Ring-opening reactions of epoxides
Sn1 and Sn2 epoxide opening discussion Sn1 and Sn2 epoxide opening discussion
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- I want to follow up the last video on the Sn2 ring opening
- of the cyclohexene oxide.
- Because the way I explained it, it might have been a
- little non-intuitive.
- Because when we first learned about Sn2 and Sn1 reactions,
- we said, well if you have an aprotic solvent with an OK
- leaving group, but a really good nucleophile, you'll have
- an Sn2 reaction.
- But if you have a protic solvent with an excellent
- leaving group and a weak nucelophile, then that would
- lead you to an Sn1 reaction.
- All of the ingredients that I showed you in the last video
- actually were more in this direction.
- This was actually an acid catalyzed reaction, so it was
- definitely protic.
- And then this ring is strained right here.
- This epoxide ring.
- And we already protonated this oxygen, so it was a good
- leaving group.
- And clearly, the water, the H2O right here, is a weak
- nucleophile.
- So if we just went off of the checklist that we learned
- about when we just explored the differences between Sn1
- and Sn2 reactions, we probably would have predicted an Sn1
- reaction here.
- We probably would have said the first step would have been
- that this bond just broke on its own, and this guy just
- left, forming a carbocation.
- And only then would the water, the weak nucleophile, would
- have attacked the carbon.
- In that case, it could have attacked from either side.
- Because all of a sudden it would lose its handedness,
- this carbon right here.
- But everything I've read-- and I've actually never performed
- this reaction in the chemistry lab.
- But everything I've read says that this is the
- reaction that occurs.
- That you actually have this, I guess you could call it
- stereospecificity.
- The trans version is actually what predominates.
- And actually what is going on is an Sn2.
- So my best explanation is that this is really more of, you
- could call it an Sn2 like reaction.
- Because it really meets the checklist more for an Sn1 in
- terms of what the reactions are.
- But because the end product is actually selective, you
- actually end up with the trans, as opposed to both the
- cis and the trans.
- We're assuming that an Sn2 type
- reaction must have occurred.
- But I really wanted to make this clear.
- Because it really kind of goes against the grain of our
- checklist of predicting whether an Sn1 or an Sn2
- reaction would occur.
- The reason why, at least in my mind, I think that the Sn1
- reaction maybe didn't occur is, in order for it to occur,
- this carbocation needs to form.
- And that carbocation is secondary.
- It's not tertiary.
- So it's maybe just not quite stable enough.
- Now Sn1 reactions can occur with epoxides.
- I'll show you two examples.
- So if I have-- let me draw two examples here.
- So this is ethane.
- If I just did a double bond here, it would be ethene.
- But if I have these bonded to an oxygen,
- this is ethylene oxide.
- And once again, if this was an alkene, it would be called
- either ethene-- that's actually the official name,
- but some people call it ethylene.
- And so the common name for this is ethylene.
- Actually I spelled it wrong.
- It's ethylene, not with the-- ethylene oxide.
- I've even seen on the web a couple of times, people call
- this ethene oxide.
- Which, in some ways, would be more consistent.
- Let's say on one hand, we have ethene oxide, and let's say on
- the other hand, we have something
- that looks like this.
- Let me try my best to draw it.
- We have something that looks like this.
- And so this part, if we just look at the main chain.
- You could either view this as a propane.
- Or having three carbons.
- 1, 2, 3.
- And then a methyl breaking off of it.
- But this y shape, where you have four carbons here, is
- usually called isobutyl.
- So you could call this isobutylene oxide.
- So let's call this isobutylene oxide.
- That's what you're more likely to see.
- Isobutylene oxide.
- And remember, they just pretend like
- this is a double bond.
- That's where we get the isobutylene oxide.
- But then to make it clear that it's really not a double bond,
- that there's really an oxygen in there, it's really not an
- alkene, you put the oxide there.
- Now let's think about the different reactions that might
- occur if we have a nucleophile in either situation.
- So if we have a nucleophile over here for the ethylene
- oxide, either of these carbons are primary carbons.
- So if you have a carbon right here.
- Let me draw all of its bonds.
- It's bonded to one other carbon.
- It's bonded to a hydrogen, and another hydrogen right there.
- So it's a primary carbon.
- It would not be a stable carbocation.
- So in this case, you would definitely predict that an Sn2
- reaction would have to occur with this epoxide.
- That the nucleophile would attack back here.
- And then this electron would be taken back by the oxygen.
- And that would open the ring.
- So this would be an Sn2 situation.
- And in my mind, this is pretty clear.
- But in this situation, you do have a primary
- carbon right here.
- It's only bonded to one other carbon.
- But over here, you have a tertiary carbon.
- It's bonded to 1, 2, 3 carbons.
- So this would be a stable carbocation.
- So in this circumstance, you would actually expect that
- this epoxide ring is so strained, that the first step
- is that the oxygen would just take the electron.
- The oxygen would just take the electron.
- And then you're left with something
- that looks like this.
- Let me do it in the same colors, so it's clear.
- You would be left with something
- that looks like this.
- And now, since it took the electron, just like that.
- Or actually, maybe let me take a step back.
- It has to be very good leaving group.
- It wouldn't necessarily just take the electron right there.
- The first step in order to make this a good leaving group
- would be if you maybe had a water around, or something
- that it could take, or some type of acid, that it could
- take a hydrogen proton from.
- So the first step is it would take a hydrogen proton.
- Let me do this a little bit neater.
- It would take a hydrogen proton.
- So it would give an electron to a hydrogen proton, so that
- it becomes a good leaving group.
- So now our molecule looks like this.
- Do it in the same colors.
- Our molecule will look like that.
- That's our carbon.
- And now this is an oxygen bonded to a hydrogen.
- And now this has a positive charge.
- This now will just be a water.
- I won't draw it anymore.
- And now this is an excellent leaving group.
- Tertiary carbon.
- This is classic Sn1.
- Classic Sn1 reaction.
- So this guy can essentially leave. And then we are left
- with down here-- let me draw it over here.
- We're left with something that will look like this.
- We have our carbon bonded, not to
- hydrogens, to other carbons.
- So these are carbons over here.
- And then this carbon is bonded to this OH.
- We now have an alcohol.
- And this is now a carbocation.
- And now this is ready to be attacked by even a weak
- nucleophile.
- Even a weak nucleophile could attack this.
- So when you have a tertiary carbon in your epoxide ring,
- look for an Sn1 reaction.
- But if you have anything less, as far as I can tell, based on
- the reactions that I've looked at in multiple books-- if you
- have secondary or primary carbons, it seems like Sn2 is
- going to be more likely to break your epoxide ring.
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