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Introduction to Conjugated Pi Systems

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Alright chapter 17 in the klein textbook everybody know we can fit in a couple minutes there i gotta fit this in in order to stay on schedule so chapter 17s on conjugated pi systems this won’t be anything new it’s just going to be an introduction to things we’ve already seen before we just really didn’t know what to call them so the idea of a conjugated system

Is a pretty easy one a conjugated system is alternating double single bonds oh very much unchanged so if we think about a conjugated system we could say that this is a conjugated system right it’s alternating double single bonds there’s another system that’s closely related called accumulated and that dot in the center is just used to describe a carbon so it’s

Actually adjacent carbon-carbon bonds that are head-on attached to one another and then there’s another one that’s also similar to these and this is called isolated so we’ve got conjugated we’ve got cumulated and then we’ve got isolated in both cases we’ve got two pi bonds it just has to do with their relative positioning to one another so in this case conjugated

Systems are always going to be a single bond apart not more than one single bond like the isolated case we’ve got two sigma bonds apart so let’s take an example here this is an example that somebody was asking me in my office hours about ir let’s take this aldehyde versus this aldehyde which one is conjugated the one on the left or the right the left we’ve got

Alternating double single bonds in it right so if we look at this one we can say all right it has a resonance structure where look where the oxygen has a negative charge the carbon has a positive charge and the double bond remains in place or we can even have it more delocalized excuse you and the positive charge is down here so conjugated systems are pretty unique

Because there’s a lot of resonance you stay or delocalization wearing this one we can’t move that pi bond we can only have one resonance structure where there’s an oxygen with the negative charge and the carbon with the positive so in this example that i gave you this one’s conjugated this one is isolated and it’s because of this d localization that we can observe

A variety of really unique reactivities because of all the different possible resonance structures so one thing i wanted you guys to do on the problem of the day that i just handed out was to go through a mechanism that we’ve already seen and then see if there’s any sort of d localization and see if you might even get more than one product out for each reaction

Instead of just one product so i want you guys to try to troubleshoot this a little bit on your own but try to think about what can stabilize a carbo cation it should be pretty simple at this point the neat thing with this is let’s kind of take a look at what’s going on in a conjugated system i’m not going to include hydrogen’s i’m just gonna include the carbon

Backbone so let’s say we’ve got this carbon backbone and it’s all sp2 hybridized like in our conjugated system we think about it all of these carbons have p orbitals right not the greatest-looking p-orbitals but in either case we can have a pi bond between the end pieces or we can conversely have a resonance structure where that pi bond is now delocalized and we

Actually have some double bond like character in between the internal carbons so now there’s a pi bond more on the center carbon rather than the exterior carbons this makes this carbon-carbon bond pretty weird because if we look at it and the way we normally draw it we draw it as a single bond but is it really a single bond no it has double bond like character right

So if we look at this carbon-carbon bond it has double bond like character so this gets interesting to think about so let’s take this system and draw it out a little bit lower so i’m gonna take this diamond actually let me slide it over a little bit it’s a dying because it’s got two alkenes on it we can apply energy to this and we can actually get that single bond

To rotate that i just said has some double bond like character and we actually call these by different names this is called an s trans dying because that’s single bond is acting like a little bit of a double bond so the groups are trans to one another the one on the right is going to be the s sis dying and it actually takes a fair amount of energy to knock this

Into the assist conformation because we know that sis we run into steric repulsion so it’s energetically destabilizing to go to the sis and it’s about 15 kilojoules per mole the interesting thing about this is as cysteine is less stable but it is more reactive and i think that’s where we’re gonna leave it today we’re gonna study a lot of reactions with this esses

Conformation where we’ve got a dying where they’re both facing each other and we’re gonna learn a really cool reaction called the diels-alder reaction where this is a very powerful way of making carbon-carbon single bonds alright i’ll see you guys tomorrow

Transcribed from video
Introduction to Conjugated Pi Systems By Jeffrey Engle