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Course: Organic chemistry > Unit 5
Lesson 2: Nucleophilicity and basicityNucleophilicity vs. basicity
Nucleophilicity vs. Basicity . The difference between what it means to be a nucleophile and a base. Created by Sal Khan.
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- Okay, so let me get this straight... Basicity is absolute and nucleophilicity depends on surrounding conditions. Is that correct?(10 votes)
- Yes, basicity is an intensive property of an atom or molecule. Nucleophilicity does depend on the molecule, but also a significant amount on it's environment.(7 votes)
- Why is Khan explaining the nucleophilicity and basicity in such a confusing way? He could have just said nucleophilicity donates electron pairs and basicity attracts protons. In other videos, they never explain like this.(7 votes)
- I would agree with you. Sal's main area of expertise is in mathematics, not necessarily other subjects. I feel like this is one of the reasons why KA hired a chemistry professor to redo most of the organic chemistry videos.(6 votes)
- Why is OH(-) more nucleophilic in a protic solution than a Fluoride ion? Surely it will be more affected by the Hydrogen bonding that will occur as the Oxygen will be more attracted to the protons in the protic solvent as it has a stronger negative charge? Or am I just completely wrong?(5 votes)
- A fluoride ion has a stronger negative character than a hydroxide ion. Fluorine is more electronegative AND the oxygen in hydroxide ion is already bonded to a hydrogen. Hydrogen bonding will occur with both ions, but more so with fluoride ion.(5 votes)
- hmm..which one is more nucleophilic. an alkoxide anion or a hydroxide anion?(3 votes)
- an alkoxide molecule has more steric hindrance and less distinct polarization, making it less attracted to the positive carbon center.(4 votes)
- Could anyone explain me why is Fluoride anion a better nucleophile than Iodide anion in an aprotic solvent?(3 votes)
- Because if its high charge density. In a protic solvent, the fluoride ion is highly solvate, and this makes it a poor nucleophile. In a polar aprotic solvent, the cation is preferentially solvated, leaving the fluoride ion relatively "bare". So it becomes a much better nucleophile.
The iodide ion, on the other hand, has a low charge density and is highly polarizable, so it is a good nucleophile even in protic solvents.(2 votes)
- In my textbook, it has been given that haloalkanes are not soluble in polar solvents. Why is it so!?(2 votes)
- Haloalkanes are slightly soluble in polar solvents (like water) because they themselves are largely non-polar as although there is a difference in electronegativity due to the alkane, it's a small one as the alkane branch usually is much larger and alkanes are non-polar and therefore not soluble in polar solvents.(2 votes)
- Is it fair to say that Iodide has more steric hindrance with the surrounding H+ protons in a protic solution, which is why it is more reactive than the fluoride?(2 votes)
- Sal has covered it here: https://www.khanacademy.org/science/organic-chemistry/substitution-elimination-reactions/nucleophilicity-basicity-sal/v/nucleophilicity-nucleophile-strength
Due to hydrogen bonding, there is a much larger number of protons around the Fluoride ion than the Iodide ion, since the Fluoride ion has a higher electron-density. Hence, it's more sterically hindered, even though in the absence of a solvent, it's smaller than the iodide ion.(2 votes)
- Is F- more nucleophilic in a aprotic solution relative to I-?(2 votes)
- Yes. Sal has covered it here: https://www.khanacademy.org/science/organic-chemistry/substitution-elimination-reactions/nucleophilicity-basicity-sal/v/nucleophilicity-nucleophile-strength
In a polar aprotic solvent, there is no Hydrogen bonding, hence kinetically, F- does not become sterically hindered. Besides that, it's already smaller than I- without the hydrogen bonding involved. So, to answer your question, F- is more nucleophilic in an aprotic solvent relative to I-(2 votes)
- How do you compare nucleophilicity of, say, hydroxide and methoxide? Methoxide isn't hindered enough for steric strain to be a major factor.(2 votes)
- When a negative charge is present on same type of atom in different nucleophiles, the order of nucleophilicity is determined by taking the strength of their conjugate acids. If the acid is stronger, its nucleophilicity is less. CH3OH is a slightly more acidic (pKa=15.5) than H2O (pka=15.74). Therefore the hydroxide ion is slightly more nucleophilic than the methoxide ion.
This is possible to remember as an exception. Generally ROH is a weaker acid than H2O, but not in the case of CH3OH.(2 votes)
- out of HI AND HF which is good leaving group?(2 votes)
- I⁻ is a great leaving group.
F⁻ is a poor leaving group, because the C-F bond is quite strong.(2 votes)
Video transcript
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.