Nucleophilicity vs. Basicity - Organic Chemistry, Chemistry Video Lecture - Class 12

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.