Nucleophilicity vs. Basicity

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.