On the second carbon of this repeating unit, there is a methyl group.

Now, compare this with our hydrogenated natural rubber:

They are structurally identical! The sequence of atoms is exactly the same.

By simply adding hydrogen to natural rubber, we have synthesized a polymer that is chemically indistinguishable from an alternating copolymer of ethylene and propylene. This is a profound realization of how different chemical pathways can lead to the exact same macromolecular architecture.

Therefore, the correct answer is unequivocally Option A.

This problem is a brilliant reminder that in organic chemistry, structure is everything. Whether a polymer is born in a tree or synthesized in a high-tech reactor, its properties are dictated entirely by the arrangement of its atoms. Keep visualizing these structures, and you'll master polymer chemistry in no time!