What I want to do in this video is differentiate between
the ideas of nucleophilicity or how strong of a nucleophile
something is, and basicity.
The difference is at one level subtle, but it's actually a
very big difference.
And I'll show you why it's kind of confusing the first
time you learn it.
When we studied Sn2 reactions, you have a nucleophile that
has an extra electron right here.
It has a negative charge.
And maybe you have a methyl carbon.
Let me draw it.
Maybe you have a hydrogen coming out.
You have a hydrogen behind it.
You have a hydrogen up top.
Then you have a leaving group right over there.
In an Sn2 reaction, the nucleophile will give this
electron to the carbon.
The carbon has a partial positive charge.
Let me draw that.
The leaving group has a partial negative charge
because it tends to be or will be more electronegative.
So this electron is given to this carbon right when the
carbon gets that, or simultaneously with it, this
electronegative leaving group is able to completely take
this electron away from the carbon.
Then after you are done, it looks like this.
We have our methyl carbon so the hydrogen is in the back,
hydrogen in the front, hydrogen on top.
The leaving group has left.
It had this electron right there, but now it also took
that magenta electron so it now has a negative charge and
the nucleophile has given this electron right over here and
so now it is bonded to the carbon.
The whole reason I did this is because this is acting as a
nucleophile.
It loves nucleuses.
It's giving away its extra electron, but it is also
acting as a Lewis base.
This is a bit of a refresher.
A Lewis base, which is really the most general, or I guess
it covers the most examples of what it means to be a base.
a Lewis base means you are an electron donor.
That's exactly what's happening here.
This nucleophile is donating an electron to the carbon.
So, it's acting like a Lewis base.
So for the first time you see that, you're like, well, why
did chemists even go through the pain of defining something
like a nucleophile?
Why don't they just call it a base?
Why are there two different concepts of nucleophilicity
and basicity?
The difference is that nucleophilicity is a kinetic
concept, which means how good is it at reacting?
How fast is it at reacting?
How little extra energy does it need to react?
When something has good nucleophilicity,
it is good it reacting.
It doesn't tell you anything about how stable or unstable
the reactants before and after are, It just tells you they're
good at reacting with each other.
Basicity is a thermodynamic concept.
It's telling you how stable the reactants or
the products are.
It tells you how badly something would like to react.
For example, we saw the situation of fluorine.
Let's think about this.
We saw the situation-- actually, I should say
fluoride, so fluoride looks like this.
Seven valence electrons for fluorine and then it swiped
one extra electron away.
You get fluoride.
So fluoride is reasonably basic.
It is more basic than iodide.
But in a protic solution-- let me write it here.
But less nucleophilic in protic solution.
And a protic solution, once again, has
hydrogen protons around.
And the reason why this is, is fluoride, it wants to bond
with a carbon or something else more badly, or maybe even
a hydrogen proton.
It wants to bond with it more badly than an iodide anion.
If it did, it actually will be a stronger bond than the
iodide anion will form, that the fluoride anion is actually
less stable in this form than the iodide is.
If it were to be able to get a proton or give its electron
away, it will be happier, but it's less nucleophilic.
It's less good at reacting in a protic solution.
The whole reason it's less nucleophilic is because there
are other things that are keeping it from reacting.
We saw in the video on what makes a good nucleophile, and
in the case of fluoride, it's because it's
a very small atom.
It's actually a very small ion so it's very closely held.
The electron cloud is very tight, and so what it allows
is the hydrogens from the water to form a very tight
shell around.
These all have partial positive charges so they're
attracted to the negative anion.
They form a very tight shell protecting the fluoride anion,
which makes it harder for it to react in a protic solution,
so it doesn't react as well.
If it was able to react, it actually will form a stronger
bond than the iodide anion.
So that's the big difference, just so we see the
difference in trends.
So basicity, it does not matter what your actual
solvent is.
It is a thermodynamic property of the molecule or
the atom of the anion.
So if you looked at pure basicity, the strongest base
you see-- and I'll just write hydroxide here.
It's normally something like sodium hydroxide or potassium
hydroxide, but when you dissolve it in something like
water the sodium and the hydroxide separates, and it's
really the hydroxide that acting as a base, something
that wants to donate electrons.
So hydroxide is a much stronger base than fluoride,
which is a stronger base than chloride, which is a stronger
base than bromide, which is a stronger base than iodide.
Now, if you were to look at nucleophilicity just to see
the difference, we saw that what the solvent is actually
matters because the solvent will affect how good something
is at reacting.
So in nucleophilicity, there's a difference between a protic
solvent and an aprotic solvent.
In a protic solvent, the thing that has the best
nucleophilicity is actually iodide because it's not
hindered by these hydrogen bonds as much.
It doesn't have a tight shell.
It has this big molecular cloud, and some people think
it also has kind of a softness.
It has this polarizability where that cloud can be pulled
towards the carbon and do what it needs to do.
So in this case, iodide is a better nucleophile, let me
just say, than hydroxide, which is a better nucleophile
than fluorine.
Now, in an aprotic solution, where all of a sudden the
interactions with the solvent are not going to be as
significant, then things change.
In this situation, basicity matters.
So in an aprotic solution, basicity and
nucleophilicity correlate.
I'll put an asterisk here because there's also one other
aspect of nucleophilicty that I haven't talked about yet,
but I'll talk about it in a second.
In this type of a situation, hydroxide will be better at
reacting than fluoride, which would be better at reacting
than iodide.
And the whole reason why in both situations hydroxide is--
I mean, even when it can interact with the solvent,
it's still a pretty good nucleophile, because if you
think about hydroxide, and I have to think about this a
lot, it has an extra electron.
If you think about it, you could imagine it's water that
took away-- let me draw it this way.
You can imagine it's water where a proton left or where
an electron was taken from a proton, so normally, you'd
have two pairs and now you have a third pair right here.
This oxygen has one, two, three, four, five, six, seven
valence electrons, one more than neutral oxygen, so it has
a negative charge.
It already has an extra electron that gives this
negative charge, but oxygen is also more electronegative than
hydrogen, so it's also able to get this guy involved a little
bit anyway.
It's a very basic molecule.
So even when it might be interfered a little bit by a
protic environment like water, it's still a better
nucleophile than something like fluoride.
If you take the solvent out of the picture, it's a super
strong base.
It's also going to be a very, very good nucleophile.
Now, the last aspect of nucleophilicity, remember,
nucleophilicity is how good something reacts.
Now, let's imagine we have something here.
We have two hydroxide molecules, right?
Let's say that this one is just a straight-up hydroxide.
And let's say this one over here has all sorts of
things off of it.
Let's say it has this big chain of stuff.
I don't know which one.
Now if you were to look at these two molecules, if you
were to try to guess which one is going to be a better
nucleophile, you should just remember: nucleophilicity is
how good something reacts, how good is it getting in there
and making a reaction happen.
This thing has this big molecule all around it.
It might actually make it very hard, if you go back to this
circumstance up here, it might make it very hard for it to
get in there.
We've talked about steric hindrance from the point of
view of the carbon, but we haven't really talked about it
from the point of view the nucleophile.
In this nucleophile right here, it might be hard for
this extra electron right here to actually get
to the target nucleus.
It will be hindered.
While in this situation, it will be much easier, even
though the group that's reacting, this oxygen that has
a negative charge, this extra electron, is on some level
fairly, fairly equivalent.
But this one right here is a much smaller molecule.
It'll be less hindered, easier to get in.
So this'll be a better nucleophile.
And that's why I didn't want to make the strong statement
that in an aprotic solution, basicity and nucleophilicity
are completely correlated, because nucleophilicity still
has that other element of how hindered is it.
Is it in an environment or is it part of a molecule that
will keep it from reacting even though it might be a very
strong base?
If it actually forms a bond, it'll be very strong.
The big thing to remember is that they're just two
fundamentally different concepts and that's why there
are two different terms for them.
Nucleophilicity, how good is it at reacting, saying nothing
about how good the resulting bond is.
Basicity is how good is the bond?
How badly does it want to react, but it doesn't say how
good is it at reacting itself.