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Introduction to Conjugated Pi Systems

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So we’re going to take a break from spectroscopy for a little bit and then there’s a small portion of the chapter that will revisit uv-vis spectroscopy and we’ll talk about that in a little bit more detail but you guys already did some of that in our spinach lab alright so this whole chapter is based on conjugated hi system so we first have to define what it means

To be conjugated does anybody have a good idea of what they think that means has resonance so it’s something we’re double bonds or pi bonds can be localized right so conjugated system is normally defined as something where you have alternating pi bonds so for example this would be a nice example of a conjugated system this is different than an isolated system right

Because in an isolated system these double bonds are separated by an sp3 hybridized carbon so this would simply be called non-conjugated or isolated and these can easily be spotted by identifying some sort of sp3 spacer or some sort of atom where there’s no p orbital where d localization can occur there’s also one last class that i wanted to talk about and these are

Called accumulated and accumulated pi bonds are very different than conjugated pi bonds right so do you guys remember back to first term when we have the allium with two adjacent five on in order to represent this we show a dot in the center as being a central carbon otherwise it just looks like one giant long double bond and so we want to look very clearly specify

Hey this is to separate double bonds attached with a carbon in between these have unique geometries but they aren’t considered conjugated molecules so let’s take a look at some of the weird properties with conjugated molecules and if you have a chance to watch the ir video you had a chance to see this example already but conjugated system show up a little bit weird

In nmr spectroscopy and ir spectroscopy so let’s do that example and what i want you guys to do is help me draw the resonance structures for this guy vs this guy so let’s draw all of the different resonance structures i’ll do the easy one all right are there any more for the one on the right no but it’s a valid resonance structure right we’ve got a negative charge

On the oxygen so i’ll bracket that this one is pretty similar apple pie bond up and then we have that one last resonance structure right so we can pull this remaining pi bond over and we can put the positive charge over on this carbon all right so now for trivia point which of these aldehydes will have a higher absorbance frequency on ir so which one will show up

At the larger wave number for the co double bond what do you guys think the right one right so if we look at the right one this aldehyde shows up at a higher wave number on ir since it’s more double bond like and this is simply due to that conjugation effect right we know we can be localized the pi bonds more in the example shown on the left and in the example on

The right so the example on the left that co bond has more single bond character than the one on the right so that makes sense there’s also some weird unique features with this too so if we look at this this bond right here has some pi bond character right versus this bond over here has no pi bond character between those two carbons that also limits the ability

Of this molecule to rotate freely right if you think about sigma bond sigma bonds are those point on interactions and they can spin freely through space but that only occurs when you have a true sigma bond and no pi bonds however if we look over here this now has pi bond character so let’s take a look at this using a little bit of a molecular orbital picture i’m

Going to call this one example a versus example b and we’re going to look at example a because we know its conjugated yeah the c double bond o will show up at a higher frequency yeah that’s a good question that’s a good point so when i’m talking about the aldehyde i should try to be more specific with which bond i’m referring to so for this one i’m referring to

The ceo of on not the ch bond in the aldehyde that’s a good point all right so let’s take a look at example a with the molecular orbital picture and right now i’m just going to really focus on the pi bond aspect so if we think about this right we’ve gotta see see see see kiki and i’m actually going to offset this oh a little and put the hydrogen out here and i’m

Not even going to show the other hydrogen’s just simply because our actually let’s go ahead and show them i’m changing my mind okay and what i’m going to do is i’m going to draw in all of these p orbitals yep thank you okay so if we look at this right at least in the resonant structure i have drawn here we’ve got some pi bond character here and we’ve got some pi

Bond character here however we said that this has a resonance structure where if you were to draw this out these electrons can actually move semi freely throughout this entire system and i’m going to show that last resonance structure that we have because that’s a nice important one if we look at this last resonance structure the double bond was right here does

That make sense so that double bond that i set up here has pi bond characters is now represented down here using this ml picture that does mean that this oxygen over here has an extra set of electrons so i’m just going to call this negative and that means that the carbon over here is now positive and just like i said because it has pi bond character this bond has

Restricted rotation and this is really just a unique feature of being a part of the conjugated system in order to allow those electrons to be localized we have to form a partial pi bond in between those two carbons so i’ll show you guys the classic example that we’re going to use a lot and we have to come up with a new feature to talk about so let’s take a look at

This conjugated system is it dying we’ve got two alkenes in here we oftentimes refer to this as an s trans dying the interesting thing about this is you can imagine you can try to rotate this and if you rotate it you can get both of those dines kind of facing the same direction when you do that this is called an s fist dying yeah it looks like a relatively unhappy

Molecule which ones more stable yeah the left one is more stable okay so more stable more stable is another way of saying less reactive right if we look at the one on the right we would say this is less stable and more reactive and these dying’s do a lot of really cool chemistry they do things called rearrangements they do a reaction called the diels-alder reaction

We’re going to be spending the rest of the chapter talking about reactions of dying’s in particular the other interesting thing is first time i said that those sigma bonds rotate freely but just now i said anytime you’ve got a conjugated system it actually takes a little bit more energy to rotate those bonds it’s not that they’re locked into place like a true pi

Bond but they do have a little bit of pi character so in this case it takes about fifteen kcals per mole to actually do this bond rotation it’s not without effort this doesn’t spin very freely on its own sometimes you have to add in a little bit of extra heat in order to get this tip at least adopt some of that assists dying confirmation the reality is if you were

To keep the sample up it would be an equilibrium right we’d have a little bit of each but the reactive species would be that s 15 does that make sense alright let’s talk briefly about preparation of dying but this will just be review and then i think we’ll call it a day so in order to prepare a dying you can start with this alkyl halide and then what reagent would

We need if we wanted to turn this into a dying a base right typically you want a nice good non nucleophilic base oftentimes you want to use a pretty bulky base because usually want to favor this hoffman product so a good example of this might be something like turkey talk side and is this going to be an e1 or e2 reaction e 2 because it’s a strong base so you can

Do preparation of these using typically to chemistry the other thing you can do but you have to be a little bit more careful with if you can start out with a dye halide and you can make a dying and again what base would work best here trippy talk side why wouldn’t i want to use sodium hydride necessarily yeah exactly if you use a small non nucleophilic base you

Run the risk of making an alkyne we want to make the ss dying so we want to use a super-duper bulky base so tert-butoxide would probably be the better base here and again this would be an e2 reaction and then the nice thing is we can actually make this from our regular alkene and how do we do that yeah just br2 so now we can convert from a regular alkene all the

Way to a dying by using careful chemistry i think we’re going to stop there tomorrow when we come in we’ll talk a little bit more about the molecular orbital theory and then get into some unique reactivity of rhymes

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Introduction to Conjugated Pi Systems By Jeffrey Engle