16. Conjugated Systems
Conjugation exists when three or more atoms with the ability to resonate are adjacent to each other (overlapping).
Definition of Conjugation
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Hey, guys, and let's talk about a new concept called conjugation. So conjugation exists when three or more atoms with the ability to resonate are next to each other or back to back. Another way that you can think of it is that they're orbital's are overlapping. Now. This idea of resonating or resonance is an old concepts from organic chemistry, one that you guys should all be relatively familiar with. We've all drawn a resident structure at this point. So might be wondering what Johnny, what's the difference between resonance and conjugation? And essentially, there isn't really a difference there. Two names for the same idea, whereas to resonate resonate is a verb, right. If you resonate something that's an action right, will conjugation or conjugated conjugated would be the adjective that describes that you could do that. Okay, so I don't want to get too much into grammar, but basically just saying that something that is conjugated has the ability to resonate. OK, so they're really the same. Similar words for the same idea. So now what does this mean? Well, conjugation provides a highway or an electron highway for electrons to d localize from one side of a molecule to another. And we all know that de localization provides stability for molecules. That's something that we learned a long time ago about resonance. Okay, so it turns out that these conjugated molecules, because there have the extra stability, they're gonna display unique chemical reactivity that we're going to spend a few topics talking about. Okay, Now, in another note, that's pretty much unrelated to everything I just said. There's a new important side note for you to know, Which is that the higher the level of conjugation in a molecule, the higher the UV wavelength is gonna be in a UV viz spectrometer. Okay, now, why am I mentioning this? So the higher the UV wavelengths with if you guys remember wavelength looks like that, Okay, Now, the reason I'm mentioning that is because I'm really not going to spend any time talking about you. Viv is spec. But this is the Onley meaningful application that you really need to know about it for organic chemistry one and two, which is that as your conjugated compounds have more and more congregation, meaning that more and more atoms can resonate together, the higher the wavelengths tend to be for this UV spectrometer, and this could be a multiple choice question. Or it could be a free response question that you get asked. So that's just something that I wanted to throw in there. Okay, so now it's actually talk about the properties of the types of molecules that are conjugated. Well, we just said that three atoms with the ability to resonate have to be back to back. So what type of atoms are the ones that can resonate? Well, we all know that pie bonds can resonate. So we're gonna put here one pie bonds. Okay, now, a pie bond doesn't just have to be a double bond. It could also be a triple bond because we know that triple bonds actually have two pi bonds in them. So double bonds and triple bonds are definitely kid bull resonating. Now, the other ones that are capable of resonating would be ones that have orbital's that air free to accept or donate electrons. So that would be, for example, if you have a basically a lone pair or an anti on. Okay, so I'm gonna put here an eye on or lone pair. Really, depending on what the formal charges of that molecule in terms of resonating the idea of having a lone pair or negatively charged an ion, really? They resonate the same exact way. Okay, the whole deal of having a negative charge just has to do with what's the formal charge of that specific Adam. So for the context of conjugation, we're gonna treat these exactly the same. Okay, so that's what happens if you have two electrons in your orbital. But we know that you don't have to always put to another idea is Well, what if I just put one electron in the orbital? What's that called? That's called a radical Radicals are also capable of resonance or conjugation. Okay. So radicals can also congregate on. The last idea would be Well, how if we put zero electrons in there, then that would be a positively charged um, Adam. So that would be a cat ion. Okay, so these air all of the types of Adams that I want you to think about when we talk about atoms that can resonate. We're just saying that here actually have five atoms listed, right? Because I have the three different charges. I have an eye on the radical to Cat ion. And then I also have the two atoms from the pi bond. So all I'm saying is that you need some combination of these three atoms in a row that would provide for for conjugation to take place. Okay, so what we're gonna do is we're gonna do this practice problem, and you have to identify which of the following molecules exist in a conjugated state. So go ahead and use what I talked about earlier, above as a reference and figure out which of these molecules are conjugated and which ones are not conjugated. So go ahead and do that now.
Quick rule to remember: the higher the conjugation, the higher the UV wavelength
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All right. So is this first molecule conjugated? Does it have three atoms back to back that are able to resonate? And actually, we notice about it is that it has a pie bond and a pie bond back to back meaning this is definitely conjugated because it has 1234 Adams in a row that are of the type that I showed above. Okay, so my question to you is would this be conjugated? Absolutely. This is conjugated okay, because it has those four atoms in a row. In fact, it more than meets the criteria, because all we needed was three atoms in a row, but this one actually has four. Okay, so let's look at this next one. Does this next one have a conjugated state or exist in a conjugated state? And it actually does. Because again, I have three atoms, at least three atoms that air back to back or adjacent that can resonate. We see that we have a double bond that counts is too. And then we have a cat iron, which counts us three. So 123 which is an empty orbital that some of the type of an empty orbital that will be able to resonate as well. So this is also conjugated. Finally, we have this last molecule. Did you say that it was conjugated or non conjugated or unconscious? Gated. And the answer is that this is not conjugated. Okay, so this would be, um sometimes what we like the word that's opposite of conjugated is isolated. Okay, so this is an isolated molecule or not Conjugated, not conjugated because of the fact that I have. Do I have three atoms that can resonate? Yes, I have 12 and three, but one of my criteria is not being fulfilled. They're not all immediately next to each other. Notice how I have this Adam in the middle. That's messing things up. It's isolating them from being ableto really d localized with each other. So because of that, that's going to cause my isolated molecule to exist. Does that make sense? So basically, we've got to conjugated and one isolated. Now, I'm gonna ask you a follow up question, which you don't need to know the answer for, but I'm just gonna throw it out there out of the two conjugated molecules. Which one? We would we expect to have the higher wavelength in a UV viz spectrometer. Would you expect Compound one to have the higher wavelength or compound to to have the higher wavelength? And the answer is compound one, because compound one is mawr conjugated than compound to What we see is that Compound one has four atoms that are able to resonate back to back, whereas two on Lee has three. Since four is bigger than three, that means that we would expect it to have a bigger wavelength, and we would expect this one toe have a smaller wavelength. Okay, so that's just a nap. Lick ation of what I was talking about earlier in terms of that analytical technique called you Viv is all right, so let's go ahead and move onto the next topic.
Review of Common Resonance Structures
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inorganic chemistry. One. We learned about a position word called a Lilic or alot. And what a Lilic simply meant was it was the position that's next to a double bond. Okay, now that position is actually a little bit more important than you might think. The reason is, because now we just stated that conjugation depends on three atoms in a row that can resonate. Right? We'll double bonds typically have two of these atoms, but they're always missing the third one. Notice this double bond. I'm gonna erase this in a second, but notice this one. I'm circling, right? It's missing. It has won two atoms that can resonate, but it's missing the third. Okay, so many times, these double bonds are looking for some kind of orbital reactive orbital to be placed on the third Adam so that they can participate in resonance. Okay, What that means is that typically, Carvell cat irons, carbon ions and radicals are usually unstable, right? Usually we say these air reactive intermediates they don't like to form. But when they're paired with double bonds on the Olympic positions, they become unusually stable due to conjugation. Okay, So meaning that whereas most of the most carbon cat ions are not very stable. The one next to Dol bond will be unusually stable. Will be better than normal. Okay, so now what I wanna do is refresh ourselves kind of on the resonant structures of these reactive intermediates because we'll be drawing a lot of resonance in this section. So let's go ahead and start off with the simplest situation, which is cat ions. Okay, so do you guys remember how Catalans move? I told you guys that cat ions always move with one arrow. I always talk about how if you have a cat ion in that a little position, you can draw it like a door opening on the door hinge. So I just say you draw it like the door opens. And now you replace the other side with a cat eye on your Catalan and your double bond switch places, and that's your resonance structure. Then if we wanted to show the complete structure, you would just show that this one also goes back. Okay, so the cat ion residence structure is the easiest one to draw. How about a basically a lone pair? Now that lone pair, if It's on a carbon. A lone pair is gonna be a negative charge. Now, it's not always gonna be a negative charge. It just depends on what Adam it is. Remember, I was saying this has to do with formal charges. So in this case, since it's a carbon, it's gonna be what we call Carbon ion. So, do you guys remember how many arrows lone pairs move with two. They always move with two. So we would actually start from the region of highest electron density. Just like any mechanism we've ever drawn, you would start off with these electrons moving towards the closest bond. Okay, Now, if we make that bond, we have to break a bond because we're violating the octet of this carbon right here. It already had four bonds were about to make the fifth one. We have to break the bond. So we're gonna get is two arrows, makeup on break a pond, and we're gonna wind up getting something. Looks like this. So negative charge here and now the dole bond is on the other side. Okay, So that would be that would be applied toe lone pairs, but also anything that's an anti am because really the same exact concept last one is radicals. So what if we have just one electron next to a single electron ICS its opon. Now remember that radicals actually moved with three arrows. They move with three half headed arrow, so it's a little bit weird. So we would start off by making part of a double bond with one. But now the double bond next to the radical breaks off into its own radicals. Then we get one radical joining us here and the final radical being dropped off at the end where it's going to become its own standalone radical. So now we have is two electrons joining to make a new pie bond and that left over radical on the side. That would be for radical. So as you can see, maybe this is like a nice um, it's a nice little pattern, but we've got one arrow, two arrows and three arrows. Okay, so these are just ways to think about kind of categorize, thes, resonant structures, resin structures or something you're still gonna have to do for the rest of organic chemistry. So you have to kind of stay on your toes about that. All right, so that's it for that. This topic Let's move on
Additional resources for Conjugation Chemistry
PRACTICE PROBLEMS AND ACTIVITIES (1)