WEBVTT

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What I want to do in this video
is differentiate between

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the ideas of nucleophilicity or
how strong of a nucleophile

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something is, and basicity.

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The difference is at one level
subtle, but it's actually a

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very big difference.

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And I'll show you why it's kind
of confusing the first

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time you learn it.

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When we studied Sn2 reactions,
you have a nucleophile that

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has an extra electron
right here.

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It has a negative charge.

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And maybe you have
a methyl carbon.

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Let me draw it.

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Maybe you have a hydrogen
coming out.

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You have a hydrogen behind it.

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You have a hydrogen up top.

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Then you have a leaving group
right over there.

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In an Sn2 reaction, the
nucleophile will give this

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electron to the carbon.

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The carbon has a partial
positive charge.

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Let me draw that.

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The leaving group has a partial
negative charge

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because it tends to be or will
be more electronegative.

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So this electron is given to
this carbon right when the

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carbon gets that, or
simultaneously with it, this

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electronegative leaving group
is able to completely take

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this electron away
from the carbon.

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Then after you are done,
it looks like this.

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We have our methyl carbon so the
hydrogen is in the back,

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hydrogen in the front,
hydrogen on top.

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The leaving group has left.

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It had this electron right
there, but now it also took

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that magenta electron so it now
has a negative charge and

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the nucleophile has given this
electron right over here and

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so now it is bonded
to the carbon.

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The whole reason I did this is
because this is acting as a

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nucleophile.

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It loves nucleuses.

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It's giving away its extra
electron, but it is also

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acting as a Lewis base.

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This is a bit of a refresher.

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A Lewis base, which is really
the most general, or I guess

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it covers the most examples of
what it means to be a base.

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a Lewis base means you are
an electron donor.

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That's exactly what's
happening here.

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This nucleophile is donating
an electron to the carbon.

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So, it's acting like
a Lewis base.

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So for the first time you see
that, you're like, well, why

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did chemists even go through the
pain of defining something

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like a nucleophile?

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Why don't they just
call it a base?

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Why are there two different
concepts of nucleophilicity

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and basicity?

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The difference is that
nucleophilicity is a kinetic

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concept, which means how
good is it at reacting?

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How fast is it at reacting?

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How little extra energy
does it need to react?

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When something has good
nucleophilicity,

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it is good it reacting.

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It doesn't tell you anything
about how stable or unstable

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the reactants before and after
are, It just tells you they're

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good at reacting with
each other.

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Basicity is a thermodynamic
concept.

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It's telling you how stable
the reactants or

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the products are.

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It tells you how badly something
would like to react.

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For example, we saw the
situation of fluorine.

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Let's think about this.

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We saw the situation-- actually,
I should say

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fluoride, so fluoride
looks like this.

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Seven valence electrons for
fluorine and then it swiped

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one extra electron away.

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You get fluoride.

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So fluoride is reasonably
basic.

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It is more basic than iodide.

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But in a protic solution--
let me write it here.

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But less nucleophilic
in protic solution.

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And a protic solution,
once again, has

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hydrogen protons around.

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And the reason why this is, is
fluoride, it wants to bond

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with a carbon or something else
more badly, or maybe even

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a hydrogen proton.

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It wants to bond with it more
badly than an iodide anion.

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If it did, it actually will be
a stronger bond than the

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iodide anion will form, that the
fluoride anion is actually

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less stable in this form
than the iodide is.

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If it were to be able to get a
proton or give its electron

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away, it will be happier, but
it's less nucleophilic.

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It's less good at reacting
in a protic solution.

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The whole reason it's less
nucleophilic is because there

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are other things that are
keeping it from reacting.

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We saw in the video on what
makes a good nucleophile, and

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in the case of fluoride,
it's because it's

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a very small atom.

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It's actually a very small ion
so it's very closely held.

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The electron cloud is very
tight, and so what it allows

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is the hydrogens from the water
to form a very tight

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shell around.

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These all have partial positive
charges so they're

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attracted to the
negative anion.

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They form a very tight shell
protecting the fluoride anion,

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which makes it harder for it to
react in a protic solution,

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so it doesn't react as well.

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If it was able to react, it
actually will form a stronger

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bond than the iodide anion.

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So that's the big difference,
just so we see the

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difference in trends.

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So basicity, it does not
matter what your actual

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solvent is.

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It is a thermodynamic property
of the molecule or

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the atom of the anion.

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So if you looked at pure
basicity, the strongest base

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you see-- and I'll just
write hydroxide here.

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It's normally something like
sodium hydroxide or potassium

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hydroxide, but when you dissolve
it in something like

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water the sodium and the
hydroxide separates, and it's

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really the hydroxide that acting
as a base, something

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that wants to donate
electrons.

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So hydroxide is a much stronger
base than fluoride,

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which is a stronger base than
chloride, which is a stronger

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base than bromide, which is a
stronger base than iodide.

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Now, if you were to look at
nucleophilicity just to see

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the difference, we saw that what
the solvent is actually

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matters because the solvent will
affect how good something

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is at reacting.

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So in nucleophilicity, there's
a difference between a protic

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solvent and an aprotic
solvent.

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In a protic solvent, the
thing that has the best

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nucleophilicity is actually
iodide because it's not

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hindered by these hydrogen
bonds as much.

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It doesn't have a tight shell.

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It has this big molecular cloud,
and some people think

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it also has kind
of a softness.

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It has this polarizability
where that cloud can be pulled

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towards the carbon and do
what it needs to do.

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So in this case, iodide is a
better nucleophile, let me

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just say, than hydroxide, which
is a better nucleophile

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than fluorine.

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Now, in an aprotic solution,
where all of a sudden the

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interactions with the solvent
are not going to be as

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significant, then
things change.

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In this situation,
basicity matters.

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So in an aprotic solution,
basicity and

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nucleophilicity correlate.

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I'll put an asterisk here
because there's also one other

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aspect of nucleophilicty that
I haven't talked about yet,

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but I'll talk about
it in a second.

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In this type of a situation,
hydroxide will be better at

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reacting than fluoride, which
would be better at reacting

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than iodide.

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And the whole reason why in both
situations hydroxide is--

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I mean, even when it can
interact with the solvent,

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it's still a pretty good
nucleophile, because if you

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think about hydroxide, and I
have to think about this a

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lot, it has an extra electron.

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If you think about it, you could
imagine it's water that

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took away-- let me
draw it this way.

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You can imagine it's water where
a proton left or where

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an electron was taken from a
proton, so normally, you'd

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have two pairs and now you have
a third pair right here.

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This oxygen has one, two, three,
four, five, six, seven

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valence electrons, one more than
neutral oxygen, so it has

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a negative charge.

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It already has an extra electron
that gives this

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negative charge, but oxygen is
also more electronegative than

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hydrogen, so it's also able to
get this guy involved a little

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bit anyway.

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It's a very basic molecule.

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So even when it might be
interfered a little bit by a

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protic environment like water,
it's still a better

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nucleophile than something
like fluoride.

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If you take the solvent out of
the picture, it's a super

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strong base.

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It's also going to be a very,
very good nucleophile.

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Now, the last aspect of
nucleophilicity, remember,

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nucleophilicity is how good
something reacts.

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Now, let's imagine we
have something here.

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We have two hydroxide
molecules, right?

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Let's say that this one is just
a straight-up hydroxide.

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And let's say this one over
here has all sorts of

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things off of it.

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Let's say it has this
big chain of stuff.

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I don't know which one.

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Now if you were to look at these
two molecules, if you

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were to try to guess which one
is going to be a better

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nucleophile, you should just
remember: nucleophilicity is

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how good something reacts, how
good is it getting in there

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and making a reaction happen.

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This thing has this big molecule
all around it.

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It might actually make it very
hard, if you go back to this

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circumstance up here, it might
make it very hard for it to

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get in there.

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We've talked about steric
hindrance from the point of

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view of the carbon, but we
haven't really talked about it

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from the point of view
the nucleophile.

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In this nucleophile right here,
it might be hard for

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this extra electron right
here to actually get

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to the target nucleus.

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It will be hindered.

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While in this situation, it
will be much easier, even

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though the group that's
reacting, this oxygen that has

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a negative charge, this extra
electron, is on some level

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fairly, fairly equivalent.

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But this one right here is
a much smaller molecule.

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It'll be less hindered,
easier to get in.

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So this'll be a better
nucleophile.

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And that's why I didn't want to
make the strong statement

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that in an aprotic solution,
basicity and nucleophilicity

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are completely correlated,
because nucleophilicity still

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has that other element of
how hindered is it.

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Is it in an environment or is
it part of a molecule that

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will keep it from reacting even
though it might be a very

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strong base?

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If it actually forms a bond,
it'll be very strong.

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The big thing to remember
is that they're just two

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fundamentally different concepts
and that's why there

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are two different
terms for them.

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Nucleophilicity, how good is it
at reacting, saying nothing

00:12:35.850 --> 00:12:38.110
about how good the resulting
bond is.

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Basicity is how good
is the bond?

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How badly does it want to react,
but it doesn't say how

00:12:43.570 --> 00:12:46.540
good is it at reacting itself.

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