IR signals for carbonyl compounds

- If you look at the dot structure for a ketone, we already know we're gonna see a signal for the ketone carbonyl on our IR spectrum, because the oxygen is partially negative, and this carbon here is partially positive. So, a relatively large separation of charge means a relatively large change in the dipole moment of the carbonyl when it stretches. And so therefore we get a very strong signal on our IR spectrum. And the signal for the ketone carbonyl shows up at a wave number of approximately 1715 or 1720. It's a little different for a conjugated ketone, so down here we have the dot structure for a conjugated ketone. Conjugation lowers the signal for the wave number, and let's see why. So we can think about resonance here, so if I move these electrons into here, push those electrons off onto oxygen, I can draw a resonance structure, so over here, we would now have a double bond in here, we would now have a single bond of carbon to oxygen, and three lone pairs of electrons around the oxygen. Formal charge is negative one formal charge for the oxygen, and this carbon right here has a plus one formal charge. So, if the electron's in magenta move over to here, like that, then the electrons in blue right here could move off onto your oxygen. So remember, the actual structure of the molecule's a hybrid of your resonance structures. And so, the resonance structure on the left, this looks like a carbon-oxygen double bond. The one on the right looks like a carbon-oxygen single bond. So in reality, it's more of a hybrid of those two. And so, over here I'm going to draw a carbon bonded to an oxygen, I'm gonna give it a partial bond right here. So I'm saying it's stronger than a single bond, and not quite as strong as a double bond. The idea is resonance lowers the double bond character of your carbonyl. So that's weakening, that's weakening the carbonyl. And so, if you're weakening the bonds, right, remember the value for K, the spring or the force constant goes down, and from earlier videos we've seen if you decrease K you're going to decrease the frequency, you're going to decrease the wave number where you find the signal. And so the signal for the carbonyl, alright, let me go ahead and use green here, so the signal for our carbonyl moves down to approximately 1680 or so, so definitely less than 1700. And once again that's because resonance, right, that's weakening, you can think about that decreasing the bond strength. Alright, what happens when you add in an electronegative atom? So now we're talking about a generic carboxylic acid derivative here. So why is some electronegative atom, like oxygen, for example, right? We could also have resonance here, right? So we could draw a resonance structure, we could move the lone pair of electrons in here, and push those electrons onto our oxygen, and we could go ahead and draw a resonance structure. So now, once again, the top oxygen would have a negative one formal charge, and now the Y would have a positive one formal charge, like that. So, for keeping with our colors here, so the lone pair of electrons moves in here, right? And then the pi electrons in here in our carbonyl move off onto our oxygen. So once again, for our carboxylic acid derivative we have resonance. And when you're thinking about the carbonyl, what is that doing to the carbonyl, right? In your hybrids, it's decreasing the double bond character of the carbonyl. So you're decreasing the strength of the carbonyl, and, we've already seen, right? Decrease K, right, decreasing the strength of the carbonyl, you decrease K, you decrease the signal where you find the wave number. You decrease the frequency of bond vibration, alright? So that's the idea of resonance. So resonance is present. But when you're talking about a carboxylic acid, you also have an inductive effect. So we talked about this in the video on carboxylic acid derivatives. There's a competition between resonance and induction. So let's think about induction here. And if we have our R group, our carbon, our oxygen, and our eletronegative atom Y, like this. If we are thinking about the double bond as being, as being decreased by resonance, right? So decreased double bond character because of resonance, what is induction? Induction, of course, refers to the electronegativity of this atom here. So if we have a very electronegative atom relative to carbon, like oxygen, right? This is going to withdraw electron density in this direction. So we can think about a lone pair of electrons in this oxygen, right, some electron density I should say, moving in here to strengthen the carbonyl. So if you're withdrawing electron density, you can think about some electron density from the oxygen moving in to increase the strength of the carbonyl. And so this has the effect of giving us more of a double bond again, right? So that's just how I like to think about it here. And so, induction, I'm gonna go ahead and write this here, induction, by withdrawing electron density, we're increasing the strength of the carbonyl, so we're increasing the spring constant or K. So we're increasing the frequency of bond vibration, we expect to find the signal at a higher wave number. And so, once again, these two competing effects are something we have to think about for carboxylic acid derivatives, right? Resonance is going to lower the signal, and induction is going to increase the signal. So let's look at a whole bunch of carbonyl compounds here. Let's look at the different IR signals and see if we can explain the signals. Here we have a bunch of compounds that contain the carbonyl, and we would expect to find the signal for the carbonyl at a wave number in a range of 1850 to 1650, so somewhere in that range we'd expect to find the signal. And the way I like to think about this is to kind of divide that in half. So 1850 and 1650, so somewhere in the middle would be about 1750, and if you go a little bit lower than that, 1740, I like to think about that as being the average signal. So therefore, an average value for K, or an average carbonyl strength, alright? So if we go a little bit higher than that, so we're looking at an estronal, in the video on the reactivity or carboxylic acid derivatives, I told you how to think about the competing effects of resonance and induction. And for an ester, the inductive effect is a little bit stronger than the resonance. So if the inductive effect is a little bit stronger than resonance we've just seen that has the effect of increasing the strength of the carbonyl. So we're going to increase the value for K a little bit, and if you increase the value for K, you increase the wave number where you find the signal. So if you increase K, you increase the frequency of bond vibration, so you increase the wave number, and so the wave number will go up a little bit from 1740 to approximately 1745. So again, this is approximately where you'd find the signal for the carbonyl for an ester. Next, let's do an acid anhydride. So for an acid anhydride, the inductive effect is even more important. And if the inductive effect is even more important, so if we think about this oxygen here as being very electronegative, that means that we're gonna get an even higher value for K. So an even higher value for K, an even stronger carbonyl. And so if we have a higher value for K, we have a higher frequency of vibration, we expect to find the signal at a higher wave number. And so the wave number goes up to approximately 1760 to 1790. And this is the first signal for an acid anhydride. So this is actually the symmetrical stretch. So we have two carbonyls, right? And if they're stretching in phase with each other, that's the signal that we would expect to find. So we also have an asymmetric stretch, so we're actually going to see a second signal for an acid anhydride, we talked about this in the previous video, it takes a little bit more energy for the asymmetric stretch, and so you find that at a higher wave number. So approximately 1810 for the asymmetric stretch. So two signals for the acid anhydride. For an acyl or acid chloride, the inductive effect completely dominates the resonance effect, so even more strongly than the previous two examples. So the chlorine is withdrawing some electron density, we're strengthening the carbonyl even more, so the force constantly goes up even more. We have a higher frequency of vibration, we get a higher wave number. So the signal for this carbonyl appears at a higher wave number, so approximately 1800, or even a little bit higher than that, so 1815 or so. So once again, that's the approximate value for the signal of the carbonyl for an acyl or acid chloride. Alright, let's go in the other direction. So if this is our average spring constant, and pretty close to our average signal for our wave number, let's look at an aldehyde next. So here we have an aldehyde, right, we have a hydrogen here, so we don't have an electronegative atom to think about. In the previous examples we had oxygen, we had oxygen, we had chlorine to think about, but here we have hydrogen, so we're not really worried about the inductive effect here. We're more concerned with the electron donating of the alkyl group, right? So we have an alkyl group right here, and alkyl groups are electron donating. So if we're donating some electron density in this direction, we're gonna lost a little bit of electron density from our carbonyl. So, like charges repel here, and therefore, we're decreasing the strength of the carbonyl. So we're decreasing the force constant K, and if we decrease the force constant of K, we would expect the signal to be at a lower wave number here. So the average value would go down to approximately 1725. And again, all these are just approximate values, I'm not saying it has to be exactly that, just somewhere around that. We would expect to find the signal for the carbonyl for an aldehyde. Comparing an aldehyde to a ketone, a ketone has two alkyl groups. So we have even more electron donating, and so we're going to weaken our carbonyl even more. And so if we're weakening our carbonyl, we can think about K decreasing even more than before, and so we should expect the signal for the ketone to be at a lower wave number. And it does go down to approximately 1715 or 1720. And so, once again, this is pretty close to where you'd find the signal for a ketone here. Alright, let's talk about carboxylic acids next. And so, if we have a carboxylic acid, let's think about it, let's draw another one here. If we have a carboxylic acid in the dimeric form, so we have another carobxylic acid here, we can get some pretty strong hydrogen bonding. So we get some pretty strong hydrogen bonding here. And that's gonna have the effect of weakening the carbonyl. So you can think about some electron density moving into here because of your strong hydrogen bonding. And so that turns out to decrease the value for the force constant even more compared to our average value. And so, for the carboxylic acid in the dimeric form, we would expect to see it somewhere around 1710, although the signal can definitely vary for a carboxylic acid, so you might not see this exact value somewhere else because everything depends on, on things like what form is it in. Here we're talking about the dimeric form. Finally, we have our amide here. And, a carboxylic acid derivative, so we think about resonance versus induction, and this is the example where resonance dominates. So resonance dominates induction, and we have to think about resonance structure. Move these electrons in here, we push those electrons off, so we have the effect of weakening the carbonyl. So we're decreasing the strength of the carbonyl, and resonance dominates, so we're gonna decrease the value for K a lot here, so we're gonna decrease the value for K a lot compared to our made-up average value. So we're gonna decrease the signal for the wave number. So we're gonna get a much lower wave number here. So approximately 1650's to 1690. So again, these are all just approximate values for where you would find your signal. But, thinking about resonance and induction, and electron, and donating alkyl groups, and hydrogen bonding, allows you to approximate the signal, and therefore approximate the strength of the carbonyl.