Relative Stability of Amides, Esters, Anhydrides and Acyl chlorides - Organic Chemistry, Chemistry Video Lecture - Chemical Engineering

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.