Relative Stability of Amides Esters Anhydrides and Acyl Chlorides Relative Stability of Amides Esters Anhydrides and Acyl Chlorides
Relative Stability of Amides Esters Anhydrides and Acyl Chlorides
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- I want to make a quick correction to the last video
- where we introduced the carboxylic acid videos, and
- then we can actually compare them in terms of their
- relative stability.
- And that one mistake I did is when I named the ester, all I
- did is I named the main carbon backbone.
- I didn't actually tell you how many carbons you have attached
- to the oxygen over here.
- Acetate really just refers to this part over here, knowing
- that it is bonded to-- or maybe I should say it refers
- to this part over here.
- But you also have to specify how many carbons
- you have over here.
- So this molecule that we drew over here is
- actually methyl acetate.
- Or if you want it's systematic name, it's methyl ethanoate.
- And this is where we get the methyl from.
- It is methyl ethanoate.
- Now, with that out of the way, let's actually compared these
- carboxylic acid derivatives, and I'll compare the
- derivatives of acetic acid.
- So the first one we saw was the amide acetamide.
- Acetamide looks just like this.
- This is acetamide.
- Then the next one we looked at was the ether.
- And that's the one I just pointed out the mistake on.
- So this was ethyl acetate.
- You have CH3 right here.
- This is methyl acetate.
- This is where the methyl part comes from.
- So this is methyl acetate.
- And then we will compare that to the anhydride version.
- So this is acetic anhydride.
- Let me do this in a new color.
- Acetic anhydride looks like this.
- And finally, we had the acetyl chloride.
- Acetyl chloride looked just like this.
- And, of course, these are all derivatives of acetic acid,
- which we drew at the top of the last video, which is right
- over there.
- Now, let's think about which of these may or may not be
- more stable.
- And to think about it, we're going to think about any
- resonance structures that these molecules might have. So
- if we first focus on acetamide, we know that
- nitrogen forms three bonds and then has two extra electrons
- that form another lone pair.
- And it's actually electron rich, because it's a good bit
- more electronegative than the hydrogens here and actually
- reasonably more electronegative then the
- carbon it's attached to.
- So you could imagine a situation where a nitrogen
- could donate an electron to this carbon right over here,
- and then that carbon can let go of one of the electrons in
- a double bond with the oxygen.
- And so it could go back to the oxygen and you would have a
- resonance structure that would look like this.
- Let me actually use the same colors.
- This oxygen just gained an electron up there.
- And, actually, let me draw the electron.
- Well, actually, I'll just put the negative charge due to
- that electron.
- And then we had this bond right
- over here to the nitrogen.
- But the nitrogen just a gave an electron to this carbon
- right here, the carbonyl, or what was the carbonyl carbon,
- and so now we have a double bond over here.
- And the nitrogen just to gave an electron, so it now has a
- positive charge.
- But this is a resonance structure of acetamide, so
- pretty stable.
- So this is maybe I should even say quite stable.
- Now, let's think about an ether here, and we could
- probably do something fairly similar.
- So if we think about this ether here, oxygen has two
- extra lone pairs.
- It is more electronegative than the things that it is
- bonded to, so it could do a very similar thing to what
- this nitrogen did.
- It could give an electron to this carbon right over here,
- and then that carbon can give back an electron to the
- carbonyl oxygen, to that oxygen over there.
- And so it's resonance structure
- would look like this.
- It would have a resonance structure, so it's almost the
- exact same thing is as what we saw with the amide.
- So it would have a resonance structure.
- This oxygen just gained an electron.
- Now it will have a negative charge.
- This bond to the oxygen is still there, but this oxygen
- just gave another electron, formed another bond with what
- was the carbonyl carbon.
- It actually forms a new carbonyl group, if you want to
- view it that way, and so it will look like this.
- And this oxygen gave away an electron so now it has a
- positive charge.
- So this seems like a pretty good resonance structure.
- So the question is which of these two are
- going to be more stable?
- And the answer there really just comes out of
- the Periodic Table.
- If you look at nitrogen and oxygen, they're right there.
- They're both near the top right.
- They're both pretty electronegative, but oxygen is
- more electronegative.
- It is to the right of nitrogen.
- So because oxygen is more electronegative, remember,
- electronegativity is just the tendency to hog electrons.
- How much do you like to hog electrons?
- Oxygen likes to hog electrons more than nitrogen likes to
- hog electrons.
- So it would be less likely, marginally less likely, to
- give away an electron than nitrogen would be, so this
- resonance structure is a little bit less likely, or
- when you think of it probabilistically, it's going
- to happen a little bit less frequently than
- this resonance structure.
- So this one right here, this is going to stabilize it less.
- So this is going to be a little bit less
- stable then the amide.
- So if we called this quite stable, I'll just say this is
- just stable over here.
- Now let's think about what's going to
- happen with the anhydride.
- The anhydride, once again, we have an oxygen right here.
- It's got its spare electrons.
- It's more electronegative than the things
- that it's bonded to.
- So it seems like you could do something very similar to what
- we saw with the actual ether.
- It could donate an electron to this bond right over here, in
- which case, you would have a resonance structure
- that looks like this.
- And if it donates an electron to this carbon over here, then
- an electron can be taken away from the carbon and given back
- to that oxygen.
- So that would give us a resonance structure that would
- look like this.
- That guy now has a negative charge.
- We now have a double bond to this oxygen.
- it still has another bond to the other acyl
- group, just like that.
- And now this oxygen gave away an electron, so it has a
- positive charge.
- So it seems like a very similar situation to what we
- saw with an ether.
- But there's another resonance structure here.
- You could also have a situation where instead of the
- electron being donated to this carbon bond and that
- happening, you could have a situation like this, where the
- electron gets donated to that carbon bond and then this
- electron gets taken back by that oxygen, so you would have
- this situation.
- You would have a situation where this acyl group still
- looks the same bonded to this oxygen, but now we have a
- double bond over here.
- This guy took away an electron.
- So one of the bonds in the double bonds goes away.
- It now has a negative charge, and then you have the rest of
- the molecule.
- So you actually have these two resonance structures.
- And, of course, this is now a positive charge since it gave
- away electron.
- So you might say, hey, more resonance
- structures, more stability.
- But the key here is to realize that both resonance structures
- are dependent from the same oxygen.
- They essentially have to share this oxygen's electrons.
- So each one of these individually is less likely to
- occur than just what occurs with the ether.
- You can kind of view it as both of these bonds have to
- share what in the ether this one gets on its own.
- And we even saw that this is still less likely to occur
- than this over here.
- So in this situation, in an anhydride, if one guy's
- getting the double bond, the other guy's-- if there's a
- resonance structure on one side of the anhydride, the
- other side is still reactive.
- If there's a resonance structure on the left side,
- then the right side is still reactive, so we'll call this
- less stable.
- This right here is less stable.
- Then finally, you have your acetyl chloride.
- And acetyl chloride, there really is no resonance
- structure here.
- Chlorine is so electronegative that it is unlikely to give
- away an electron.
- It is sitting pretty with eight electrons.
- That's actually true from these other
- characters right here.
- But chlorine is very, very unlikely to contribute a
- resonance structure here.
- So this gets no stability, so this is the least stable of
- the carboxylic acid derivatives that
- we're looking at.
- Now, why am I even bothering with all of this hierarchy of
- stability for carboxylic acid derivatives?
- And I'm bothering with it because we know-- or we don't
- know quite yet, but I've already shown you some
- mechanisms that can take us from a carboxylic acid to an
- ether or carboxylic acid to an acyl halide.
- And, in general, you can go between all of these and a
- carboxylic acid.
- But since we know their relative stabilities, we know
- that if you start with an acetyl chloride, you're much
- more likely to go to an acetamide if you have the
- proper ingredients in there.
- If you have some amides floating around than the other
- way around.
- You're much more likely to go from that to that, because
- this is much less stable than that, then you are to go from
- acetamide to acetyl chloride.
- Likewise, you're much more a likely to go from a carboxylic
- acid to-- let me put it this way.
- You're much more likely to go from a carboxylic acid to an
- acetamide, and I haven't drawn the carboxylic acid here, but
- this has better resonance than even its original carboxylic
- acid than the other way around.
- Or you're much more likely to go from an
- anhydride to an ether.
- So that's why I'm doing this hierarchy.
- You're much more likely to go from something on the right to
- something on the left.
- And we'll explore some of those mechanisms in the next
- few videos.
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