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- So I've drawn out these four scenarios here.
- What we want to talk about is what type of reaction is
- likely to happen in each of these scenarios, and if there
- is some type of reaction, how fast is it likely to occur.
- So let's look at this first scenario up here.
- I have an iodide anion, which we saw on the last video is a
- very strong nucleophile.
- So this right here is a very, very strong nucleophile.
- And actually, we have the iodide anion in every one of
- these scenarios.
- In every one of these scenarios, we have this very
- strong nucleophile, and let's see the other reagent that it
- might react with.
- Here I have a methyl carbon that's attached to a fluorine,
- or if we actually name this actual molecule,
- this would be what?
- Fluoro.
- This would be fluoromethane, because we only have one
- carbon there.
- But when I say it's a methyl carbon, that means that this
- carbon is only attached to other hydrogens.
- It's CH3.
- It's not attached to any other carbons.
- Let me label all of them right now.
- So in this scenario right here, I have a methyl.
- I have a methyl carbon.
- It's a methyl carbon, not attached to any other carbons.
- It is attached to this fluorine, and if we kind of
- pattern match it based on other things, maybe fluorine
- is our leaving group.
- But what's left over, the substrate, we have this methyl
- carbon in the middle of it.
- Over here, this would be a primary carbon.
- It is only attached to one other carbon, so this is a
- primary carbon.
- It's attached to one other carbon.
- This one right here is secondary.
- It's attached to two other carbons, the secondary carbon.
- And then finally, this is a tertiary carbon.
- It's attached to one, two, three, other carbons.
- So that right there is tertiary.
- So we've already identified the
- difference between the scenarios.
- All of them have this very strong nucleophile, and then
- the reagent, the thing that it might react with, the carbon
- in the middle, has different levels of connectivity to
- other carbons.
- This has no connectivity.
- This is connected to one other carbon, two other carbons,
- three other carbons.
- So what's likely to happen here?
- Well, strong nucleophile, and I'm not going to go too much
- into the solution just yet.
- I'll do a future video on that, probably the next one.
- But we have a strong nucleophile.
- Let's just say this is a good solution
- for doing Sn2 reactions.
- And so there's no reason why an Sn2
- reaction can't occur here.
- Strong nucleophile, it can give its extra-- let's say
- it's giving that electron right there, it could give its
- extra electron to the carbon, to this carbon,
- to this methyl carbon.
- And right as it's doing it, this fluorine could take away
- the electron from carbon, could take away that electron,
- so that electron could go to fluorine.
- What I've just drawn here is an Sn2 reaction.
- Once again, S stands for such substitution, N stands for
- nucleophile, and then the two means that both of the
- reagents are active in-- this really is only one step of the
- process, but in the slowest part of the process.
- And this is the slowest part of the process.
- This is actually the whole process right here.
- So we get an Sn1 reaction.
- We get Sn1 reaction going on, so that we have a substitution
- with a-- sorry, sorry, Sn2 reaction.
- I want to be very careful here.
- Sn2 reaction.
- We have substitution with a nucleophile, and both
- reactants are present during the rate-determining step.
- And there's only one step here, so both
- of these are involved.
- Now let's think about what happens in this scenario over
- here on the right.
- What's going to happen over here?
- Well, once again, strong nucleophile.
- It's not too different.
- Now this is a primary carbon.
- We have little CH3 group over here, but this thing should
- still be able to get its way into that carbon.
- It still should be able to give its electron.
- It still should be able to give its electron to that
- carbon, and then this electron that the carbon had had could
- now go to the fluorine to make a fluoride anion.
- And I haven't drawn the final step, but we've seen Sn2
- reactions before.
- But once again, this is an Sn2 reaction.
- Now, we said not only what would happen, but we also said
- how fast might it happen?
- Or how can we compare them?
- I'll tell you right now, that this one on the left is going
- to happen very fast relative to this one, which will only
- happen fast. I'm not giving any absolute numbers, but
- we're just comparing them relatively.
- And the reason why this is very fast and this is only
- fast is because this one has this CH3 group in the way.
- So this is a big part of a molecule.
- This is big.
- This right here is big, especially relative to just a
- hydrogen atom.
- So this is big.
- This allows less directions with which-- remember, all of
- these chemical reactions in organic chemistry and
- chemistry in general, you always draw them as these nice
- organized things.
- But these are really just these atoms and molecules
- bumping into each other in just the right way with just
- the right energies.
- And if you have this big thing blocking one of the entry
- paths onto the carbon, you're not going to have this
- reaction happen as quickly.
- There's going to be-- sometimes, it may be the
- iodide ion was coming from the direction, and it gets
- deflected by this over here.
- In this case, it wouldn't have, because that hydrogen
- isn't big enough to deflect it.
- So this is going to happen just not as fast as this one
- right here when we're dealing with the methyl carbon.
- And I think you see where we're going here.
- So what happens when we have two of these carbons right
- over here when we're dealing with a secondary carbon, a
- carbon attached to two?
- If the solution is right, and we're going to talk about the
- types of solutions that favor Sn2 reactions, we will have an
- Sn2 reaction still.
- We'll still have an Sn2 reaction, but it will be
- slower than these guys up here.
- So we still are going to have an Sn2 reaction,
- but it will be slower.
- So far, this is the fastest, this is the next fastest, and
- this is slower than either of those two.
- And it's the same exact logic.
- This only had one CH3 blocking the entry into
- this primary carbon.
- This has two CH3's.
- It has this one over here, it has this one over here, and it
- has this one up here.
- So it lowers the likelihood that an iodide anion will be
- able to bump into this carbon and in the
- exact the right direction.
- So that's why it's going to occur slower.
- This iodide anion has to come in just the right direction
- for it to occur.
- That's less likely, so overall the reaction is slower.
- Now, finally, what's going to happen here?
- You might be tempted to say this is going to be really
- slow, but the reality is that this tertiary carbon is
- blocked really in every direction.
- It's really in every direction.
- It's blocked there, blocked there, and blocked over there.
- So this iodide anion is actually not even going to be
- able to get to it.
- So I'll write no Sn1.
- And we'll see in the next video that maybe there's
- another type of reaction that we've learned-- sorry, I
- should say Sn2.
- No Sn2.
- I'm sorry if part of my brain makes me think that I might
- have said Sn1 earlier.
- No, everything in this video is Sn2.
- Everything in this video, we have the nucleophile attacking
- the other reagent.
- They're both present, so it's Sn2, although my brain
- sometimes wants to say Sn1.
- These are all Sn2 reactions, and you're not
- going to have an Sn2.
- And actually, if your solution is right, you actually might
- have an Sn1 reaction here, and we'll talk about that later.
- But you have no Sn2 here, because the nucleophile cannot
- get to the carbon.
- It's blocked I guess you could say by these methyl groups.
- Now, the term for this blocking, I guess you could
- say, of the point that the thing needs to attack or kind
- of give its electron, or move into is
- called steric hindrance.
- You know what it means to hinder something.
- You're making it hard for it to happen.
- And steric just means three dimensional, or the shape, the
- three-dimensional shape of things: hindrance.
- So by the virtue of the three-dimensional shape of
- this or the way that this is made up, it's blocking the
- reaction from happening.
- It's hindering the reaction.
- So the big takeaway from here is you're only going to have
- an Sn2 reaction.
- You're going to have a very fast Sn2 reaction if you're
- dealing with a methyl carbon, a little bit slower if you're
- dealing with a primary carbon.
- If you have a secondary carbon that's blocked by two other
- carbons, it might happen if the solution is just right,
- but it's going to happen very slowly.
- And if it's completely blocked, then you're going to
- have no reaction.
- And the one last thing, you might be saying, well, hey,
- how can I not come from the other side?
- You can't come from the other side because the point that
- you want to give the electron that's taking away is kind of
- being blocked by the fluorine in this situation.
- That Sn2 reaction will only happen on the opposite side
- from the leaving group, just as a reminder.
- This is the nucleophile.
- This is the leaving group.
- In Sn2 reactions, the nucleophile goes from one end,
- and then the leaving group leaves from the other.
- It cannot happen on the same side, which actually makes a
- lot of sense if you think about it.
- That's as opposed to the Sn1 reaction, where it can happen
- on the same side.
- And what we'll see is if the solution is right, this type--
- having a mixture of this and this would actually favor Sn1.
- I'll do a whole video on that, but you could
- think about what happens.
- If the solution is right, the fluorine can take this
- electron first, then this will become a carbocation.
- And this would actually be a very stable carbocation.
- The very same thing that was causing steric hindrance will
- actually make this a stable carbocation, and then that
- will make it favorable for a nucleophile, and really even a
- weak nucleophile.
- to attack it, or to bond to it, or
- to give it its electron.