Conjugation Chemistry - Online Tutor, Practice Problems & Exam Prep

Definition of Conjugation

Hey guys. Now let's talk about a new concept called conjugation. Conjugation exists when 3 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 their orbitals are overlapping. Now this idea of resonating or resonance is an old concept from Organic Chemistry 1 that you guys should all be relatively familiar with. We've all drawn a resonance structure at this point. You might be wondering, well Johnny, what's the difference between resonance and conjugation? Essentially, there isn't really a difference. There are two names for the same idea. Whereas to resonate, right? Resonate is a verb, right? If you resonate something, that's an action, right? Well, conjugation or conjugated conjugated would be the adjective that describes that you can 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. Okay. They're really the same similar words for the same idea. Now what does this mean? Conjugation provides a highway or an electron highway for electrons to delocalize from one side of a molecule to another. We all know that delocalization provides stability for molecules. That's something that we learned a long time ago about resonance. It turns out that these conjugated molecules because they have the extra stability, they're going to display unique chemical reactivity that we're going to spend a few topics talking about.

Now, in another note that's pretty much unrelated to everything I just said, there's an 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 going to be in a UV-Vis spectrometer. Okay? Now why am I mentioning this? So the higher the UV wavelength with if you guys remember, wavelength looks like that. Now the reason I'm mentioning that is because I'm really not going to spend any time talking about UV-Vis spectroscopy, but this is the only meaningful application that you really need to know about it for organic chemistry, which is that as your conjugated compounds have more and more conjugation, meaning that more and more atoms can resonate together, the higher the wavelengths tend to be for this UV-Vis spectrometer. This could be a multiple-choice question, or it could be a free response question that you get asked. That's just something that I wanted to throw in there.

So now let's actually talk about the properties of the types of molecules that are conjugated. Well, we just said that 3 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 π bonds can resonate. So we're going to put here π bonds. Now a π 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 2 π bonds in them. So double bonds and triple bonds are definitely capable of resonating. Now the other ones that are capable of resonating would be ones that have orbitals that are free to accept or donate electrons. That would be, for example, if you have basically a lone pair or an anion. Okay? So I'm going to put here anion 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 a negatively charged anion, really they resonate the same exact way. The whole deal of having a negative charge just has to do with what's the formal charge of that specific atom. For the context of conjugation, we're going to treat these exactly the same. That's what happens if you have 2 electrons in your orbital. But we know that you don't have to always put 2. Another idea is, well, what if I just put 1 electron in the orbital? What's that called? That's called a radical. Radicals are also capable of resonance or conjugation. Radicals can also conjugate. And the last idea would be, well, what if we put 0 electrons in there? Then that would be a positively charged atom, so that would be a cation. These are all the types of atoms that I want you to think about when we talk about atoms that can resonate. We're just saying that here, I actually have 5 atoms listed because I have the 3 different charges. I have the anion, the radical, the cation, and then I also have the 2 atoms from the π bond. So all I'm saying is that you need some combination of these 3 atoms in a row that would provide for conjugation to take place. What we're going to do is we're going to do this practice problem, and you have to identify which of the following molecules exist in a conjugated state. Go ahead and use what I talked about earlier as a reference and figure out which of these molecules are conjugated and which ones are not conjugated. Go ahead and do that now.

Conjugated states

All right. So is this first molecule conjugated? Does it have 3 atoms back to back that are able to resonate? And actually, what we notice about it is that it has a pi bond and a pi bond back to back, meaning this is definitely conjugated because it has 1, 2, 3, 4 atoms in a row that are of the type that I showed above. My question to you is, would this be conjugated? Absolutely. This is conjugated because it has those 4 atoms in a row. In fact, it more than meets the criteria because all we needed was 3 atoms in a row, but this one actually has 4. 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 3 atoms, at least 3 atoms that are back to back or adjacent that can resonate. We see that we have a double bond that counts as 2 and then we have a cation which counts as 3. So 1, 2, 3, which is an empty orbital, so that's a 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 unconjugated? And the answer is that this is not conjugated. This would be, sometimes the word that's opposite of conjugated is isolated. Okay? This is an isolated molecule or not conjugated. Not conjugated. Because of the fact that I do I have 3 atoms that can resonate? Yes. I have 1-2-3. But one of my criteria is not being fulfilled. They're not all immediately next to each other. Notice how I have this atom in the middle that's messing things up. It's isolating them from being able to really delocalize with each other. Because of that, that's going to cause my isolated molecule to exist. Does that make sense? Basically, we've got 2 conjugated and one isolated.

Now I'm going to ask you a follow-up question which you don't need to know the answer for, but I'm just going to throw it out there. Out of the 2 conjugated molecules, which one would we expect to have the higher wavelength in a UV Vis spectrometer? Would you expect compound 1 to have the higher wavelength or compound 2 to have the higher wavelength? The answer is compound 1 because compound 1 is more conjugated than compound 2. What we see is that compound 1 has 4 atoms that are able to resonate back to back whereas 2 only has 3. Since 4 is bigger than 3, that means that we would expect it to have a bigger wavelength and we would expect this one to have a smaller wavelength. That's just an application of what I was talking about earlier in terms of that analytical technique called UV Vis. Let's go ahead and move on to the next topic.

Review of Common Resonance Structures

In Organic Chemistry 1, we learned about a position word called allylic or allyl. What allylic simply meant was it was the position that's next to a double bond. 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 3 atoms in a row that can resonate. Double bonds typically have 2 of these atoms, but they're always missing the third one. Notice this double bond. I'm going to erase this in a second. But notice this one I'm circling, right? It's missing it has 1, 2 atoms that can resonate but it's missing the third. Many times, these double bonds are looking for some kind of orbital, reactive orbital to be placed on the third atom so that they can participate in resonance. What that means is that typically, carbocations, carbanions, and radicals are usually unstable. Usually, we say these are reactive intermediates. They don't like to form. But when they're paired with double bonds on the allylic positions, they become unusually stable due to conjugation. Meaning that whereas most carbocations are not very stable, the one next to the double bond will be unusually stable. It will be better than normal. Now what I want to do is refresh ourselves kind of on the resonance structures of these reactive intermediates because we'll be drawing a lot of resonance in this section. Let's go ahead and start off with the simplest situation which is cations. Do you guys remember how cations move? I told you guys that cations always move with one arrow. I always talk about how if you have a cation in that allylic position, you can draw it like a door opening on a door hinge. So I just say you draw it like the door opens and now you replace the other side with a cation. Your cation 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 cation resonance structure is the easiest one to draw.

How about basically a lone pair? Now that lone pair, if it's on a carbon, a lone pair is going to be a negative charge. Now it's not always going to be a negative charge. It just depends on what atom it is. Remember I was saying this has to do with formal charges. So in this case, since it's a carbon, it's going to be what we call carbanion. Do you guys remember how many arrows lone pairs move with? They always move with 2. 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 4 bonds. We're about to make the 5th one. We have to break the bond. So what we're going to get is 2 arrows. Make a bond, break a bond. And we're going to wind up getting something that looks like this. So, negative charge here and now the double bond is on the other side. So that would be applied to lone pairs but also anything that's an anion because really they're the same exact concept.

Last one is radicals. So, what if we have just one electron next to its O bond? Now remember that radicals actually move with 3 arrows. They move with half-headed arrows, so it's a little weird. We would start off by making part of a double bond with 1, but now the double bond next to the radical breaks off into its own radicals. Then we'd 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 what we have is 2 electrons joining to make a new pi bond, and that leftover radical on the side, that would be for radicals. So as you can see, maybe this is like a nice pattern, but we've got one arrow, 2 arrows, and 3 arrows. These are just ways to think about kind of categorize these resonance structures. Resonant structures are something you're still going to have to do for the rest of organic chemistry. You have to stay on your toes about that. That's it for this topic. Let's move on.