Chapter 31.6, Problem 11P

Interpretation Introduction

Interpretation:

Predict the result of the catalytic hydrogention of natural rubber and will the product obtained show syndiotactic, atactic or isotactic behaviour.

Concept introduction:

Isotactic, syndiotactic, and atactic are the stereochemical forms. The polymer having the (-R) groups on the same side of the macromolecule backbone (zigzag bone) is known as isotactic polymer. An isotactic macromolecule consists of 100% meso diads.

Syndiotactic are the macromolecules in which the (-R) groups are arranged in an alternate manner along the long chain of the polymer. Gutta-percha is also an example for Syndiotactic polymer.

In atactic form, the substituents are placed in a random manner along the long chain.

The important point to note here is that the polymer obtained from the chain-growth polymerization mechanism must have chiral carbons to possess these structures. If the polymer formed does not have chiral carbon then it cannot occur in isotactic, syndiotactic and atactic forms.

The catalytic hydrogenation natural rubber requires the breaking of double bond and the addition of hydrogen. The double bond of alkene consists of a sigma (σ) and a pie bond (π). The carbon -carbon pie bond is relatively weak.

Therefore, it breaks and hydrogen adds to it. When the hydrogen adds, then the double bonds are replaced with the single bonds. The hydrogenation of double bond releases energy therefore, known as an exothermic reaction. The heat released is called the heat of hydrogenation.

The catalyst used for the purpose of hydrogenation can be Ra-Ni (Raney-Nickel), PtO₂ (Platinum oxide), Pd-C (Palladium on carbon) etc. They can be used to enhance the rate of reaction.