Video Transcript
Consider the following polymer. Which of the following halogenated
hydrocarbons can be used to make it?
This is a question about
polymers. Specifically, it’s about making
polymers. Which of the five answer choices is
the repeated subunit or monomer that makes up the polymer? A first question we can ask is, is
this polymer an addition polymer or a condensation polymer? The key difference between an
addition polymer and a condensation polymer is that when we make an addition
polymer, no by-product forms. But when we make a condensation
polymer, a by-product will form. Often, that by-product is
water.
If we look at the answer choices,
several key features stick out. First, there is a lack of a hydroxy
group in any of the answer choices. No hydroxy group means no water
molecule can form as a by-product, suggesting that it is not a condensation
polymer. And while another by-product is
technically possible, the presence of double bonds in several of the answer choices
suggests that it is an addition polymer.
As we will see in a moment, the
extra electrons of the double bond allow for the formation of new bonds that make a
polymer without a by-product. To put it simply, addition polymers
are made of monomers with unsaturated bonds. Usually, that means a double bond,
although it could mean a triple bond. So, we can eliminate choice (A) and
choice (E) from consideration as they contain only saturated single bonds. Let’s erase choice (E) to give
ourselves more room to work with. The remaining answer choices each
have the same central structure, two carbon atoms with a double bond in between
them. This structure is a common
structure for the monomer of an addition polymer, as the 𝜋 bond from the double
bond can break, and the electrons can be used to form a new bond to the next monomer
in the chain.
The pattern can continue to link
more and more monomers together. The end result is a continuous
chain of carbon atoms with single bonds between them. But what about the atoms that are
attached to the carbon chain? These atoms are different in each
of the three answer choices remaining. Well, the attached atoms remain
unchanged throughout this process. While the carbon atoms change the
way they bond with one another, the attached atoms simply remain attached as the
central structure changes. So, if our ending polymer has a
repeated unit with three hydrogen atoms and one chlorine atom attached to the carbon
chain, that means our starting polymer also had three hydrogen atoms and one
chlorine atom attached to its carbon atoms.
The structure shown in choice (D)
is the correct answer. It has the double bond necessary to
form the spine of the addition polymer, as well as the correct attached atoms. So, which of the following
halogenated hydrocarbons can be used to make this polymer? That’s choice (D).
