Animated Solution for Chemistry - Polymers: Which polymer has 'chiral' monomer?

When we talk about chirality in chemistry, we are looking for a very specific geometric property. A molecule is chiral if it cannot be superimposed on its mirror image. For organic molecules, the most common source of chirality is a chiral center—an $s{p}^3$ hybridized carbon atom that is bonded to four completely different groups.

In this problem, we are asked to identify which of the given polymers is made up of chiral monomers. To solve this, we must act like chemical detectives, breaking down each polymer into its fundamental building blocks (monomers) and inspecting them for chiral centers.

Let's start by examining PHBV, which stands for Poly(3-hydroxybutyrate-co-3-hydroxyvalerate). As the name suggests, it is a copolymer formed from two distinct monomers:

1. 3-hydroxybutanoic acid 2. 3-hydroxypentanoic acid

Let's draw the structure of the first monomer, 3-hydroxybutanoic acid:

If we focus our attention on the 3rd carbon atom (the one attached to the hydroxyl group), we can list the four groups attached to it: - A hydrogen atom ($-\mathrm{H}$) - A hydroxyl group ($-\mathrm{OH}$) - A methyl group ($-{\mathrm{CH}}_3$) - A carboxymethyl group ($-{\mathrm{CH}}_2\mathrm{COOH}$)

Because all four of these groups are entirely different, this carbon atom is a chiral center.

Now, let's look at the second monomer, 3-hydroxypentanoic acid:

Again, focusing on the 3rd carbon atom, we find it is bonded to: - A hydrogen atom ($-\mathrm{H}$) - A hydroxyl group ($-\mathrm{OH}$) - An ethyl group ($-{\mathrm{CH}}_2{\mathrm{CH}}_3$) - A carboxymethyl group ($-{\mathrm{CH}}_2\mathrm{COOH}$)

Just like before, all four groups are distinct. This means the second monomer also possesses a chiral center. Since both monomers of PHBV are chiral, we have found our answer!

For the sake of completeness, let's briefly see why the other options fail the chirality test:

- Buna-N: Made from 1,3-butadiene and acrylonitrile. Both of these monomers contain double bonds ($s{p}^2$ hybridized carbons) and lack any carbon bonded to four different groups. - Nylon 6,6: Made from hexamethylenediamine and adipic acid. Both of these molecules are highly symmetrical linear chains without any chiral centers. - Neoprene: Made from chloroprene (2-chloro-1,3-butadiene). Like the monomers of Buna-N, it is planar and achiral.

Thus, PHBV stands alone as the polymer with chiral monomers.