Side-Chain Oxidation

Concept: Benzylic Oxidation

Video Transcript

In this video we're going to cover a pretty easy concept called side chain oxidation. So the alkyl group that's attached directly to a benzene is known as the alkyl side-chain. This is a site where special reactions can take place because it's right next to a benzene and it turns out that regardless of the length of the alkyl benzene side chain. No matter how long it is it can always be oxidised just to a benzoic acid using hot potassium permanganate or K M N O 4. So let's look at this reaction really quick. I got the general reaction here, we've got R group and notice that I'm not specifying how long it is, it could be a single C H 3 as in toluene or it could be like a hundred carbons long and go off the page of your, off your screen and onto the wall behind you and around the room. It doesn't matter guys how long that R group is, hot K M N O 4 is going to cleave the entire chain of the benzylic site and it's going to replace it with the carbocylic acid. When you have a carbocylic acid in a benzene ring that's specifically called benzoic acid. Cool.

Now one thing that professors might start to get picky about at this point of your course, in ORGO 1 we would usually just say K M N O 4 and be done with it, strong oxidising agent in ORGO 2 we're typically a little bit more cautious about how we talk about the reaction condition. So usually it's actually like four things that we write, it's going to be K M N O 4, then base and heat and finally afterwards you use acidic reaction to basically you know to finish off the reagent, the reaction. So from now on when I write K M N O 4 which will happen more in this course I'm always going to be using these reagents in this specific order and this just has to do with these are the lab conditions that are the most favourable to produce an oxidation. Now there is one exception that you need to know but it's pretty easy all it is is this. There always must be at least one benzylic hydrogen present for oxygen oxidation to occur meaning that it's not enough to just have an R group, I also need to have an H on the R group. If I don't have an H, I'm not going to get an oxidation so you could already start thinking in your mind of what kind of alkyl groups would not give you an H? Well once they have lots of carbons in that position so we're going to do an example about that.

Now you might be wondering about the mechanism, guys, don't worry about the mechanism here. That's not the point of this chapter, this chapter is not your oxidation reduction chapter it's your aromaticity chapter so we're not going to worry about any mechanism here and in fact I never worry about the mechanisms at this point of the course just know to recognize the reagent, know how to draw those four reagents in order. Your professor might be one of those that actually is picky about how you draw K M N O 4 and then know what the product is going to be. So here we are, which of the following alkylbenzenes would not yield benzoic acid in the presence of hot K M N O 4? So go ahead let you guys think about it for a little bit and I'll come back and answer the question.

Example: Which alkylbenzene would NOT yield benzoic acid, treated with KMnO4

Video Transcript

Alright, so we said that regardless of the length of the R group it's always going to turn to a benzoic acid unless you're missing hydrogens.

So essentially three of these have a benzylic hydrogen, one doesn't. A has a hydrogen, C has a hydrogen and D actually has 2 hydrogens but B has none so B is the answer. B would actually display no reaction when reacted with K M N O 4 because it does not have a benzylic hydrogen available to oxidize so is not going to be able to respond to the oxidizing agent, alright? Awesome guys.

So like I said, pretty easy concept not much to know. Let's move on to the next one.