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Sn1 Amine Reaction

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Sn1 Amine Reaction

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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.

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Sn1 Amine Reaction

Sn1 Amine Reaction