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Organic chemistry
Elimination vs substitution: reagent
How to figure out if a reagent will act as a strong or weak base or a nucleophile in a reaction.
Want to join the conversation?
- why can't H-S(-) act as a base as mentioned at. Its conjugate acid is not that acidic. 3:40(8 votes)
- what's the difference between Elimination and substitution ?(1 vote)
- In a substitution reaction an existing group on the substrate is removed and a new group takes its place. In an elimination reaction the group is simply removed and no new group comes to take its place and this usually results in a double or triple bond forming in the substrate instead.
Hope that helps.(2 votes)
- Thinking very carefully, I discovered the upper nitrogen to be sp2, not sp3. So those electron pairs are actually in the p orbital. And I think the nitrogen is unable to bond further at all? In the first place, it cannot function as a base.(1 vote)
- He just showed the bottom nitrogen acting as the base..what’s the issue with that?(1 vote)
- How can you tell if something is a strong/weak acid or base?(1 vote)
- Other than just memorising what is a strong acid or strong case? You can't really(1 vote)
- I understand if you had a protic solvent, it would stabilize the strong base (to form weak acid) or the strong nucleophile. The protons would react with them. So in order to have an Sn2 or an E2, so you need an aprotic solvent. Aprotic solvent will favor Sn2 or an E2 reaction. But why does Sn2 favor strong nucleophile, and Sn1 favor weak nucleophile from the first place? In addition, does E2 favor strong base, and E2 favor weak base as well? I also don't get the reason.(1 vote)
- why can't the sp3 nitrogen delocalize its charge throughout resonance like the sp2 one? I know tertiary amines are less basic than secondary ones but I still can't make sense of this. 6:40(1 vote)
- Hi, at, you said that it is not acidic due to resonance, but doesn't resonance increase acidity, therefore making it a weak base? 6:30(1 vote)
- Water is a weak nucleophile. But it can polarize easily being a solvent.
Is the charge and polarizability of a nucleophile someway connected??
Can't the hydride ion be a nucleophile due to its high charge ?(1 vote)
Video transcript
- [Instructor] When
you're trying to determine between a substitution and
an elimination reaction, it's important to consider
the function of the reagent. Does your reagent
function as a nucleophile or does it function as a base? So first let's look at nucleophile. Let's consider the idea of charge. We know that water can
function as a nucleophile so we have a region of
high electron density around the oxygen. The oxygen is partially negative because oxygen is more
electrode negative than hydrogen so these hydrogens are partially positive. So water can function as a nucleophile, however it is a weak nucleophile because we do not have as high
a region of electron density as we would with the hydroxide ion. So the hydroxide ion is
a strong nucleophile. It has a full negative
charge on the oxygen instead of only a partial negative charge. Next let's consider the
polarizability of the nucleophile. Let's compare the hydroxide
ion with hydrogen sulfate. So we've already seen the hydroxide ion is a strong nucleophile
with its negative charge on the oxygen, but hydrogen
sulfate also turns out to be a strong nucleophile, even though it doesn't have a negative
charge on the sulfur. And that's because of the
concept of polarizability which is related to the size of the atom, the distance of electrons
from the nucleus. Sulfur is a larger atom than oxygen, and we have several electrons that are far away from the nucleus, and since those electrons
are far away from the nucleus the nucleus doesn't have
as much of a pull on them and it's easier to
polarize those electrons. So it's easier for, let's
say, a pair of these electrons to function as a nucleophile to get closer to an electrophile. And so that's the reason
why hydrogen sulfate is a strong nucleophile. Hydroxide, we've already
seen, is a strong nucleophile. The electrons are closer to the nucleus so the nucleus has a
stronger pull on them. So it's not as polarizable
but it still turns out to be a strong nucleophile
because of the negative charge. When you get to something
like the hydride ion, this is very small. We know that hydrogen is the smallest atom so these two electrons are
pretty close to the nucleus, so the nucleus has a
very strong pull on them. So because this electron cloud is so close to the nucleus it's not polarizable, even though it has a
negative charge on it. So this does not function as a nucleophile so the hydride ion can't
function as a nucleophile because it's not polarizable. When you're trying to figure
out the nature of the reagent there are four different categories and you want to assign your reagent to one of those four categories. The first one is where your reagent acts only as a nucleophile and not as a base, and a good example of that
would be the chloride anion. So it can act as a nucleophile because we have a
negative one charge here, we have a region of high electron density. But it can't act as a
base, and think about why. The conjugate acid to the
chloride anion would be HCl. Just add an H plus to Cl
minus and you get HCl. And we know that HCl is a strong acid, and we also know the stronger the acid the weaker the conjugate base, so the chloride anion is a very weak base, and that's why it's only gonna function as a nucleophile in our reactions. So the same idea for the bromide
anion and the iodide anion. And then we also have
our sulfur nucleophiles which we just saw. This hydrogen sulfate here
is a strong nucleophile because of the polarizability of sulfur. But these are also only gonna function as a nucleophile in our reactions, and that's because the conjugate
acids are fairly acidic. So the same reason we
talked about over here. The second category is when your reagent only functions as a base
and not a nucleophile. And the example of that
would be the hydride ion. We've already seen why the hydride ion does not function as a nucleophile but now let's talk about
why it's a strong base. If you think about the
conjugate acid to H minus, just add an H plus and
you, of course, get H2. We know that H2 is a very stable molecule which makes it a very weak acid. And the weaker the acid the stronger the conjugate base, which makes the hydride
anion a very strong base. And if you see it in a reaction think base only as the reagent. You would get the hydride
ion, like sodium hydride, so NaH would be your source. Another example is this molecule which has the abbreviation DBN. So DBN functions as a base
only and not a nucleophile. You might think a lone pair of electrons on the nitrogen could
function as a nucleophile, but not when you have this
fused ring system here. That would be too bulky and prevents this from acting as a nucleophile. It does act as a base though. And let's figure out which
nitrogen gets protonated. Is it this sp3 hybridized nitrogen or this sp2 hybridized nitrogen? It turns out to be the
sp2 hybridized nitrogen, and let's look at why. So I'm gonna make this lone
pair of electrons magenta, and that long pair of electrons is gonna pick up a proton and form a bond. So that lone pair turns
into this bond here, and now this nitrogen would have a plus one formal charge. So plus one formal
charge on this nitrogen, this is the conjugate acid to DBN. And this conjugate acid
is resonance stabilized. I could push these electrons into here and then push these electrons
off on to the nitrogen, and let's follow those electrons. Let's make these electrons red. So this lone pair moves into
here to form a double bond, and then let's make these
electrons in here blue. The blue electrons come
off onto this nitrogen and we still have a bond
to our hydrogen in here, which moves the formal charge
to this other nitrogen. So this nitrogen now has
a plus one formal charge. So our conjugate acid
is resonance stabilized, the positive charge is
delocalized over two nitrogens. And because our conjugate
acid is resonance stabilized it's not very acidic, which means that the
conjugate base is strong, and that's why this is
a strong conjugate base even though it's a neutral molecule. So protonation occurs at
the sp2 hybridized nitrogen and not the sp3. If you tried to protonate sp3 it wouldn't be able to delocalize the positive charge over both nitrogens. So there's a similar molecule to DBN, which is abbreviated DBU,
and it acts in the same way, only as a base in a reaction. Our third category is where our reagent is a strong nucleophile and a strong base, and a good example of
that is the hydroxide ion. We've already talked about
why the hydroxide ion is a strong nucleophile, and
we notice from experience that hydroxide is a strong base. So something like sodium hydroxide is used all the time in general chemistry. If we replace the hydrogen
with an alkyl group, we form an alkoxide ion which functions in a similar way to the hydroxide ion. So they're both examples of strong nucleophiles and strong bases. Our fourth and last category is weak nucleophile/weak base. And the water molecule, we
know, is a weak nucleophile. It does not have a negative one formal charge on the oxygen. And water, of course, is a weak base. So the conjugate acid
will be H3O plus, right? Just add an H plus to
H2O and you get H3O plus. And we know the hydronium
ion is fairly acidic. So it would have a weak conjugate base. If you replace one of the hydrogens with an alkyl group,
then you form an alcohol, which is also a weak
nucleophile and a weak base.