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- In the last few videos, I showed you amines, and, in
- particular, it was ethylamine, involved in Sn2 reactions.
- But what I want to show you in this video, it doesn't always
- have to be an Sn2 reaction.
- In fact, in this video, we'll look at an Sn1 reaction.
- So let's say I had some methylamine.
- So let's say it looks like this.
- So this is our methyl group bonded to a nitrogen, which is
- bonded to two hydrogens, just like that, and it
- has its lone pair.
- So this is methylamine right there.
- And let's say it's in solution with something
- that looks like this.
- Let's say we have something that looks like this.
- So then we have a bromo group.
- Now, there's a couple of different ways that you can
- name this, but maybe the simplest way, you find the
- longest chain here.
- It has one, two, three carbons, so that is a propane.
- And then on the two carbon, regardless of whether you
- start counting from here or there, so this is one, two,
- three, on the two carbon, you have a bromo group and you
- have a methyl group right there.
- So this would be 2-bromo.
- You want to write it in alphabetical order.
- 2-bromo-2-methylpropane.
- What we care about is what would happen over here?
- Now, over here, you have a situation where you have a
- carbon right here attached to the bromine.
- And if your brain is really in Sn2 mode, you might say, hey,
- maybe these electrons attack this carbon and then this
- electron that's with the carbon right now gets taken by
- the bromine.
- But if you really think about what are the conditions for an
- Sn2, you'll probably remember that this won't happen because
- it's sterically hindered.
- You have these three other carbons here.
- And actually, these are not just carbons, these are CH3's.
- The hydrogens aren't even drawn.
- But this is a CH3 here.
- This is a CH3 over here.
- This is a CH3.
- These things are going to block the nucleophile's access
- to this carbon, so you're not going to see an
- Sn2 reaction here.
- But what you can see is, because this guy is a tertiary
- carbon, he is bonded to three other carbons, it would
- actually be a fairly stable carbocation.
- So what you could see in step one of this reaction is just
- the bromine leaves.
- So if you have just the bromine leaves, then our
- situation looks like this.
- You have a tertiary carbocation.
- This electron leaves, so it now has a positive charge
- right over there.
- That's why it's a carbocation now.
- And now the bromine has taken an electron.
- It already had one, two, three, four, five, six, seven
- valence electrons.
- One, two, three, four, five, six, seven.
- And it just took one more electron from that carbon.
- It now has a negative charge.
- It's now a bromide anion.
- But now that the bromine has left, now the methylamine can
- get involved.
- This would be methylamine right over here.
- So now this, as we learned in the previous video, has a
- partial negative charge, has these extra electrons, it will
- be attracted to this carbocation.
- So let me draw the methylamine.
- So you have the nitrogen, hydrogen, hydrogen bonded to a
- methyl group right like that.
- And in an Sn1 reaction, that's what this is.
- Remember, the rate-determining step right here only involves
- one of the reactants.
- That's where the number one comes from.
- In an Sn1 reaction like this, the nucleophile could attack
- from either side.
- This no longer is a tetrahedral shape.
- It's now a triangular planar shape, so it could go in from
- this direction or from that direction.
- So, in this case, well, I'm just going to
- draw it in this direction.
- So this nucleophile can give this electron right here to
- the carbocation, in which case, we will end up with
- something that looks like this.
- We have our nitrogen.
- It has bonded to two hydrogens over there.
- It has a methyl group, just like that.
- It has this electron over here.
- Now, this magenta electron has been given to this carbon over
- there, and then that carbon is bonded to three other carbons:
- one, two, three.
- And so, just like that, we have created--
- oh, we have to remember.
- We've given away an electron to this carbocation.
- The carbocation is now neutral.
- It gained an electron.
- Let me write the the carbon right there.
- But since the nitrogen gave away an electron, it now has a
- positive charge.
- So what will we call this molecule right over here?
- Now, this is a little bit tricky.
- So you have a methyl group over here.
- Well, this is no longer amine.
- It's an ammonium because it has this positive charge.
- We have four bonds on the nitrogen.
- So we could call this right here N-methyl.
- That tells us that the methyl is bonded to the nitrogen.
- It's not bonded to the other carbon chain.
- N-methyl.
- And then this right here, these four carbons, one, two,
- three, four, this you could view as a tert-butyl group.
- There's other ways to name it, but that's
- the most common one.
- So it's N-methyl-tert-butyl ammonium.
- Remember, we have that positive charge there.
- And as we saw in the previous video, this isn't ammonium
- right now, but you had some of your original methylamine
- floating around.
- If you have some of your original methylamine floating
- around, and this could in theory, or it can in practice
- even, nab away one of these hydrogens, or at least the
- proton part.
- So it gives its electron to that hydrogen, and then the
- hydrogen's electron gets taken back by what was an ammonium.
- And then if you have that-- and this is
- a reversible reaction.
- It can go in either direction.
- This methylamine is now methyl ammonium.
- So it now looks like this.
- It now has three hydrogens on it because this guy bonded
- with that hydrogen.
- It now has three hydrogens, so this is methyl ammonium.
- And then this thing over here will now look like this.
- It lost a hydrogen.
- So you have a methyl group bonded to an N, bonded to an
- H, which is then bonded to a tert-butyl group.
- So now this has become, as I just said,
- this is methyl ammonium.
- But this over here is N-methyl.
- Once again, the N tells us that this methyl group is
- bonded to the nitrogen.
- N-methyl-tert-, and now instead of calling it butyl
- ammonium, and actually we didn't have to have this dash
- over here; this is butyl ammonium.
- It's now butylamine, tert-butyl, and then obviously
- because we have that nitrogen there, butylamine.
- Anyway, I just wanted to show you that, that the mechanisms
- or the reactions I showed in the last video, they're not
- the only type of things that can occur.
- You can also have Sn1.
- And, in general, it can act as a weak base.
- It's definitely not a strong base.
- But it's the most basic of all of the neutral functional
- groups, which I've said over and over again.
- So there's many different types of reactions.
- I've just tried to give you a sample of a few.