EAS: Sequence Groups

Concept: Concept: Sequence Groups

Video Transcript

In this video we're going to discuss a synthetic technique having to do with benzene reactions that's called sequence groups. So one of the major goals of this chapter is to get you to do organic synthesis so we're actually going to be learning how to turn a regular benzene into some much more complicated molecule, and it has to do with aromatic reactions in order to accomplish this you're going to need to use sequence groups, sequence groups are defined as groups that have the ability to alter the sequence of an aromatic synthesis due to the fact that they have directing effect changes. So what I'm looking at is that these are groups that can be easily transformed from one type of director to another, so these are going to be groups that in one form they might be an O P director but then after you transform them they become an attractor, why is that important? Because that's going to allow us to substitute different places of my benzene depending on which directing effect I choose. So let's look at the first one I think it will become more clear, so reduction of nitrogroups is a major sequence group why? Because notice that a nitrogroup is one of the strongest what type of director is it? it's a meta director right, so this is one of the strongest meta directors we know that nitrogroups love to add towards the meta position but nitrogroups are also very easily reduced we learned earlier in this chapter that you can use pretty much any reducing agent but you know specifically L I lithium aluminum hydrate you can use catalytic hydrogenation you can use stainless chloride and you even just use iron and H C L all of these would turn a nitrogroup into an annulene so what kind of directing of directing group is in annulene or in N H 2 guys we know this is one of the most powerful O P directors.

So this is what we would consider a sequence group because the fact that this transformation will determine where do I add my next group where do I add my next regent, so for example if I had reacted another E S mechanism on the first molecule I would get a meta substantially if I add on it on the second one I get most likely a para substantial right so it's definitely going to be important that we know exactly when we're going to try to react with the secondary and tertiary reagents. Let's move on to another really important sequence group and that's a clemmensen reduction so clemmensen reduction is going to turn basically an acyl group into an alkyl group but let's look at the directing affects of that in acyl group has a partial positive at the carbon so we know that's going to be a pretty good meta director after we reduce and we turn it into hydrogen's what type of director is in our group? Guys it's an O P director so this would be another example of a sequence group it's a group that you have to think about the sequence of the regents before you actually react them because if you want a meta substituents you would react it as an acyl if you want an ortho or para you would react it as an alkyl. Here's the last one that is really one of the most common ones and that is the side chain oxidations, so remember that we talked about how a side chain of any length can be oxidized by potassium permanganate into benzoic acid well this happens to also be a sequence group because remember an R group has what kind of directing effects its ortho para after I oxidize that side chain it's now going to become.

A pretty strong meta director right so this will also be a sequence group because I have to determine when to add that second reagent depending on whether I want it ortho para or whether I want it meta, now remember guys there was an exception with the side chain oxidation which was that if there are absolutely no H's on the benzilic position then it won't work so obviously I'm talking about situations where it would work if it would work if it would oxidize then that is a sequence group. Awesome so this is the information that we're going to take into our questions about organic synthesis cause we are going to need to know when do you add that second reagent. So let's move on to the next topic.