Give the structure of the molecule that gives rise to the foll...

Question

Give the structure of the molecule that gives rise to the following IR and mass spectra.

Accepted Answer

All right. Hello everyone. So this question is asking us to give the structure of the compound whose IR and mass spectra are shown below. So the first thing we see here on the screen is the IR spectrum of the unknown compound. And if I scroll down here, we can also see the mass spectrum now because we can only see one spectrum at a time. Let's go ahead and talk about the information we find from the IR spectrum first because the IR spectrum, if you recall gives us information about the relevant functional groups present in our unknown. So drawing our attention right now to the right side of the IR spectrum, we see this one relatively sharp peak here, this relatively sharp band at just around 1600 inverse centimeters. Now, this peak, if you recall actually corresponds to the presence of a car bottle within this molecule. Now, I just want to go ahead and address that traditionally, right, it is a value of 1700 inverse centimeters that tends to indicate the presence of a car B. But I just want to point out that when a car bal is adjacent to an electron donating group, it can actually appear at values less than 1700. That's why here, for example, it's appearing at just about 1600. So we have a car Boel and because of the value that we found the car bal in there must be an electron donating group. And so now with that thought in mind, let's go towards the left side of the spectrum because on the left side of the spectrum, there's one peak right here at around 3400 inverse centimeters, there's another one at 3000, about 200 in centimeters. Now, both of these peaks actually correspond to an H bond. And so the reason or rather because we have two peaks in the Irish spectrum corresponding to the NH bond itself, this confirms that we have an NH two that is a primary amino group somewhere in this molecule. Now, to be specific, the peak at about 3400 inverse centimeters represents asymmetric stretching. And at 3200 inverse centimeters, it's for or it represents the symmetric stretching. And so because this particular this particular amino group, excuse me is capable of both symmetric and asymmetric stretching, we can confirm that it's an NH two group. And so given the fact that we have an amino group and our prior discussion about the peak that we saw corresponding to the car bono, this essentially confirms the fact that we have an amide functional group somewhere in the unknown compound because in order for the amino group, which is electron donating to exert an effect on the car bono, they have to be in close proximity. So an amide functional group would make sense. So now as for the rest of the molecule that we still have to account for, let's draw our attention to the series of overlapping peaks just to the right of the two NH peaks. So here in between around 36,080 inverse centimeters, we see some overlapping peaks. Now this would represent carbon hydrogen bonds specifically when the carbon in question is sp two hybridized. And so the fact that we see these overlapping peaks and also the presence of some more overlapping peaks just to the left of the car Bonal stretch confirms essentially that we have an aromatic ring. And so given that there are no other significant peaks in the IR spectrum, we can conclude that our unknown has an aromatic ring with an amide functional group attached to it. Meaning at this point, our unknown is most likely Benzo, but we can confirm this using the mass spectrum. So let me go ahead and scroll down. All right. So first and foremost, because our proposed unknown is Benz amide, let's go ahead and talk about the molecular formula and also potential combinations that we could expect to see in the mass spectrum. So here's our benzene ring with our aide functional group attached to the ring itself. Now, the molecular formula of benz amide is let's see, C seven H seven no, and the molecular mass of this compound would be approximately 121. So as for the combinations, right, the cations that we might see in the mass spectrum of benzo, we can imagine for example, that the amino group is separated from the rest of the molecule. And so when the benzene ring itself and the car bono contribute to the formation of this carbo cion, that molecular formula ends up being C seven H 50 and this would have a positive charge to be seen in the mass spectrum. So this species writes C seven H 50 positive would have a mass of about 105. So what if we took it a step further and removed the car bal as well? Well, if we remove the car bal as well, then that new molecular formula is going to be C six oops C six H five. And once again, this would have to be a creble cion in order to be seen in the mass spectrum. So now this species C six H five positive would have a mass of about 77. And notice how we see this prominent peak in the mass spectrum at a mass to charge equal to 77 which corresponds to the mass of this particular species in addition, at mass to charge equal to 51 we see a new prominent peak corresponding to AC 4h 3 positive ion. So all this is to say that our correct answer is indeed going to be Benz. So that marks our final answer and there you have it. So with that being said, thank you so very much for watching and I hope you found this helpful.

Give the structure of the molecule that gives rise to the following IR and mass spectra.
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Key Concepts

Infrared (IR) Spectroscopy

Mass Spectrometry

Functional Group Identification

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