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Amine as Nucleophile in Sn2 Reaction
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- We spent a couple of videos naming amines.
- In this video, let's actually see how they might react.
- Now, to just kind of understand what happens with
- an amine, let's study one in particular.
- Let's look at ethylamine.
- That's its common name, and that's obviously going to be a
- two-carbon chain, so that's the ethyl part, connected to a
- nitrogen, so that is the amine part.
- And then the nitrogen will have two other bonds, two
- hydrogens, just like that.
- And if you look at the Periodic Table, so here's our
- Periodic Table right here, you see nitrogen is in Group 5:
- one, two, three, four, five.
- This says Group 15, but we're going to ignore the transition
- metals for now.
- So it has five valence electrons.
- When you look at it, when you look at the bonds here, you
- only see one, two, three valence electrons.
- So there's going to be two more and they're going to be
- in a lone pair, just like that.
- When you also look at the Periodic Table, you also see
- that nitrogen is pretty electronegative.
- In fact, there's only a few elements that are more
- electronegative.
- But remember, electronegativity is just how
- much does it like to hog electrons?
- So it's more electronegative than carbon.
- It's to the right of carbon and it's way more
- electronegative than hydrogen.
- So if you compare it, not only does it have this lone pair of
- electrons right here, but it's also going to hog-- let me do
- it in a different color.
- It's also going to hog these orange electrons that are on
- the other side of the bond.
- So, as we know, implicitly there's a
- carbon right over here.
- There's another carbon over here.
- There's two hydrogens over here.
- We haven't drawn all of that stuff.
- But it's more electronegative than everything it's bonded to
- so its also going to attract those electrons, so it's going
- to have a partial negative charge.
- Let me make it very clear.
- The partial negative charge-- that's carbon right there.
- The partial negative charge is at the nitrogen itself.
- And you're going to have slightly partial positive
- charges of the carbons and the hydrogens,
- especially at the hydrogens.
- And now if we just talk about the properties of it, because
- it's polar, this would actually give it a reasonably
- high melting and boiling point.
- You could imagine, amines when they are in a solution with
- other amines, they are going to be attracted essentially
- via this type of hydrogen-type bonding.
- The negative part of partial negative nitrogens are going
- to be attracted to the partially positive hydrogens
- of other amines.
- Now, the hydrogen bonding for an amines is not going to be
- as strong as it will be for, say, waters or alcohols
- because waters and alcohols involve oxygen, and oxygen is
- even more electronegative than nitrogen.
- So these partial charges are even stronger.
- So that's why hydrogen bonding is even stronger in the case
- of things that have oxygen in it like alcohol or water.
- Now, with that said, let's think about
- how this might react.
- I have a lone pair over here.
- I also have a partial negative charge because this guy is
- more electronegative.
- It seems like we have a lot of negative action going around
- the nitrogen there.
- And because it has that, maybe it would either like to give
- away electrons or maybe take hydrogens from other things.
- And that, in fact, is the case with amines.
- So in general, and let me write this down, this is
- ethylamine right here, just so we get our naming practice.
- Ethylamine, this is the common name.
- We just said this is an ethyl group attached to an amine.
- And this is obviously a primary amine.
- It's only attached to one carbon right here.
- But in the case of an amine like this, and actually out of
- all of the functional groups we will look at, out of all
- the neutral functional groups we look at, because of what we
- just described, because it has this partial negative charge,
- it has this lone pair, the nitrogen's pretty
- electronegative, this is actually the most basic.
- And this isn't just ethylamine.
- This is all amines.
- Let me write this down.
- Amines are the most basic of the neutral functional groups.
- Obviously, something like a hydroxide anion, it has a
- negative charge.
- It will want to attract protons even more.
- But this one's pretty good,
- considering that it is neutral.
- So given that it's basic, and we'll also say, and it's a
- related notion, it's also nucleophilic.
- And just as a reminder, when people talk about basicity,
- they're talking about how much does something want to give
- away electrons?
- Or how much does it want to take protons?
- When you talk about the nucleophilicity, or how good
- of a nucleophile something is, it's how good are you at
- giving away electrons?
- Or how good are you at essentially being attracted to
- protons or giving away electrons?
- And so when you talk about basicity, you're talking about
- a thermodynamic property.
- When you're talking about nucleophilicity, you're
- talking about a kinetic property.
- How good is it at interacting?
- The basic tells you how stable is it once it has interacted.
- So given that it's a nucleophile, you could
- imagine, I think you see where I might be going with this.
- Let me draw the ethylamine again.
- So ethylamine, I'll draw it in a little bit simpler.
- So let me draw the hydrogens like that, and then it has
- this lone pair.
- Let's think about what might happen if we have a solution
- of ethylamine.
- If we have ethylamine and it's mixed in with some
- bromoethane.
- So that's one, two carbons right there.
- Let me draw the bromo group.
- And it might not be obvious the way I drew this
- bromoethane right here.
- Let me write this down.
- This is bromoethane.
- But I think it will be obvious if I draw this carbon right
- here and if I draw the other two hydrogens, which are
- implicitly there.
- So that is one hydrogen right there and then you have
- another hydrogen over here.
- And you have a good nucleophile in the ethylamine.
- So what do you think will happen?
- Oh, yeah, I also have a good leaving group.
- So you see I'm talking in the language of Sn2 reactions.
- Or really, I'm talking in the language of nucleophilic
- substitution reactions, and I'm going to lean towards the
- Sn2 direction because I have a primary carbon here, good
- leaving group, pretty OK nucleophile.
- If this was a secondary, especially if this was a
- tertiary carbon, then I would be thinking Sn1.
- Then I would say, oh, maybe this leaves on its own.
- But in an Sn2, what we're going to have is, this
- nucleophile will attack or it will give away its electron,
- so it'll give its electron to this carbon.
- And simultaneously, since this electron is going towards the
- slightly positive, the partially positive carbon
- here, this bromine can finally take away that carbon's
- electrons, and so we just experienced an Sn2 reaction.
- This is an Sn2 reaction.
- And then after that is done, the bromine will have left.
- It has taken this electron.
- It already had this one over here.
- It already had that one.
- And actually, it had another six electrons.
- One, two, three, four, five, six, seven valence electrons
- for bromine.
- Now, it will have eight.
- So, one, two, three, four, five, six, seven, now it will
- have eight.
- It just took this one right here.
- It gained an electron, so it has a negative charge.
- And now let me draw this part right here.
- So we had this carbon, that carbon.
- That hydrogen is sticking-- let me draw it like this.
- That hydrogen is sticking out, just like that.
- The other hydrogen is behind it.
- It's bonded to the other carbon up here.
- That's really a CH3.
- It has three hydrogens off of it implicitly there.
- And now the ethylamine has attached to it.
- So let me draw that.
- So that's the nitrogen.
- It's bonded to one, two carbons just like that.
- And then it has a hydrogen, a hydrogen, and then you have
- this electron is still with the nitrogen.
- And then this magenta electron has been given to the carbon,
- just like that.
- And since the nitrogen has given away an electron, it now
- has a positive charge.
- So when you think about these two molecules, this molecule
- now has a positive charge.
- This molecule right now has a negative charge.
- So they would actually be ionically attracted to each
- other, and we've actually just formed a salt.
- And this would be called-- and this is interesting here, at
- least from a naming perspective.
- You have two ethyl groups.
- It might not be completely clear.
- You have one, two carbons here, and then you have
- another one, two carbons there.
- We could re-draw this as-- or we could rewrite it as NH2.
- And then we have two carbons, and then another two carbons
- just like that.
- So we could call this diethyl-- we have one ethyl,
- two ethyls-- so this is diethyl.
- Let me color this.
- Let me do it in a different color here.
- So you have an ethyl group right there, an ethyl group
- right there, so we call this-- this is the common name for
- it-- diethyl.
- And then you might want to call it diethylamine, but it's
- not ammonia or it's not derivative of ammonia anymore.
- It's a derivative of ammonium.
- And that's because it's now positively charged.
- This nitrogen now has four bonds.
- So this is diethylammonium.
- Diethylammonium is this molecule right here.
- And if you view it as a salt, and a salt is just an
- ionically-- well, I don't want to go into the definition
- completely of a salt, but you view this as a salt because
- it's a ionically bonded molecule, where one of the
- molecules is diethylammonium, the other one will be bromide.
- This is the bromide anion right here.
- And you always list the cation first, the positive charged
- first. So this is diethylammonium bromide, which
- we were able to form with the Sn2 reaction between
- ethylamine or ethylamine and bromoethane.
- Now, we can now go under another step here.
- If we have a ton of this ethylamine floating around, if
- we have an excess, and early on we will have a lot more of
- that than we will have of this thing right here, if we have a
- ton of ethylamine floating around-- let me draw some
- ethylamine.
- If we have a ton of the ethylamine floating around, it
- might be able to grab one of the hydrogens from the
- diethylammonium over here.
- So you could imagine a situation
- where it grabs a hydrogen.
- It essentially gives its electron to the hydrogen, and
- then that hydrogen's electron can be
- taken back by the nitrogen.
- And I'll draw this as an equilibrium because it's not
- really that much more likely that this guy takes away that
- guy's electron than that guy can go and take the electron
- back, so it's a reversible reaction.
- But if that were to happen, then this guy, this guy right
- over here, will then look like this.
- You have the ethyl group bonded to a nitrogen that only
- has one hydrogen on it.
- Let me just draw it like this.
- A nitrogen that only has one hydrogen on it.
- And then he's bonded to another ethyl
- group right over there.
- And now he has his-- so if you view this as an electron right
- there, he has his lone pair.
- So he took that-- it looks like yellow ochre, just kind
- of a yellow-green color-- took that back from the hydrogen
- and is now neutral, got an electron back.
- So now, this is just diethyl ammonia.
- Actually, no.
- I'm sorry.
- It wouldn't be diethyl ammonia.
- It would be diethylamine.
- Sorry about that.
- Let me erase that.
- This would be diethylamine.
- It would be ammonia.
- Ammonia is just NH-- let me draw it like this.
- Ammonia, just to make things clear, is this, and that's why
- my brain was telling us to go ammonia.
- And then ammonium is this.
- Give away one of these electrons to a hydrogen
- proton, you have a positive charge.
- Now, when you do the common naming, when it's kind of
- ammonium derivative, you do say diethylammonium.
- And then my brain said, OK, now that we got rid of the
- hydrogen, I went back to ammonia, so I said diethyl
- ammonia, but that's wrong.
- Obviously, this is now an amine, so we used the common
- naming for amines, and we get back to diethylamine.
- And then this guy over here will turn into ethylammonium.
- Sorry for all the confusion.
- So this guy over here is a nitrogen, two hydrogens, ethyl
- group, and then he grabbed another hydrogen.
- So he grabbed this hydrogen over here.
- So now he has a positive charge since he gave away that
- electron over there.
- So now he is ethylammonium.
- But this is reversible right here.
- But what you can see, through this Sn2 reaction, we were
- able to essentailly, get rid of one of these hydrogens and
- substitute it with another ethyl group.
- We went from having a primary amine-- this is bonded to one
- carbon-- to having a secondary amine
- through this Sn2 reaction.
- It's bonded to two carbons.
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