Infrared Spectroscopy Table - Video Tutorials & Practice Problems
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1
concept
Common IR Frequencies
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Now we're going to dive right into the nitty gritty of IR spectroscopy. Basically, for every absorption, there's two things you need to know. You need to know. First of all, what's the frequency or wave number that it's absorbing at and to what the shape is? Right? Well, we're going to focus on right now is just that first part of knowing exactly which wave number is going to correlate with different types of bonds. Now, before we begin, you already see something pretty complicated in front of you. I need to make a disclaimer for you guys. Okay, which is this on this section? I'm gonna need toe ask you guys something which is I'm gonna need for you to be a little bit flexible with me. Now, I know that that's hard toe ask, because a lot of you guys out there are super perfectionists. And you wanna make sure that you have all the information even before you walk into the classroom. I think that's what who you are. But anyway, the problem with that is that analytical techniques are not taught in a perfectly standardized way. And in fact, many professors explanations of analytical techniques will depend based on their personal experience based on their personal interest in the subject and based on their teaching preference. Okay, so what I'm gonna do is I'm gonna default to the more complex explanation. The one that covers all the bases. And it's gonna be your job to calibrate what I'm saying. Toe what your exact professor wants. If he doesn't care about some of these values, that's okay. But I'm going to teach them just in case. Okay? Also, if I happen to tell you that a specific absorption happens at 17. 50 and your professor says it happens at 17. 40 it's not the end of the world. Okay? We're going for big picture here. We're not going for the exact specific. I don't not want you to get into an argument with your professor about how Johnny on Clutch Prep said that it was 17. 50. Okay, we just want to know the general over arching themes, and you can tailor this to your specific situation. Okay? That being said, this should really work for most of your most of the absorption is you ever need to know. Okay, so let's go ahead and just look at this chart. And as you can see, this is our familiar ir spectrum. We have wave number on the X axis, and we have percent transmitted on the y axis. But notice that I'm not focusing on shapes here. I don't have any stalactites. What I'm focusing on is the actual numbers and memorizing the specific wave numbers that happen at different with different types of bonds. Okay, now, what you also can recall is that this is still split into those regions that we talked about. So notice that anything below 1500 I didn't even include right, Because what is that called the fingerprint region, which we're not going to discuss at Clutch Prep. Okay, then everything after 1500 follows the trend that we were talking about, how we have our double bond region, which goes to about 2000. So you can see that right here these air are double bonds, right? And then notice that from 2000 to about 2500, we have our triple bonds, okay? And this was double bonds, okay? And noticed that pretty much everything after 2500 and beyond was our bonds to h I'm just gonna put a single bond. H it doesn't necessarily have to be see just anything single bond into H. Okay, so we're keeping the same general picture, but now we're gonna memorize the specific absorption is Okay, so first of all, you can already see a discrepancy. If you guys were paying close attention, we're going to start at the very bottom at the lowest frequency. And what you see is that I have all came here. That's gonna be the C double bond. See, in my dull one region, resulting at 1600 on a previous lesson, I said 16. 50. It's a range. It's going to be somewhere between 1600 and 16. 50. Um, don't worry too much about it as long as it's somewhere in that lower range. You know, you haven't Al Keen. Okay, so now we get to this big section called the Carbon Eel Region. Okay, now there's a chance that your professor just wants you to know that carbon eels around 1700 that's it. Okay, That actually might be about half of you guys. Might be that lucky. The other half might need to know the exact types of carbon eels and all of their absorption. Okay. And for that, I devised a little pneumonic, a little memory tool that you're not going to see anywhere else called Corn. So our favorite vegetable, this section is gonna be corn, because corn helps us to remember the order in which these different carbon deals, um, will present themselves. Okay, so let's just talk about this. What are the different categories? Well, first we have acid chlorides. Okay, so we have acid chlorides and those air resulting at the highest wave number of 17. 90. Then we have carb oxalic acid. We just get a different color there. And Esther's, which were resulting at 17. 50. I'll explain corn in a second. Okay, then we have Aldo hides and key tones at 17. 10. And then finally we have am IDs. Uh, 16. 80. Okay. Now, where the heck did I get corn? Well, if you think about the things that are attached to the carbon eel in all of these for acid chloride, have a C l for O h or for car oxalic acid. Er, Esther, I haven't Oh, for Aldo hides and key tones. I haven't are or an age, right? And then for am IDs, You have an end. Okay, So getting rid of that stupid little L I get the acronym corn, and maybe that helps you guys to memorize the order in which they're at it doesn't tell you the exact numbers, but maybe you remember. Oh, acid chloride is higher than am I because of corn? All right, so you're going to sound like a total vegan this semester. Okay, so then what else do we have going on here? Notice that. Have these two bullet points which have to do with general rules, Okay. In general congregation, which is basically the ability of any of these carbon yells to resonate with something else. So I'm just gonna put here in case you don't know the definition of conjugation. That's okay. That just means can it resonate with something else? Okay, so the possible the presence of conjugation is gonna lower the number by 20. So that means that, for example, here I have a conjugated key tone. It's right by my head's gonna move out of the way a little bit notice that the reason I'm calling this a conjugated key tone is because I have a key tone with a double bond that could resonate with it. Okay, if you drew resident structures, you'd be able to draw charges and all that stuff. Okay, Now, I noticed that this kondracke tone is resulting at I covered it. 16 90. What's the normal absorption of a key tone? 17. 10. So any time you have conjugation present is gonna lower your predicted value by 20. About 20. Is that cool? And you could apply that to the other ones as well. So a conjugated acid chloride would result at 17. 70. Awesome. Okay, so you guys were getting this. So other thing, this is getting even weirder. What the heck is a banana bond? Okay, so banana pond again. This is so that I could be comprehensive. Guys, I'm just trying to make sure that I cover all my bases. Banana bonds happened in small strain rings. So, for example, cyclo butane has banana bonds. It just means that you have overlapping bonds or overlapping orbital's have to shape kind of like bananas in order so that they can overlap. OK, so now we've got vegetables. You got fruits. What the heck is going on. It feels like I'm going walking through a produce aisle. But anyway, Banana bonds, Because of their spatial arrangement, they're highly strained. They're gonna tend to result about 100 higher. So you can imagine that I am cycle a key tone right that has highly strained bond in it is going to result about 100 higher. So instead of 17 10 it's a 18 10. Okay, now, that's just a general rule. Your professor might have a slightly different number, but that's just something for you to remember that if it has these highly strained rings, that would mean like a three member ID ring or four member drink thes have been Anna Bonds, and they're going to get a higher, higher absorption frequency. Cool. So far, case that was the carbon you'll region. Maybe that was too much information, depending on your class. But now you know. So then we're gonna close off the double bond region with one more molecule that I did mention earlier accumulations. Okay, accumulating is just two double bonds that air back to back, okay, And it could be any accumulating. It doesn't just have to be end of one end on one end, as I have in this example, um, it could be C o N. It could be There's a lot of different molecules that fall into the queue. Million area. They're gonna results around 1900 to 2000. Cool. They still belong in the double one region because they have on Lee double bonds present. Okay, so now let's move on to the triple bond region, where we have al kinds and night trials sitting between 2200 and 2300. Just remember that night trials results a little bit higher than al kinds, but typically, um, the way that we can differentiate it a NALC kind from a night trial is Is there even a nitrogen in this molecule? If there are not any nitrogen in the molecule, you know you can have a night trial. Okay, so it's usually not a very difficult task Thio to figure out if you have one of the two. Okay, so now we have bonds to H and boy, there's a lot of them. Okay, so let's start off with carb oxalic acid carb. Oxalic acid is a bond of O. H. But it's a bond of Ohh That happens on a carbon, Neil. Okay, so car oxalic acid would look like this. Okay, Now, a carb oxalic acid is an example of something I'm gonna be referring to as a complex carbon Neil. Why is that? Because a carb oxalic acid doesn't just have one absorption. It has to notice that this double bond is a new absorption that I already mentioned. It's right above my head. Actually. The double bond between the sea and the oh, because attached to an O should result at 17. 50 right? We just talked about that. So then why am I bringing it up here? Because notice that carb oxalic acids also have an O. H. Bond. This o H is also going to result somewhere. And it turns out it's gonna be a very broad peak between 2500 and 3000. It's literally gonna take up a huge swath of the spectrum. Okay, so a complex carbon eel is when you get more than one absorption from a carbon deal. Okay, so we're gonna be looking at thes, and we're gonna have toe make sure that we can kind of locate them and recognize them because this is gonna happen a lot with IR spectroscopy. Our other example of a complex carbon. He'll actually occurs in the same range. Aldo, hide. Okay. Have we talked about Aldo Hide already? Yes, we did. We talked about how an alga hide should result. Where, according to my Carbonell region two results at 17. 10. Okay, but it also has a bond to H. That bond to age is actually gonna result right here at 2700. Okay, so it's another thing to be aware of that both of these are examples of complex carbon eels. And we need to look out for both of their peaks. One in the carbon you'll region and one in the hydrogen region. Single bond to age region. Okay, Sounds good. So far. Awesome. So now let's move on to the other bonds age. So we've got our hydrocarbons. We've got our basically, Let me write this here. We've got our al canes. We've got our Al Keene's and we've got our AL kinds. Okay, notice that the terminology I'm using for this is s p three sp two s p. Why am I referring it to do it like that? Well, That's just the shortest way that I could tell you. That s p three means that it's gonna have four groups, so it should be an alky. It should have single bond with, you know, for a carbon with four bonds around it. So that would be an Al Cane Al Keane's RSP to hybridize. So if you guys recall, that means it has three groups around it or should have three different types of bonds. Ah, great example, being the clutch logo. Benzene. As I put at the top, notice that each of these carbons would have just three groups would have a hydrogen. Ah, carbon and carbon. Okay, so this hydrogen, because it's an AL Keen because SP two hybridized is gonna result higher. Usually, Al Keynes results around 2900. Okay, and al Keynes results around 3100. So about 200 wave numbers higher, then finally, all kinds are going to result the highest. Now, all kinds are triple bonds, right? But there is kind of a caveat to this, which is that it's on Lee Terminal al kinds terminal. All kinds will result here. Why? Well, because if you remember, this is what it all kind looks like, right? We already talked about the wave number for that bond. Where does the triple bond result? The triple bond should result at 2200. Okay, The Onley time you're going to get an additional absorption is when you have an age present. And that only happens when you have a terminal Al kind. Okay, so if you're Al Qaeda is in the middle, you only get 2200. But if your terminal But if you're Al Qaeda is on the end of a chain, so has an age. Then you get a 22 100 plus a 33 100. Okay. So in that sense, this is also a complex, functional group. Not a complex carbon Neil, but a complex, functional group. Okay, hoof. We're almost to the end. We're almost finished with our hydrogen, so we've got a means and we've got alcohol's now notice that they actually have overlapping ranges. My alcohols are a little bit higher, but they tend to overlap these air, mostly gonna be distinguished by their shape, which we will go into, but in general alcohol's arm or broad. Okay. As you can even see here as I drew. The range is very large with alcohol. In fact, it looks like a parabola. It takes almost the entire 3000 area. Okay, Whereas the means tend to have much smaller peaks that air, um, in the same range. But they definitely don't look like an alcohol. So we kind of assume that it's in the meat. Okay. And that's pretty much the end of that. Okay, so that was a mouthful for the rest of today's. You know, anything you're watching in terms of ir feel free to use this as an open book. Exams feel free. Thio, use this as a reference because my biggest goal for you is not to memorize this right now, but to get familiar with it so that once you're working with it, the value start coming back to you. Okay. Awesome. So let's move on to some practice problems really quick in these practice problems, What I'm gonna have you guys do is I'm going toe. Have you guys look at each of these molecules and I don't need anything about shapes. I just need to figure out how many different absorption what I see for these molecules. What I see one what I see to what I see for and what would be the values of those absorption based on the values that I taught you above. So go ahead and look at the first problem, okay? And try to predict how many different values am I going to get there and what would be the actual numbers of those values? Okay, so go ahead and take a second to do that, and then I'll go ahead and answer the question.
2
example
Major IR absorptions
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2m
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Let's see if you got this first one, right? So for this molecule, how many different types of bonds were you able to spot? It turns out there's more than one type of bond here. There's actually two types of bonds. Let's go ahead and see. The first one that I'm looking at here is a C C. Single wand. Okay, that would be all of the bonds. You know, they're making up the Alcan. Okay, the second kind of bond that I could spot is a C H. Bond. Because there's hydrogen is everywhere, right? But we specifically have to include what's the hybridization of that ch? Because remember that the peak is going to change depending on what type of hybridization is. So being an out cane thes would all be R S p three c h bonds. All right, so now I know what you're thinking. You're saying, Johnny, but I thought the C C. Bond didn't matter. And that's exactly right. You got it right. It turns out that this first bond I'm just gonna cross out because it doesn't matter. This is a single bond. That's not to H. So where would it result? You got it. Fingerprints. This is fingerprints. And for right now, I still want you to identify the fingerprint region bonds. But don't worry about memorizing anything with them. I just want you to identify it and say that's irrelevant. Okay, So exactly did that right. This doesn't matter. I'm just gonna cross it out like we did. The Onley. One that really matters is this one. So we've got one absorption. And where would that absorption be? That would be around 2900. So if you got that, if you got that wave number, if you got that one peak, you're good. If you forgot to identify the single bond, that's okay. Don't worry about it. But for this next one is we keep moving to the new examples, try to identify every single bond that there is, and then throw out the ones that are in the fingerprint region so you can just focus on the functional group region bonds. All right, so move on to the second question.
3
example
Major IR absorptions
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1m
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So, how many different types of bonds did you guys find for this one? Well, it's more than two will tell you that. Let's go ahead and start off with ones that don't matter thes We're going to be single bonds. The stuff that isn't h so that would be, um, won. I've got a C C single bond fingerprint region again. Two. I've actually gotta see single bond. Oh, that's also relevant because that's a single bond. That's not the hydrogen. So I can immediately cross both of these out. Okay? Do we have anything else? Yes, Well, we have three. We've got our C h bond again. Okay. And once again, this entire thing is an al cane. So it's gonna be s p three, So sp three ch. Okay. And we do have a last fourth one, which is O. H. Does that one count? Yes, because that is a single bonds of hydrogen. So that means here you have to identify the wave numbers of two different bonds. So for our sp three ch, we know the value is again 2900. And what would be the value of the O h? Yes. Remember, it's actually a really big range. You can't just say one number. You can never say just one number for a broad peak because a broad peak, by definition spans multiple wave numbers. So the range you guys need toe learn for alcohol is 3200 to 3600. Okay, so we're just always gonna say 32 to 36 because it is such a broad signal. All right, so I think as you're getting the hang of this, let's move on to the next question.
4
example
Major IR absorptions
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2m
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So here's another one. I hope that you guys were able to find four different types of bonds. Let's first of all talk about, I guess, the one that doesn't matter, which is C single bonds. See, we know that we see that around, but there are a few ones here that do matter. So, for example, a new one that we had in this question is a C double bond. C Remember that? That gets its own signal. That's a double bonds that would be in the double one region. So that's something to hold on to. We'll do values in a second. Um, three. There was R S P three ch. That would be the entire Al Cane portion of this molecule. What's the fourth peak? Good job. So the fourth peak is actually gonna be SP two ch. Why? Because there's two hydrogen on this molecule. That air located on an AL Keen. Those hydrogen are gonna have ah higher wave number than the rest of the surrounding hydrogen. Why? Because they have a different hybridization. Okay, so now we have to talk about Well, how many signals or how many different absorption frequencies do I need to stay here. Three, We're not gonna worry about fingerprint, but what's the what's the wave number that we have to memorize for an all keen CC double bond? That would be 1600. Okay, we're just gonna go with the value of 16 as one stated value. Okay, it's a pretty sharp peaks. That's fine for SP three ch 2900 and lastly for SP two ch. This should be easy if you remember that. I just said it's gonna increase by 200 for each hybridization. So that means the SP two would be 30. 1 100. Got it. Awesome. So that was a little bit more tricky, But you guys got this. Let's move on to the last question.
5
example
Major IR absorptions
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2m
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on this question, you might notice an added complication because I threw in what we called earlier a complex carbon eel. Remember? I told you guys how complex carbon deals are the ones that are not just going to result in one area, they're going to result in two. And we need to be able to identify that. So, first of all, let's go with the easy ones here. We can get rid of our C C single bond. We know that there's gonna be an S P three C h bond. Okay, that would be, um, just quickly identifying it. That would be like a hydrogen right there. That would be SP three ch. Okay, but now we get to the part of the complex carbon eel. Okay, so, three, let's just talk about first of all, the C double bond. Oh, where is that going to result? Now, keep in mind, See? Told 10 is a carbon Neil. So that means that it could result. Remember, we have a big range for our carbon you'll region. I said it was something like 16. 80 all whipped 18 40. So pretty big range there, but specifically because this is an alga hide because of corn. Where would you expect to find it? Remember that an alga hide would count as the are in corn because it could either be Alva hydro ketone R h. Okay, so that would result at 17. 10. Okay, so we've got the the frequency for the alga hide. But what else do we have to worry about the C h. Bond? Because it's a complex carbon eel. So we're also gonna have a bond to h to worry about. So specifically, the alga hide H gets its own unique peak. And that's going to be at 2700. Okay, so hopefully I'm not in the way there. You can see what I'm writing here. We've got the complex carbon deal with two different peaks just as a pop quiz. What's the other complex carbon deal that we're gonna worry about in this course? Car broke cilic acid Because carbon cilic acid also has a peak in the Carbonell region and has a very broad peak in the hydrogen region. Okay, so, guys, that's it for this practice problem. I think you guys got the picture of what you need to do now. You're doing great. Let's move on to the next topic.
6
Problem
Problem
Answer each of the following questions based on the images below.
a) Which compounds show an intense peak ~ 1700 cm-1?
b) Which compound shows an intense, broad peak at ~ 3400 cm-1?
c) Which compound has a peak at ~1700 cm-1, but no peaks at 2700 cm-1?
d) Which compound has no signal beyond the fingerprint region?
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7
Problem
Problem
Identify which carbonyl group will exhibit a signal at a lower wavenumber
A
A
B
B
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