Welcome to Chem Help ASAP.
Let’s focus on one type of R-group that
we commonly encounter on aromatic rings, the
electron-donating group.
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In the upper right, we see a recap of the
previous video.
Electron-donating groups (or EDGs) are R-groups
that behave both as ortho,para-directors and
activators.
Great! – but what are the common electron-donating
groups?
I put electron-donating groups into two categories.
The first type of EDG is the alkyl group.
Let’s react toluene (an alkyl benzene) with
a strong electrophile and put the electrophile
on the para carbon.
If we explore the other possible resonance
forms, we can draw another structure.
We’ll put the positive charge right next
to our electron-donating alkyl group.
The alkyl group stabilizes the carbocation
through hyperconjugation.
This is how alkyl groups act as electron-donating
groups, ortho,para-directors, and activators
on benzene rings.
Let’s look at the second type.
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The second type of EDG includes oxygen and
nitrogen groups.
Note that the oxygen or nitrogen must be both
directly attached to the ring *and* have a
lone pair.
Let’s try a simple example, a methoxy on
the ring.
The oxygen that we show here in this structure
is both attached to the ring and has a lone
pair – in this case two lone pairs.
Again, we’ll react the ring with the electrophile,
and we can draw a resonance form with the
positive charge is right next to our oxy group.
How can the oxygen with its lone pair stabilize
the carbocation?
It’s not by hyperconjugation.
With the lone pair, we can instead stabilize
the carbocation with resonance.
We can draw an extra resonance form.
The new resonance form is more stable because
all the atoms now have full octets (even though
we had to put a positive charge on oxygen).
A nitrogen with a lone pair behaves the same
way and also acts as an electron-donating
group by resonance to stabilize the carbocation
intermediate.
I’ll put one qualifier on nitrogen groups.
Electrophilic aromatic substitution reactions
tend to occur under acidic conditions.
We have acid in this reaction.
Simple nitrogen groups are basic.
That means something like a simple NH2 or
NR2 will be protonated under these acidic
conditions.
If so, then the nitrogen will no longer have
a lone pair.
No lone pair, no electron-donating group.
Therefore, amine groups typically don’t
act as electron-donating groups for EAS reactions.
*Amide* nitrogens, however, are effective
electron-donating groups.
Here is an amide.
Amide nitrogens can be both attached to the
ring and have a lone pair.
Amide lone pairs are less basic.
They are not protonated by acids and yet can
still stabilize a neighboring carbocation.
Here’s our electrophile.
Here is our cationic intermediate.
Our nitrogen with a lone pair can act as an
electron-donating group and stabilize the charge.
Nitrogens with lone pairs can act as electron-donating
groups, but the nitrogen needs to be some
kind of *amide* nitrogen, not an *amine* nitrogen.
That covers the common EDG groups for aromatic
reactions.
They include alkyl groups, oxygens, and amide
nitrogens.
All act as ortho,para-directors and activators.
