載入中...
相關課程

登入觀看
⇐ Use this menu to view and help create subtitles for this video in many different languages.
You'll probably want to hide YouTube's captions if using these subtitles.
相關課程
0 / 750
- Let's think about what might happen if we had a solution of
- this carboxylic acid here.
- We might as well name it just to get some practice.
- We have one, two, three, four, five, six, seven carbons.
- So this is heptan-, and then we don't write heptane,
- because this is a carboxylic acid.
- It is heptanoic acid.
- So let's see what happens if we have heptanoic acid
- reacting with-- this is one, two carbons, and then it has
- an OH group, so this is ethanol.
- That's what the OH group does.
- It makes this an alcohol.
- And it's in the presence of a sulfuric acid catalyst. This
- right here is sulfuric acid, one of the stronger acids.
- I'll actually draw its structure, because I always
- find it frustrating when people just write just the
- formula here without the actual structure, because the
- structure actually shows you why it's so acidic.
- Sulfur has six valence electrons, just like oxygen.
- So it has a double bond to an oxygen, another double bond to
- an oxygen, and then it has a single bond to an OH group,
- and then it has another single bond to an OH group.
- And notice, it has one, two, three, four,
- five, six valence electrons.
- Now, the reason why this is such a strong acid, is that if
- either of these oxygens take the electron from this proton,
- so actually give away the proton to the solution,
- there's a ton of resonance structures here.
- And maybe I'll make a whole video on sulfuric acid, just
- to show you all of the resonance structures.
- But, in general, whenever you see a reaction when they say
- it's catalyzed by an acid, all you have to do is realize it's
- just going to make the surrounding solution a lot
- more acidic, just a ton more acidic.
- Maybe we're in a solution of ethanol, and if we are in a
- solution of ethanol, it'll just add protons to the
- ethanol itself.
- So you can imagine this guy right over here.
- Let me draw the sulfuric acid a little bit differently.
- Let me draw these oxygen-hydrogen bonds, so you
- have this oxygen and it is bonded to a hydrogen there.
- This is floating around the solution.
- You have your ethanol that looks like this, so two
- carbons and then bonded to an oxygen, and then that oxygen
- is bonded to a hydrogen.
- The oxygen has two lone pairs, just like that.
- And so this guy really is good at getting rid of the protons.
- So you have a situation where this electron can be taken
- back by this oxygen, and then it can actually be given here,
- and there's all these resonance structures.
- But it's just very good at taking that electron.
- And that can happen at the exact same time that one of
- the ethanols capture the hydrogen proton, at the exact
- same time that this oxygen captures that hydrogen proton.
- And if you just look at this part right here, this will
- just result in-- this part alone will just result in a
- ton of having these protonated ethanols flying around.
- Actually, let me draw it over here.
- I want to make sure I'm using my
- screen real estate properly.
- So this will result in a ton of these protonated ethanols.
- So one, two carbons, it has this original
- hydrogen over here.
- But now it has one electron in that original pair, and it
- gave the other electron to this hydrogen, so it gave the
- other electron to that hydrogen nucleus.
- Let me do it in that same color.
- So it took a proton.
- It gave away an electron, so it now has a positive charge.
- And now what was a sulfuric acid now has a
- negative charge over here.
- So if I were to draw it, it would now look like this.
- Plus sulfur, two double bonds, two oxygens.
- You still have this OH group.
- Actually, it could still donate.
- This is still acidic, but now this oxygen right now gained
- an electron.
- It now has a negative charge.
- Now, the whole reason I did this is I wanted to give you a
- tangible sense of what sulfuric acid looks like and
- why it's acidic.
- But really, you just have to kind of internalize that
- you're just going to have a bunch of hydrogen protons
- floating around.
- They could be attached to an ethanol.
- If this was a water solution, they could create hydronium,
- so you have a bunch of hydrogen protons floating
- around that will catalyze this reaction.
- They will be used to facilitate the reaction we're
- going to explore, and then they will be let go.
- So hopefully, this right here, by having this right here, if
- I start involving some of these protonated ethanols in
- our reaction, you won't view that as a huge stretch of the
- imagination, because they would have gotten protonated
- by the sulfuric acid.
- Or if I just actually grab protons in our reaction, that
- actually might make it a little bit simpler.
- So let's start with the actual reaction.
- So let me read redraw my heptanoic acid.
- So I have one, two, three, four, five, six, seven
- carbons, double bond to an oxygen, and then we have an OH
- group right over here.
- Now, the first step of this reaction, this oxygen right
- here, we have all these protons floating around, very
- acidic environment.
- We have sulfuric acid there just giving protons away to
- the ethanol or to other things.
- So this guy can grab a proton either directly from sulfuric
- acid or maybe from one of the protonated
- ethanols, either one.
- So we could just draw it like this.
- He just grabs a hydrogen proton.
- The hydrogen proton might have had an electron associated
- with it that would then go back to a sulfuric acid, but
- just to make things simple, I'll just say grabs a proton
- from something else.
- So once he grabs that proton, then it looks like this.
- So I'll draw my two, three, four, five, six, seven
- carbons, double bond to an oxygen, single bond to an OH.
- This oxygen had two lone pairs, but now one of the lone
- pairs is broken up because it gave an electron to this
- hydrogen proton right over there.
- Hydrogen proton, it was positively charged.
- It gained an electron.
- Now it is neutral.
- But this oxygen right over here gave away the electron,
- so it is now positive.
- Now, the next step, we have all this
- ethanol floating around.
- So let me introduce some ethanol, and I'll do this in a
- different color.
- So we have this ethanol floating around, and this is
- one, two carbons.
- This is our ethanol.
- You have two electron pairs on that oxygen.
- And then you could imagine, especially because this is now
- a good leaving group, you can imagine that this acts as a
- nucleophile.
- It would do a nucleophile attack on this carbonyl carbon
- right here.
- And so what you could imagine is that this electron attacks,
- or gets given to, this carbon right here on the carbonyl.
- And at the exact same time that this happens, this oxygen
- is positively charged.
- It wants an electron.
- It wants to get neutral again.
- It's already hogging these electrons.
- That's why you have a partial positive
- charge on this carbon.
- That's why this guy might be attracted to this.
- We've see this in Sn2 reactions many, many, many,
- many times already.
- So this electron will be taken up by this
- top oxygen over there.
- So after that happens, what does our newly formed
- molecule look like?
- Let me just scroll down and have some clear space.
- After that, our newly formed molecule will look like this.
- We have two, three, four, five, six, seven carbons to
- get to the carbonyl carbon, and now it will have a single
- bond to that oxygen up there.
- That oxygen had one lone pair already.
- It had this bond to this hydrogen that it nabbed from
- the solution, that proton that it nabbed from the solution,
- and now it has another pair.
- It had this electron that was participating in a double bond
- with this carbonyl carbon, but now it took the other side of
- that bond back.
- So now it has that electron and the other one that it took
- from the carbon, so it has two lone pairs again.
- It had a positive charge, but now it took back an electron.
- Now, it is neutral.
- Now the rest of the molecule, we have this OH
- group right over here.
- We have that OH group right over there.
- And now we have the actual ethanol that has attached
- itself, so it is no longer ethanol.
- It's attached itself to this larger molecule.
- So this oxygen right over here, it still has one lone
- pair, but the other lone pair has been broken up, and it has
- given an electron to this carbon.
- It is now bonded to the larger molecule, and then the rest of
- it, you have these one, two carbons right there, and then
- you have this hydrogen right over there.
- Remember, we have a bunch of protons floating around.
- Actually, I should make it clear.
- These are all reversible reactions, so I actually
- shouldn't even draw one-way arrows here.
- Let me scratch that out.
- Actually, delete that.
- A better thing to do, instead of drawing these one-way
- arrows is to show that the reaction could actually go in
- either direction.
- It's just as likely to go from here to here as it is from
- there to there.
- So let me draw that.
- These are kind of in equilibrium with each other.
- So you can imagine the next thing that could happen is
- another oxygen could grab a proton from the medium.
- And actually, before I do that, let me make sure this
- guy can lose a proton to the medium.
- Actually, this guy could take the proton from that guy, but
- I won't do it that way.
- So you could imagine a situation where this proton
- just jumps off.
- I should have pointed it out.
- This guy gave away an electron to this carbonyl carbon.
- So this right here has a positive charge.
- In general, the oxygen, very electronegative, it's going to
- be hogging the electrons of this proton, so it
- can take them away.
- So far, we gained a proton, and now we
- can give back a proton.
- So this electron right here can go back to this oxygen,
- making it neutral.
- And the proton can be picked up by anything.
- It could be picked up by another molecule of heptanoic
- acid through this first protonation we saw.
- It could be picked up by another molecule of ethanol.
- Let me draw it like that.
- Let me draw it as getting picked up by another molecule
- of ethanol.
- It just gets thrown back in the solution.
- So this guy gives an electron to the proton, but the end
- result is the hydrogen leaves.
- The electron goes back to the oxygen.
- It now has a neutral charge.
- And once again, these are all reversible reactions.
- It can go in either direction.
- But we're now over here, and let me redraw
- my heptanoic acid.
- Two, three, four, five, six, seven single bond now to the
- oxygen, single bond to this OH.
- And now we have this bond right over here to this now
- deprotonated, what was ethanol, so you have O, and
- then we have one, two carbons, just like that.
- And this guy has a hydrogen attached to it.
- And I won't even bother to draw this protonated ethanol.
- The protons are flying around everywhere.
- Now, the next step, this guy, this OH group, and especially
- the oxygen in it, he could grab a proton from the
- surrounding solution.
- So he's got these two lone pairs.
- He can grab a proton from one of these protonated ethanols
- from the sulfuric acid, or maybe from one of these other
- intermediate molecules, from anywhere.
- That's the whole point of having this acid catalyst, so
- he could donate an electron to a proton and then
- form a bond with it.
- I'll keep it in that color.
- And so if that happened, then the next step in our
- reaction-- and remember, these can all
- go in either direction.
- The next step in our reaction will look like this.
- You would have your two, three, four, five, six, seven
- carbons, single bond to an oxygen, and then you have your
- bond to this oxygen, which is bound to two carbons, one, two
- carbons, just like that.
- And this guy on top is bonded to an oxygen.
- He's got two lone pairs.
- And this guy over here grabbed a proton.
- He gave an electron to a hydrogen proton.
- He had two, but now he gave one of them to this proton.
- And so he gave it to that hydrogen.
- That hydrogen is now neutral.
- It gained an electron.
- This oxygen is now positive, because it
- gave away an electron.
- It still has this other lone pair over here.
- It is now positive, and frankly, it is now a good
- leaving group.
- So, in the next step, you can have someone else.
- Remember, other people need protons
- earlier in this reaction.
- So this proton might get lost, maybe by one of the other
- ethanol molecules, so let me draw that.
- Let me do a color I haven't used yet.
- I'll use orange.
- So maybe one of the other ethanol molecules, or one of
- the other intermediaries in this whole reaction.
- So ethanol, I'll just do ethanol because
- it's easier to draw.
- It might give an electron to just the nucleus.
- And then this guy can take the electron back.
- This guy could take that hydrogen's electron back and
- give it to this carbon that was the carbonyl carbon
- several steps ago.
- And then since he's got that electron, he can then give
- back this electron to this, what was an OH group, but now
- it's got another hydrogen there, so he can then give it
- back to him.
- And then the resulting products will look like this.
- And this is all in equilibrium.
- We now have two, three, four, five, six, seven carbons.
- Now, this guy has a double bond again.
- So you have an oxygen right there.
- It is now a double bond.
- I'll draw this newly formed double bond in magenta.
- This guy has left as water, so I can just draw that, so now
- you have this other OH group that is bonded to this
- hydrogen over here.
- He has now left as water.
- And now you have this other thing over here.
- You have what was that ethanol group has now attached itself.
- That ethanol has lost its hydrogen.
- It has attached itself to what was a carboxylic acid, so now
- it looks like this.
- It now is bonded to an oxygen and is bonded to one, two
- carbons, just like that.
- And this whole reaction that I showed you is called
- esterification, one of those words that I
- have trouble saying.
- This one in particular is called the Fischer
- esterification.
- And he won the Nobel Prize in 1902 generally for his work in
- organic chemistry.
- And the reason why it's called that is we started with the
- carboxylic acid.
- We started with a heptanoic acid, and
- now we have an ester.
- An ester is something that, instead of an OH group like
- you have in carboxylic acid, you have an OR.
- You have an oxygen with an actual alkyl
- group attached to it.
- And this ester right here, and we'll probably talk more about
- esters in future videos, this ester right here, just to give
- you a hint of how to name it, you first start with the group
- that's attached to this oxygen right here.
- There are two carbons over there, so we'll use ethyl to
- name that over there.
- And then we have the rest of this, and we already know that
- that is one, two, three, four, five, six, seven carbons,
- including the carbon in the carbonyl group.
- And so that is hepta-, heptan-.
- And you would be tempted to call it heptanoic acid, but it
- is no longer a carboxylic acid.
- It is now an ester.
- So you call it heptanoate.
- And this is what tells you that you are
- dealing with an ester.
- And this tells you what's on the other side of the oxygen
- in the ester.
- This is telling you how many carbons you have attached to
- the carbonyl chain of the actual ester.
- Anyway, hopefully, you found that useful.
- I just wanted introduce you to a well-known reaction
- mechanism of creating an ester out a carboxylic acid.