Leah here from leah4sci.com and in this video we're going to look at the resonance and acidity
of phenol.
In the last few videos, we discussed the physical properties as well as acid based characteristics
of alcohols.
You can find this entire video series along with the alcohols practice quiz and cheat
sheet by visiting my website leah4sci.com/alcohol.
While phenol is an alcohol, it's unique enough to warn for deeper discussion.
But first, what is phenol?
Phenol is a benzene ring with an alcohol attached not to be confused with the substituent phenyl
which is simply a benzene ring as a substituent on another molecule.
The way to tell them apart is that phenol ends in "ol" which is the suffix for the alcohol
functional group and phenyl ends in "yl" which is what we put at the end of an alkyl group
when it's acting as a substituent.
What is so exciting about phenol?
It's the benzene ring and specifically the resonance that happens as a result.
Benzene is a conjugated system which means that its pi-electrons can resonate around
and around and around.
In fact, you'll often seen benzene drawn as a hexagon with a circle in the middle rather
than three pi-bonds to show that the pi-bonds are constantly circulating as they resonate.
Phenol has the additional resonance of oxygen resonating into the ring as follows; as we
resonate, pay careful attention to the colors of each electron and bond so you'll see where
they came from and where they're going.
Resonance begins when one of the lone electron pairs on oxygen collapses down towards the
benzene ring to form a pi-bond between itself and oxygen.
This would give carbon too many bonds so kick out the green pi-bond towards the carbon on
the left, the purple and blue bonds haven't moved but the green electrons are now sitting
on carbon with a negative charge.
But they don't stay there, they want to move around the ring.
If we show them moving back in the direction of the oxygen, we'll just reform this structure
so let’s instead focus on what happens moving forward.
The green electrons will collapse down towards the blue pi-bond, putting too many bonds on
this carbon atom which is going to kick out the blue pi-bond.
This gives us a molecule with a purple pi-bond hasn't moved, the green electrons are now
sitting as a pi-bond on the left and the blue electrons are sitting as lone pair with a
negative charge on the lower carbon.
But they don't stay there, they continue on towards the purple pi-bond, putting too many
bonds on this carbon causing the purple electrons to collapse onto the carbon atom.
The resulting structure has a green pi-bond that hasn't moved, a blue pi-bond now forms
towards the right, and the purple electrons are sitting as a lone pair with a negative
charge on the upper right.
The purple electrons will continue moving, in this case, towards the carbon holding the
oxygen which will ultimately kick up the pi-bond between carbon and oxygen.
This reforms the initial structure with one slight difference and that is that the pi-bonds
all shifted over one position.
But it's still considered the same structure because remember, the pi-bonds and benzene
are always moving round and round, giving us a total of four resonance structures for
the phenol molecule.
So what does this have to do with acidity?
Say we're asked to compare the acidity of two similar structures, phenol, and cyclohexanol.
Everything about them looks the same except that phenol has pi-bonds in the ring and cyclohexanol
does not.
If we refer back to CARIO which is explained in detail in the acid bases series link below,
we'll notice that the charge is the same, they're both neutral, the acidic atom is the
same because we're comparing oxygen but resonance is where we have a difference, the phenol
has resonance, cyclohexane does not.
It doesn't impact the neutral molecule but look at what happens when we deprotonate the
alcohols.
The cyclohexanol gives us some molecule that has an oxygen with a negative charge and that's
it, it's stuck carrying that charge.
The phenol gives us a phenoxide that has an oxygen with a negative charge but oxygen is
not stuck carrying the burden of negative charge by itself, instead oxygen can resonate
those electrons into the ring, allowing that negativity to be shared by not one, but four
atoms, three carbon, and one oxygen.
Having resonance to stabilize the negative charge makes the structure much more stable
and if it's more stable with a negative charge, it's more likely to form, making the starting
acid stronger or more acidic.
This oxygen on the other hand is comparatively unstable, very unhappy, and much more likely
to reform the starting acid.
This makes cyclohexanol a weaker acid but it gives us a stronger conjugate base.
This came from understanding what we're looking at, the PKA values justify our findings, where
phenol has a PKA of ten and cyclohexanol has a PKA of sixteen.
Recall that every PKA unit is an increase or decrease in a factor of ten, that means
phenol is ten to the sixth times more acidic than cyclohexanol just because of their resonance.
To be clear, a PKA of ten doesn't make it a very strong acid, it just makes it more
acidic than other alcohol atoms.
If we dissolve phenol in water, we get the phenoxide anion along with H3O+ since phenol
is not a very strong acid, equilibrium for this reaction still lies to the left.
If we take phenol and react it with a strong base like sodium hydroxide, the hydroxide
anion is strong enough to grab the proton, give oxygen back it's electrons and form that
phenoxide, along with water and solution where the sodium ion is just sitting as a spectator
to counter the negative oxygen.
You may be asked to rank the acidity of phenols against other alcohols, for example, cyclohexanol
which we already discussed but what if you have to rank it against a known alcohol?
For example, which is the stronger acid, phenol or ethanoic acid, if we look at cario, the
charge is the same, they're both neutral, atom is the same because we're comparing the
oxygen holding a hydrogen, resonance, well they both have resonance so which one wins?
Instead of looking at the acid, I like to analyze the conjugate base.
If we remove a proton from phenol, we've already shown that the negative oxygen can resonate
into the ring so that we have total of four resonance structures.
If we remove a proton from ethanoic acid, the resulting molecule has a negative charge
on oxygen which can resonate just once for a total of two resonant structures.
If you're thinking that resonance equals stability, more resonance should be more stable.
Hold on for a second, phenol is actually the weaker acid because the PKA for phenol is
ten but the PKA for ethanoic acid is four point eight.
As a reminder, a high KA or low PKA is how you identify the stronger acid, so how is
that possible without memorization?
Don't stop just at resonance, take a look at what else is happening.
In the phenoxide resonance structure, we have four resonance but that charge is going to
be distributed between one oxygen atom and three carbon atoms that ultimately hold the
negative charge.
This is something I teach in detail in the EAS series link below.
So what are we looking at?
One oxygen and three carbon.
The resonant structure for ethanoate or it's common name acetate is between two oxygen
atoms.
This is where we have to go back to atom but don't worry about doing this for all sorts
of molecules, you're likely going to see this or very similar examples you'll already know
what to look out for.
This is where we have to recognize that the oxygen is much more electronegative than carbon
and therefore oxygen is more equipped to carry that burden of charge and more stable with
a negative charge.
And the way I like to think of it is imagine that the negative charge that we initially
formed is a burden to be carried, say you just went grocery shopping and you have twenty
bags in the trunk of your car, you have to carry that burden of twenty bags from the
car to your house.
You are the oxygen atom, you're an adult, you're strong, you can carry bags not all
at once, so you make a few trips.
The three carbon atoms helping you are little kids.
The little kids may be get away with one bag at a time because they're so heavy so even
though there are twenty bags to carry, having three little kids each make two or three trips
will help you, but it's gonna take awhile.
Now imagine that you have another oxygen, another adult as big and strong as you helping
you with groceries.
Yes, it's only one person as supposed to three kids but that person is able to carry more
bags at the same time, making that chore of carrying groceries from the car to your house
so much faster and more efficient.
So two adults versus an adult and three little kids, the two adults win.
And so two oxygen atoms sharing that negative charge makes a ethanoate more stable and therefore
happier, less reactive and that is why ethanoic acid or acetic acid is stronger than phenol
even though phenol has more resonance.
Another problem you might see is ranking the acidity of phenol versus substituted phenol.
For example, NO2, if we look at CARIO, charge is the same, they're both neutral, atom is
the same, it's an oxygen holding the acidic hydrogen.
Resonance looks the same because we have two benzene rings so what do we do from here?
If the molecule and question is exactly the same but we have another substituent, we want
to ask how that substituent impacts the acidity.
This will typically be due to induction or added resonance, comparing the molecule in
the right or left, phenol versus nitro phenol, it appears that we're comparing nitrogen to
nothing but actually it's not nothing, it's a hydrogen atom.
How does having a nitrogen versus a hydrogen impact the acidity?
Nitrogen is an electronegative atom, does that mean it adds electronegativity to the
ring?
Absolutely not.
You have to know that the nitro group is a trick question.
The nitro group is an N double bound to one oxygen single bound to another, the double
bound oxygen has two lone pairs and is neutral, the single bound oxygen has three lone pairs
with a negative charge, balanced by a positive charge on nitrogen.
The nitro group is a positive atom attached directly towards the ring and positive groups
like to pull electronegativity towards themselves because they want to balance that charge,
making it an electron withdrawing group, EWG.
You're gonna see a lot more of this in electrophilic aromatic substitution, but for now let's just
see how it impacts the ring.
When we look at the acid based reaction, we know that removing a proton gives us a negative
charge that then resonates into the ring.
How can we speed up or slow down the reaction?
If we somehow put negativity into the ring, blocking the oxygens negativity from coming
in, that would slow down the reaction, they're both competing for resonance.
But if we put something positive near the ring like a nitro group that pulls electron
density out of the ring, that makes the ring itself slightly more positive and more accepting
of the negative charge, In other words, having an electron withdrawing group on phenol makes
the ring slightly more positive, makes the resonance more favorable, making the nitro
benzene a stronger acid.
And in fact, the PKA of nitro benzene is approximately seven and by now we know the PKA for phenol
is ten, that means nitro benzene is about a thousand times more acidic than phenol.
The PKA will change depending if a nitro group is positioned ortho meta or para though the
concept is the same.
For even more practice problems like this, make sure you check out the acid base practice
quiz on my website and for even more on alcohols, first give this video a thumbs up then make
sure you visit my website leah4sci.com/alcohol.