Leah here from leah4sci.com and in this video we're going to look at the aromatic side chain
oxidation reaction.
When you react the substituted benzene like toluene with a strong oxidizing agent like
chromic acid, sodium dichromate or potassium permanganate the product is a carboxylic acid.
You've already seen these oxidizing agents in the past for example, the oxidative cleavage
of an alkene using potassium permanganate to give us 2 carboxylic acids or the oxidation
of an alcohol into a carboxylic acid but what if I expose an alkane to a strong oxidizing
agent?
My product, no reaction.
What if I expose benzene to a strong oxidizing agent?
Once again, no reaction but if I put them together and have an alkyl group on benzene
the same reagents that individually gave us no reaction gives us a fully oxidized side
chain, oxidizing the methyl group all the way to a carboxylic acid.
Notice that I had to add H+ in the second step because potassium permanganate operates
under basic conditions giving us the carboxylic anion adding H+ in the next step will protonate
the oxygen to give us the neutral carboxylic acid.
So what are we looking at on this reaction?
Both alkene and benzene are very stable and not susceptible to oxidation but when we put
them together, the benzylic carbon which is the carbon directly attached to the benzene
becomes more reactive and more susceptible to oxidation.
The bad news is that the mechanism is very complicated and not fully understood, it involves
a radical intermediate grabbing the proton some serious changes happening to the manganese
but the good news is you don't have to know this mechanism in undergrad orgo.
So what do you have to know?
The takeaways from the mechanism and the biggest take away is that the mechanism uses a benzylic
hydrogen atom to initiate the oxidation, that means without that hydrogen, no oxidation
and so without worrying about learning the mechanism just imagine that the oxidizing
reagent which could be some version of chromic acid or potassium permanganate grabs on to
the benzylic hydrogen as a means of latching on to a molecule cuts everything else off
and then oxidizes it all the way to a carboxylic acid and it's not just methyl, it's any alkyl
side chain that has a benzylic hydrogen.
If you're asked to oxidize a ring with an ethyl group, you identify the first carbon,
verify that it has a hydrogen, cut everything else off and the product is a carboxylic acid
specifically benzoic acid meaning it's not the last carbon that gets oxidized, it's the
benzylic carbon where everything else is cut off.
If you're asked to oxidize benzene with an isopropyl group, that's a benzylic carbon
with 2 substituents, it doesn't matter so long as there is one hydrogen atom, the oxidizing
agent can grab onto that, cut off everything else and give you a benzoic acid.
What if you're given a benzene ring with a tert butyl group and asked to oxidize it?
Well we have a benzylic carbon but we don't have any benzylic hydrogen.
Carbon is bound to the benzene ring and then 1, 2, 3, 4 carbons, that means it already
has four bonds there's no fit on to hydrogen and that means there's no reaction but it
doesn't stop here.
If you're asked to react a benzene that has 2 different substituents with KMn04 so long
as each substituent has a benzylic carbon with at least one benzylic hydrogen present,
the same thing will happen, each carbon will get oxidized to a carboxylic acid and the
remaining carbons that were attached are broken off.
Each carbon gets oxidized to a carboxylic acid regardless of what else was attached
so long as we have the benzylic hydrogen.
So what if I give you a molecule that looks like this?
We have a fused ring system where one ring is aromatic and one is not and we're told
to react it with KMn04.
We treat this the same way, don't let that second ring scare you, if you see KMn04 with
something attached to a benzene ring, ask yourself, what is KMn04 do?
It's a strong oxidizing agent.
What is it looking for?
A hydrogen on a benzylic carbon.
If we look at the benzene ring we have 2 benzylic carbons, each has at least one hydrogen atom
and that means when we react, everything else gets cut off giving me a final product with
2 carboxylic acids coming out of the benzene ring and the second ring broken in the process
of this oxidation.
You can find this entire video series along with my redox practice quiz and cheat sheet
by visiting my website leah4sci.com/redox