Leah here from leah4sci.com and in this video we'll take a more advance look at Acids and
Bases in Organic Chemistry by looking at the acidity of aromatic compounds.
You can find my entire series on orgo Acids and Bases along with my practice quiz and
cheat sheet by visiting my website leah4sci.com/AcidBase.
Say you're asked to compare two different sets of molecules to determine which is stronger
in each pair.
You'll recognize that they have very similar structures but the right molecule on each
pair has a benzene or aromatic ring.
The first thing that comes to mind is Resonance.
And that's not wrong but you want to make sure that the resonance is a possibility.
For the first set we have Cyclohexanol and Phenol.
If we deprotonate the acidic hydrogen, we get a conjugate base with a negative Oxygen
where the one on the left is not aromatic,it can't resonate making the Oxygen the sole
carrier of the negative charge.
But the right one has the negative Oxygen on the benzene ring which means it can resonate
into the ring distributing that charge and making it more stable.
So for the first set, the molecule on the right is the stronger acid and the molecule
on the left is the weaker acid.
If you're not comfortable with this, go back to the resonance video on the series.
For the second set, we have an acidic Hydrogen once again sitting on an Oxygen but the functional
group here is the Carboxylic acid.
So even though there is a resonance within the carboxylic acid itself you can't resonate
the conjugate base into the benzene ring.
If you don't believe me, try it!
But make sure you recognize you cannot resonate into the benzene ring.
So what's the difference between them?
For a problem like this you have to look at the inductive effect of the Benzene ring rather
than the resonance.
Benzene, having those extra electrons resonating in the ring, makes it more of an electronegative
group and therefore a stronger influencer or the negative charge and that means the
molecule on the right is the stronger acid.
The molecule on the left just has a standard carbon hydrogen chain, it has some induction
but not as much as the benzene ring making the molecule on the left the weaker acid.
So we've established that Phenol is dis and acid because the negative Oxygen can resonate
into the ring.
But now what happens if I put Nitrogen containing group onto that Benzene ring.
For the first one, we'll put in NH2 in the para position, for the second one we'll put
an NO2 in the para position, and for the third one we'll put in NO2 in the meta position.
If you'll look at the CARIO mnemonic and just compare the negative conjugate base, we have
charge negative one, atom, oxygen holds the charge, resonance - they can all resonate
into the benzene ring, Inductive effect - we have something to look out with the Nitrogen
atoms and the orbitals are the same.
This is where a simple mnemonic is not enough, you actually have to practice the resonance
to see what happens.
The NO2 is a little bit of a suspicious group because Nitrogen is an electronegative atom
so you want to think that it has those electron donating group and therefore puts negativity
into the molecule but not in NO2.
NO2 are Nitro group has a Nitrogen double bound to an Oxygen and single bound to a second
Oxygen.
A quick formal charge will show you that the single bound Oxygen has the charge of negative
one and the Nitrogen has a charge of positive one.
That's your first clue.
Having a positive group on the benzene ring, near an Oxygen that wants to resonate into
the ring is a good thing because a negative charge resonating towards a positive charge
is much more stable than say a negative charge resonating towards an electronegative Nitrogen.
So think of a lone pair on the NH2 as depleting or trying to push away that negative Oxygen
resonance making this one the worst or the weakest acid.
If resonance makes it acidic and you're discourage resonance then you're discouraging the acidity
and that's the weakest acid.
So we have the same NO2 on the meta position which means we have the same concept of positive
Nitrogen, negative Oxygen.They're both decent or stronger acids but which one is stronger?
Iis it a question of proximity or something else?
And in this case it's not proximity, it's resonance again.
So watch what happens.
If I take the Oxygen and I resonate it, notice we have these two electrons which move over
this way and then we'll show these two electrons, but where do they go?
Standard resonance is to continue around the ring so that we have a total of 4 resonance
structures and we can show the same thing here.
These electrons go down, these electrons move over, these electrons move to the next position
and once again for resonance.
But if we look back to the meta structure, those black electrons did not have to continue
in the ring, there is a bonus resonance when you have the Nitro group in the para position
and those electrons can actually go down to form a pi bond between carbon and Nitrogen.
Nitrogen would now have 5 bonds violating the octet rule so we'll show the electrons
between the nitrogen double bound to oxygen collapsing onto the oxygen and that right
there gives you a bonus resonance structure.
Remember, resonance gives the molecule the ability to distribute the charge over more
atoms.
So if you have four resonance that means four atoms are carrying the charge and here you
have five resonance that means the molecule with more resonance has the charge distributed
even more and that makes it even more stable.
If the conjugate base is more stable then it comes from the most willing or simply the
strongest acid.
And when you have slightly less resonance, it's slightly less stable and that means it's
a weaker acid.
It's still a decent acid where compare to phenol but it's slightly stronger.
So if we throw phenol into the mix and then rank it from weakest to strongest, the weakest
acid would be the one with the NH2 because the NH2 is slowing down the negative resonating
into the ring.
Then we have the phenol itself because we don't have any group to make it stronger or
weaker than phenol itself.
Then for number three we have the one with the Nitro group in the meta position because
the nitro is positive and that means it stabilizes the positive coming into the ring but it doesn't
give us a bonus resonance structure.
And finally a Nitro group in the para position with a bonus resonance structure is the strongest
acid in this sequence.
Be sure to join me in the next video where we look at the basicity of aromatic compounds.
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