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Polymerization of alkenes with acid
Polymerization of Alkenes with Acid. Created by Sal Khan.
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Why would a positive charge be on the C bonded to Cl? Because CL is more electronegative than C, Cl will pull C's electrons, making C more positive and unstable. If my understanding is correct, to have an intermediate that is stable, wont it make (CH2+)-(CH2Cl) ?(14 votes)
Correct. You want to form the more stable intermediate.You have a choice between (+)CH2-CH2Cl and CH3-C(+)HCl. In this case, the second intermediate is more stable because of resonance: a lone pair of electrons on the Cl can be delocalized into the vacant p orbital on the C. This is not ideal, because it puts a + charge on the Cl. But to the extent that it happens, it lowers the energy of the intermediate. The other structure has no such delocalization, so it is higher in energy.(12 votes)
Is there a definition for acid?(2 votes)- Yes, something which donates protons (H+) ions. So HCl in an aqueous solution is an Acid because in the aqueous solution the H+ and Cl- ions seperate, so there are loads of H+ ions (protons) floating around, making the HCl aqueous solution an acid - Hydrochloric Acid (HCl (aq))
Note that if HCl was NOT in an aqueous solution it would NOT be considered an acid because the H+ ions and the Cl- ions are bonded firmly together in a bond, so the H+ ions (protons) wouldn't be able to perform their 'acidic' function.(8 votes)
It seems intuitive to me that the carbon attached to the chlorine in the original molecule would be LESS likely to "give up an electron" from the pi bond. This is because the attached chlorine would be causing it to already have a partial positive charge answered only by (hogging) the electron in the pi bond. Can someone explain why this is not the case?
EDIT: To clarify, it seems suspect to me that Khan claims that the chlorine in the molecule would be "willing to share electrons" with the carbon to which it is bonded. I understand Markovnikov's Rule, except I don't see why it would apply to a carbon bonded to a highly electronegative atom.(3 votes)- While you are correct that chlorine is electronegative and would destabilize the positive charge on the carbocation...you are considering inductive (minor) electronic effects but not resonance effects (major). Later, in benzene chemistry, you will learn that resonance effects dominate inductive effects.
But for now....Doesn't chlorine have lone pairs? Then you can draw resonance structures of chlorine and the carbenium ion (carbocation). This is obviously a stabilizing interaction.(3 votes)
- During polymerization, the CH2 in the chloroethene bonds with the building polymer instead of the CHCl side. Doesn't that violate Markovnikov's rule that the more substituted C will bond with the more substituted reactant?(4 votes)
The C+ cabocation took the most available electron from the next monomer. Markovnikov's rule is important in the first step, and the last step since it mainly applies to the addition of hydrogen halides to alkenes.(1 vote)
- In the beginning of the video, Sal mentioned that Vinyl Chloride is also called Chloroethene. However, the polymer in the end, does not have any double bonds so why is it referred to as Poly Vinyl Chloride when the structure only has single bonds? Isn't that an alkane molecule?(2 votes)
That’s just how the nomenclature of polymers works. They’re named after the monomers not what they look like in the polymer. Look up polyethylene for example.
When the polymers form it’s the double bond that is broken to link the monomers together.(2 votes)
Does this also work when there are other halogens in the place of the chloride?(2 votes)- Yes, most strong acids catalyze the polymerization of alkenes.(2 votes)
why didn't the chlorine get bonded to the carbocation ?(2 votes)- At2:20why is the H bonding to the left carbon and not the right? Didn't the electron come from the right carbon?(2 votes)
- One of the bonds in the double bond between the carbons is made up of an electron from the left carbon and an electron from the right carbon. If the right carbon gives that electron which makes up the bond to the hydrogen (as the hydrogen is separated from the Cl) The left carbon and the hydrogen will both have a valence electron and create a bond. Or you could think of it as when the right carbon's electron goes to the hydrogen it carries the bond with it(1 vote)
- Sal says that forming a carbocation on the carbon with Cl attached to it will be more stable but in school we were taught that due to inductive effect (electromeric effects) the formation of a carbocation on the carbon with withdrawing groups (e.g. halogens, nitrogen,etc.) will be highly unstable. I'm super confused can someone please help me out?(1 vote)
- A Cl attached to a carbocation destabilizes the cation by its inductive withdrawal of electrons, but it stabilizes the cation by resonance donation of electrons.
The resonance effect (transmitted through the π orbital) outweighs the inductive effect (transmitted through the σ bond).
A β or γ Cl is destabilizing because there is an inductive effect but no resonance effect.(3 votes)
How do we know that the chlorine will not form bond with carbon, instaed there will be many PVC's and then the end would be with chlorine?
why chlorine didnt just simply bonded with carbon?(1 vote)- There were two good answers that was written for the question about stealing an electron from a double bonded carbon vs a chloride anion. I guess another way to put it is that carbocation in the chain keeps finding more chloroethene to elongate the PVC chain because there's a lot chloroethene in the mixture. If there's a slimmer chance of the chain interacting with a chloroethene due to a diminished concentration of this reactant vs the amount of Cl- anions available as the reaction proceeds, then the chain would bind to the chlorine. Just remember that the carbocation would favor binding with another chloroethene due to the weaker pi bonds and the way C doesn't mind sharing an electron as much as the Cl- anion would. Also, halogens tend to be happy being in their octet forms.(2 votes)
2:20Video transcript
Let's say we have some
chloroethene here, and you wouldn't have to call this
1-chloro-eth-1-ene, because if you just go with chloroethene,
there's only one way to draw this. And the common name for chloroethene is vinyl chloride. So let's say we have a bunch
of chloroethene molecules along with or mixed with
some hydrogen chloride. And I've drawn all of the
valence electrons for the chlorine atom and I've drawn a
little magenta electron, the one that the hydrogen atom
brought to the table. So we've seen something
like this before. What is likely to happen? Well, maybe one of these carbons
is willing to give up an electron. That electron goes to the
hydrogen because this electron is already being hogged by the
chlorine, so this hydrogen has a partially positive charge. Chlorine has a partially
negative charge, so that electron would be attracted
to the hydrogen. Then this electron
can be completely hogged by the chlorine. And if we had to decide which
of these carbons is more likely to give up the electron,
you just have to say which one is bonded to things
that it can share electrons a little bit with. This carbon is only bonded to
hydrogens, so it's already hogging their one electron each,
and there are no more electrons to share with it. This guy is bonded to a
chlorine, so the chlorine has a bunch of valence electrons. It might be able share a little
bit with this carbon if this guy became a carbocation. So this guy will lose
an electron. This carbon will form the
bond with that hydrogen. So let's draw it out. So let's say this carbon's
electron is that blue thing right there. Well, we could draw
it like this. It goes to the hydrogen, and
then the hydrogen's magenta electron goes to the chlorine. This is just a plausible
mechanism. Now, once that happens, what
will our setup look like? What will it look like? It will have this carbon over
here bonded to two hydrogens. It has its single bond to that
other carbon that just lost its electron, which is bonded to
a hydrogen and a chlorine. And now this carbon on
the left, it is now bonded to the hydrogen. That electron went to the
hydrogen and it formed a bond with it, so then it
forms a bond. So that little blue electron is
at this end of the-- I want to make it blue. That little blue electron is at
this end of the bond, and it is now the hydrogen's
electron. And that magenta electron went
to the chlorine, so now it is a negative ion. It is a chloride ion. So we have a chloride ion. It has its standard seven
valence electrons that it started off with, but now it
took that magenta electron from the hydrogen,
and so now it has eight valence electrons. It gained an electron. It now has a negative charge. This guy over here
lost an electron. He now has a positive charge. Now, the next thing that you
might expect to happen, if we just followed the pattern of
the last several videos, is you would say, hey, this guy
will now take an electron from the chlorine, which is-- or the
chloride anion, I should say, which is completely
plausible, but there's also a bunch of the chloroethane. This isn't the only molecule
of chloroethene. I should say chloroethene,
not chloroethane. Chloroethene sitting around. So let's let us throw another
one of those in there. So we have more molecules
of this. So he could take an electron
from this chloride ion, or he could take an electron from
this guy over here. Remember, this guy, just like
this guy, who was this guy, this guy is OK. It doesn't require a super
amount of energy to make this guy lose his electron. He's bonded to other things that
are willing to share with him a little bit. Maybe he's willing to lose his
electron as opposed to the chloride ion. So this guy has-- let me draw
it in-- so this end of this bond is green. And then this goes and bonds
with this carbon. So this will be a long
bond right here. So this goes and bonds with
that carbon, essentially giving that electron
to that carbon. And then what will our
setup look like? So after that happens,
we'll look like this. I probably should have copied
and pasted this from the get go. Actually, let me do that before
I-- let me copy and paste this. So now let me just draw, copy
and paste this whole thing. Nope, that's not what
I wanted to do. Let me select it again. All right. Copy and then paste. There you go. So then we have that thing. And let me redraw what I
had erased, so that I could copy and paste. And then we have this guy went
over to this carbocation, so he's no longer a carbocation, so
let me erase this, because now he's gained an electron. He gained that green electron
right there. He gained that green electron. I'll just draw it right there. And now he's formed a bond
with this carbon. And I'll make it blue, just so
we know which carbon we're talking about. He's formed a bond. This bond now moved over to
that carbon because the electron went with it. So now that bond is to this
carbon right here. That carbon right over there,
which is bonded to two hydrogens, and now has a single
bond to the electron that gave up the carbon, has a
single bond to that character right over here, who is bonded
to a hydrogen and a chlorine. And since he now lost an
electron, he now has a positive charge. So if you look at this setup
right here, it looks very similar to this setup, although
we've added one more vinyl chloride to the mix. And the one that we added lost
its electron, or this carbon lost it electron, and now
it's a carbocation. So what could happen next? Well, we have more of this vinyl
chloride sitting around. Let me draw another
vinyl chloride. So I have a carbon, a hydrogen,
a hydrogen, and then it is double bonded
to a carbon, a hydrogen, and a chlorine. And let me copy and
paste this. I think you see where this might
be going, how this could keep on going and
going and going. So copy. Well, I just copy
that for now. So what's going to happen now? This guy could go and give an
electron to this guy and form a bond, or we could have the
same process happen over and over and over again. Let me get my pen tool going. So this electron could be given
to this carbocation right there. And then what happens? Well, if that happens, then
we're going to get-- I'll move to the left now. We have our original molecule. I'm going to run out
of space soon. We have this original molecule,
and now this guy has bonded to that. So this carbon right here is
going to be this carbon, and now it is bonded to this guy. That orange electron
is now given to this guy who was positive. So he now has-- let me make
it a little bit neater. I can do a better
job than that. So the carbon's here. The bond goes to this guy. He now has the orange
electron. He no longer has a
positive charge. He's got all of his valence
electrons now. And now this guy is bonded
to two hydrogens. That guy is bonded
to two hydrogens. And he has a single bond, this
single bond right here to the carbon that just lost his
electron, who's bonded to a hydrogen and a chlorine. And because he lost his
electron, he now is the carbocation. He is now a carbocation. So I think you see where
we're going. We can just keep adding and
adding and adding to this chain of vinyl chloride. So if this process just went
on and on and on, we could make it like this. It would look something
like this. Let me see how well
I can draw it. So it would look like this. So this is a CH3. So I'll just draw it as H3C, and
then this is bonded to a carbon, that is bonded to a--
well, maybe I'll call it a CH, which is bonded to a chlorine. So we're that far
in the molecule. And then we have-- let's see
the part that repeats. This part right here is going
to keep repeating. That part right there is going
to keep repeating, and I'm going to do it like this. So I'm just going to draw one
of them, so you have a CH2. That's that right there,
connected to a CH, which is that right there, which is
connected to a chlorine. And so that part right there
will keep repeating. And then maybe the very last
one, so you have this guy right here, but maybe the very
last one that joins on-- I mean, this can happen
millions of times. I just it made it happen
two or three times. It could happen millions of
times and form a super long chain or a polymer. And what we are describing in
this video is actually a polymer that you have probably
dealt with at some point in your life. In fact, I guarantee there's
some of it in your house right now. So then we'd have that
part right there. And we could just make
that as CH2, CH, Cl. And now, the way we've drawn
it, it's a carbocation, but maybe we've run out of all of
the vinyl chloride molecules, or we could also call them
chloroethene molecules. And now finally, when everything
is said and done, this last guy, since he's run
out of vinyl chloride molecules to take their
electrons from, he now finally takes it from the chlorine. So you can imagine this happens
many, many times. So this repeats many times. After this repeats many times,
then finally, one of these electrons from the chlorine go
to that final carbocation, because they've run out of other
vinyl chlorides, and then he attaches right over
here to the chlorine. Now, this is called-- so when we
say that this might happen many times, you might write an
N here, just to show that it repeats many, many times. If you know how many times it
repeats, if you know that there were a thousand molecules
here, you would write a thousand repetitions,
but this is called a polymer. And the name for this molecule
right here is-- each of these units is vinyl chloride,
right? Vinyl chloride. I guess the official name is
chloroethene, but the typical name, the one people actually
to use, is vinyl chloride. That's for each of
these units. It's a polymer. We have many of them, so we'll
put a poly- in front of it. So this molecule right here is
polyvinyl chloride, or, and now I think it'll ring a
bell, or PVC for short. And you've probably heard
of PVC piping. It's what most people have
for their plumbing. It's those plastic pipes. And that's what it is. It's polyvinyl chloride.