Question

14. What are stereoisomers? Give an example of a commodity thermoplastic that can exists in three forms and explain the difference between each form.

Answer 1

*STEREOISOMERS*

In stereochemistry, stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space.

Stereoisomers are again subdivided into different isomers.

***Enantiomers, also known as optical isomers, are two stereoisomers that are related to each other by a reflection: they are mirror images of each other that are non-superimposable.

***Diastereomers are stereoisomers not related through a reflection operation. They are not mirror images of each other. These include meso compounds, cis–trans (E-Z) isomers, and non-enantiomeric optical isomers.

***Cis-trans isomer --- Stereoisomerism about double bonds arises because rotation about the double bond is restricted, keeping the substituents fixed relative to each other. If the two substituents on at least one end of a double bond are the same, then there is no stereoisomer and the double bond is not a stereocenter, e.g. propene, CH₃CH=CH₂ where the two substituents at one end are both H.

***Conformational isomerism is a form of isomerism that describes the phenomenon of molecules with the same structural formula but with different shapes due to rotations about one or more bonds.

Commodity thermoplastics

The more commonly used thermoplastic materials are known as commodity plastics as they are traded and used in great quantities. Examples are polystyrene (PS), polyvinyl chloride (PVC), polypropylene (PP) and polyethylene (PE).

Consider the case of POLYSTYRENE.

Polystyrene can exists in three form namely

1.syndiotactic

2. Isotactic

3. Atactic

**These can be differentiate according to their melting point.Among the three isotactic will have the highest melting point due to its high packing efficiency and high interchain forces.

**Isotactic havehigh strength and rigidity and can even be made into fibres. syndiotactic is semi crystalline and atactic is amorphous, rubbery and flexible.

**X-ray crystallography can be used to elucidate the structures and look for different degrees of crystallinity.

¹H or ¹³C NMR can be used to assign tacticity.

Checking the melting point and density will verify which one is isotactic; the isotactic one will have both the highest mp and the highest density.