Amine as Nucleophile in Sn2 Reaction

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Amine as Nucleophile in Sn2 Reaction

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We spent a couple of videos naming amines.
In this video, let's actually see how they might react.
Now, to just kind of understand what happens with
an amine, let's study one in particular.
Let's look at ethylamine.
That's its common name, and that's obviously going to be a
two-carbon chain, so that's the ethyl part, connected to a
nitrogen, so that is the amine part.
And then the nitrogen will have two other bonds, two
hydrogens, just like that.
And if you look at the Periodic Table, so here's our
Periodic Table right here, you see nitrogen is in Group 5:
one, two, three, four, five.
This says Group 15, but we're going to ignore the transition
metals for now.
So it has five valence electrons.
When you look at it, when you look at the bonds here, you
only see one, two, three valence electrons.
So there's going to be two more and they're going to be
in a lone pair, just like that.
When you also look at the Periodic Table, you also see
that nitrogen is pretty electronegative.
In fact, there's only a few elements that are more
electronegative.
But remember, electronegativity is just how
much does it like to hog electrons?
So it's more electronegative than carbon.
It's to the right of carbon and it's way more
electronegative than hydrogen.
So if you compare it, not only does it have this lone pair of
electrons right here, but it's also going to hog-- let me do
it in a different color.
It's also going to hog these orange electrons that are on
the other side of the bond.
So, as we know, implicitly there's a
carbon right over here.
There's another carbon over here.
There's two hydrogens over here.
We haven't drawn all of that stuff.
But it's more electronegative than everything it's bonded to
so its also going to attract those electrons, so it's going
to have a partial negative charge.
Let me make it very clear.
The partial negative charge-- that's carbon right there.
The partial negative charge is at the nitrogen itself.
And you're going to have slightly partial positive
charges of the carbons and the hydrogens,
especially at the hydrogens.
And now if we just talk about the properties of it, because
it's polar, this would actually give it a reasonably
high melting and boiling point.
You could imagine, amines when they are in a solution with
other amines, they are going to be attracted essentially
via this type of hydrogen-type bonding.
The negative part of partial negative nitrogens are going
to be attracted to the partially positive hydrogens
of other amines.
Now, the hydrogen bonding for an amines is not going to be
as strong as it will be for, say, waters or alcohols
because waters and alcohols involve oxygen, and oxygen is
even more electronegative than nitrogen.
So these partial charges are even stronger.
So that's why hydrogen bonding is even stronger in the case
of things that have oxygen in it like alcohol or water.
Now, with that said, let's think about
how this might react.
I have a lone pair over here.
I also have a partial negative charge because this guy is
more electronegative.
It seems like we have a lot of negative action going around
the nitrogen there.
And because it has that, maybe it would either like to give
away electrons or maybe take hydrogens from other things.
And that, in fact, is the case with amines.
So in general, and let me write this down, this is
ethylamine right here, just so we get our naming practice.
Ethylamine, this is the common name.
We just said this is an ethyl group attached to an amine.
And this is obviously a primary amine.
It's only attached to one carbon right here.
But in the case of an amine like this, and actually out of
all of the functional groups we will look at, out of all
the neutral functional groups we look at, because of what we
just described, because it has this partial negative charge,
it has this lone pair, the nitrogen's pretty
electronegative, this is actually the most basic.
And this isn't just ethylamine.
This is all amines.
Let me write this down.
Amines are the most basic of the neutral functional groups.
Obviously, something like a hydroxide anion, it has a
negative charge.
It will want to attract protons even more.
But this one's pretty good,
considering that it is neutral.
So given that it's basic, and we'll also say, and it's a
related notion, it's also nucleophilic.
And just as a reminder, when people talk about basicity,
they're talking about how much does something want to give
away electrons?
Or how much does it want to take protons?
When you talk about the nucleophilicity, or how good
of a nucleophile something is, it's how good are you at
giving away electrons?
Or how good are you at essentially being attracted to
protons or giving away electrons?
And so when you talk about basicity, you're talking about
a thermodynamic property.
When you're talking about nucleophilicity, you're
talking about a kinetic property.
How good is it at interacting?
The basic tells you how stable is it once it has interacted.
So given that it's a nucleophile, you could
imagine, I think you see where I might be going with this.
Let me draw the ethylamine again.
So ethylamine, I'll draw it in a little bit simpler.
So let me draw the hydrogens like that, and then it has
this lone pair.
Let's think about what might happen if we have a solution
of ethylamine.
If we have ethylamine and it's mixed in with some
bromoethane.
So that's one, two carbons right there.
Let me draw the bromo group.
And it might not be obvious the way I drew this
bromoethane right here.
Let me write this down.
This is bromoethane.
But I think it will be obvious if I draw this carbon right
here and if I draw the other two hydrogens, which are
implicitly there.
So that is one hydrogen right there and then you have
another hydrogen over here.
And you have a good nucleophile in the ethylamine.
So what do you think will happen?
Oh, yeah, I also have a good leaving group.
So you see I'm talking in the language of Sn2 reactions.
Or really, I'm talking in the language of nucleophilic
substitution reactions, and I'm going to lean towards the
Sn2 direction because I have a primary carbon here, good
leaving group, pretty OK nucleophile.
If this was a secondary, especially if this was a
tertiary carbon, then I would be thinking Sn1.
Then I would say, oh, maybe this leaves on its own.
But in an Sn2, what we're going to have is, this
nucleophile will attack or it will give away its electron,
so it'll give its electron to this carbon.
And simultaneously, since this electron is going towards the
slightly positive, the partially positive carbon
here, this bromine can finally take away that carbon's
electrons, and so we just experienced an Sn2 reaction.
This is an Sn2 reaction.
And then after that is done, the bromine will have left.
It has taken this electron.
It already had this one over here.
It already had that one.
And actually, it had another six electrons.
One, two, three, four, five, six, seven valence electrons
for bromine.
Now, it will have eight.
So, one, two, three, four, five, six, seven, now it will
have eight.
It just took this one right here.
It gained an electron, so it has a negative charge.
And now let me draw this part right here.
So we had this carbon, that carbon.
That hydrogen is sticking-- let me draw it like this.
That hydrogen is sticking out, just like that.
The other hydrogen is behind it.
It's bonded to the other carbon up here.
That's really a CH3.
It has three hydrogens off of it implicitly there.
And now the ethylamine has attached to it.
So let me draw that.
So that's the nitrogen.
It's bonded to one, two carbons just like that.
And then it has a hydrogen, a hydrogen, and then you have
this electron is still with the nitrogen.
And then this magenta electron has been given to the carbon,
just like that.
And since the nitrogen has given away an electron, it now
has a positive charge.
So when you think about these two molecules, this molecule
now has a positive charge.
This molecule right now has a negative charge.
So they would actually be ionically attracted to each
other, and we've actually just formed a salt.
And this would be called-- and this is interesting here, at
least from a naming perspective.
You have two ethyl groups.
It might not be completely clear.
You have one, two carbons here, and then you have
another one, two carbons there.
We could re-draw this as-- or we could rewrite it as NH2.
And then we have two carbons, and then another two carbons
just like that.
So we could call this diethyl-- we have one ethyl,
two ethyls-- so this is diethyl.
Let me color this.
Let me do it in a different color here.
So you have an ethyl group right there, an ethyl group
right there, so we call this-- this is the common name for
it-- diethyl.
And then you might want to call it diethylamine, but it's
not ammonia or it's not derivative of ammonia anymore.
It's a derivative of ammonium.
And that's because it's now positively charged.
This nitrogen now has four bonds.
So this is diethylammonium.
Diethylammonium is this molecule right here.
And if you view it as a salt, and a salt is just an
ionically-- well, I don't want to go into the definition
completely of a salt, but you view this as a salt because
it's a ionically bonded molecule, where one of the
molecules is diethylammonium, the other one will be bromide.
This is the bromide anion right here.
And you always list the cation first, the positive charged
first. So this is diethylammonium bromide, which
we were able to form with the Sn2 reaction between
ethylamine or ethylamine and bromoethane.
Now, we can now go under another step here.
If we have a ton of this ethylamine floating around, if
we have an excess, and early on we will have a lot more of
that than we will have of this thing right here, if we have a
ton of ethylamine floating around-- let me draw some
ethylamine.
If we have a ton of the ethylamine floating around, it
might be able to grab one of the hydrogens from the
diethylammonium over here.
So you could imagine a situation
where it grabs a hydrogen.
It essentially gives its electron to the hydrogen, and
then that hydrogen's electron can be
taken back by the nitrogen.
And I'll draw this as an equilibrium because it's not
really that much more likely that this guy takes away that
guy's electron than that guy can go and take the electron
back, so it's a reversible reaction.
But if that were to happen, then this guy, this guy right
over here, will then look like this.
You have the ethyl group bonded to a nitrogen that only
has one hydrogen on it.
Let me just draw it like this.
A nitrogen that only has one hydrogen on it.
And then he's bonded to another ethyl
group right over there.
And now he has his-- so if you view this as an electron right
there, he has his lone pair.
So he took that-- it looks like yellow ochre, just kind
of a yellow-green color-- took that back from the hydrogen
and is now neutral, got an electron back.
So now, this is just diethyl ammonia.
Actually, no.
I'm sorry.
It wouldn't be diethyl ammonia.
It would be diethylamine.
Sorry about that.
Let me erase that.
This would be diethylamine.
It would be ammonia.
Ammonia is just NH-- let me draw it like this.
Ammonia, just to make things clear, is this, and that's why
my brain was telling us to go ammonia.
And then ammonium is this.
Give away one of these electrons to a hydrogen
proton, you have a positive charge.
Now, when you do the common naming, when it's kind of
ammonium derivative, you do say diethylammonium.
And then my brain said, OK, now that we got rid of the
hydrogen, I went back to ammonia, so I said diethyl
ammonia, but that's wrong.
Obviously, this is now an amine, so we used the common
naming for amines, and we get back to diethylamine.
And then this guy over here will turn into ethylammonium.
Sorry for all the confusion.
So this guy over here is a nitrogen, two hydrogens, ethyl
group, and then he grabbed another hydrogen.
So he grabbed this hydrogen over here.
So now he has a positive charge since he gave away that
electron over there.
So now he is ethylammonium.
But this is reversible right here.
But what you can see, through this Sn2 reaction, we were
able to essentailly, get rid of one of these hydrogens and
substitute it with another ethyl group.
We went from having a primary amine-- this is bonded to one
carbon-- to having a secondary amine
through this Sn2 reaction.
It's bonded to two carbons.