Mass Spect:Fragmentation

Hey, guys, In this video, we're going to discuss common fragmentation patterns found within mass spectrometry. So basically, we're going to discuss the step of ionization and everything that follows after that. So before we can really understand why some fragments are going to be more favored than others, we need to grasp this concept called ionization potential. And what ionization potential tells us is how likely a new electron is to be knocked off by this electron beam. It turns out guys that not all electrons are made equal. Some of them are gonna be held very tightly by molecules, so they're more difficult to knock off, and some of them are going to be held very loosely. So that means that they're the first ones to go when they hit this electron beam. These patterns, these ionization potentials can help us help us to predict what's going to be the major cat ion that's formed or what's gonna be the major radical cat I, in its form after ionization has taken place. So guys here have just made a very simple trend that I love for you guys to memorize. And all it is is this that basically one of the easiest ones to knock off is the lone pair of a nitrogen. And guys, this is for the same reason that we've kind of always thought of the nitrogen is having a very reactive lone pair. It's very loosely held. It's very easy to ayan eyes that lone pair. Okay, so if you have a nitrogen with a lone pair on your sample, that lone pair is the most susceptible to getting knocked off. One of those electrons is likely You're going to get knocked off during ionization during the ionization stage. Okay, Now, in terms of this kind of spectrum, we're just going to kind of go down one by one, the different types of compounds that get a little bit harder toe ionized with every step so the next one would actually be aromatics. Okay, So aromatics. These are the general category of benzene and benzene like molecules. Okay. And it turns out that any of the single bonds directly attached to an aromatic are actually relatively easy toe lionize. Okay, so this is going to be something else that we're gonna see. Aromatics tend to give a radical cat ion that doesn't involve breaking the ring because that ring is very stable. So we wanna keep that ring intact. We wind up just breaking off one of the ends, one of the single bonds attached to it that's coming off of it. So in terms of ionization, we wouldn't ionized the actual bonds of the ring. We would ionized maybe one of the hydrogen or something that's that's attached to the benzene ring. For similar reasons. Double bonds come next, So basically, this is what we call vinyl. This is a vinyl position of vinyl. Position means directly touch total bond. It's a little bit harder than benzene, Um, but it's still not that bad. We would prefer to knock off a vinyl position, something that's on a double bond rather than something that's not on a double bond. So if you're directly touched your double bond, that's the one that you are likely going to. I I in eyes. Then we get, um, lone. Parents has oxygen for the similar reasons as nitrogen just less reactive, less susceptible. So this one would have a higher ionization potential. And then finally, the hardest one guys kind of the ion of last resort would simply be a single bond that's attached toe other single bonds. Okay, so in this case noticed that I'm not attached to a double bond. I'm not attached to a ring. I'm not attached to a benzene. I'm just attached to something that is an al cane. Okay, so I'm just gonna put here in Al Cain. Now you might be wondering, Johnny doesn't matter that it's a ring. No, I'm just using rings here to keep everything consistent, because what I'm trying to show you is that it's not the ring that matters. It's really the stability of that ring, whether it's a benzene, whether it's a double bond or whether it's just a al cane. So in Al Cane, similar to the methane that we used in our intro video, that would be one of the more difficult ones toe ionized because there's really nothing. There's nothing helping that those those those radicals to get loose. That's just gonna take brute force to remove one of those electrons and make the radical cat ion Okay, there's no extra stabilizing factors for an Al cane. All right, awesome guys, let's ionization potentials. Now let's move on to simple fragmentation mechanisms

so, guys, for the purposes of talking about fragmentation, let's keep working with methane for a second, since it's the one that we used in our intro and recall that the radical cat ion or the molecular ion that wasn't the only peak on the mass spectrum, remember that there were other peaks present. So how do we get those other peaks? Well, guys, that happens, because a lot of times your radical cat iron is going to fragment into more stable ions. Okay, so it's gonna basically fall apart and deconstruct itself, blow up in the sense so that it can become more stable. And even though there's a lot of different ways that, let's say a molecule like methane could break apart, there is one overarching theme, which says that the cat ion fragment usually determines the relative amounts of the fragments found in the sample. Okay, so that means that every radical Catalan or molecular ion can separate into a radical and a cat ion, and to determine which side is going to get the cat ion, you would think about carbo carry instability similar to the car bokhari instability. We've always used inorganic chemistry, so let me show you guys exactly what I mean. Here is our molecular ion that I'm bringing over from our intro video. Right? The same exact molecule. We know that this has an M Z equal to 16. Okay, but it turns out that there's two different ways that this thing can fragment. If it wants thio, it could just stay. Is the radical Catalan which means it hasn't fragmented yet but it could also choose to fragment. Now The way fragmentation will work is that right now there's a radical in between two atoms. But that radical would choose to go toe one or the other. So it would either go to the H or would either go to the carbon. Let's see what happens if the radical goes to the H. Well, you would draw a arrow that has basically a fishhook arrow because only one electron is moving. And what that means is that you would get a h radical because the radical move to the H. And now my carbon is missing an electron, so you'd get a cat ion fragment. Okay, Now notice that not both of these were going to be detected by the mass spectrometer, right? We said what types of ions are detected by mass spectrometry? Onley positive charges. So that means that the M two C ratio that I'm going to see for this molecule is gonna be 15. It's gonna be 15 because we lost the hydrogen. This one does not is not observed in my mass spectrum because not positively charged. Cool. Awesome. But it turns out that there's another alternative mechanism that could have happened, which is that instead of going to the H, what happens if the radical ghosts to the carbon instead totally fair. This could also happen. Well, I'm still going to get a radical and a cat ion separately now because it fragmented. I'm gonna get my radical that has Basically, it's a metal radical and I'm going to get a positively charged H. So that means that now, my m two Z, because it's on Lee detecting the positive charge is going to be equal toe one. Okay, so these are the two possibilities that could happen for a single if a single electron is removed. And if a single arrow is used for the radical in terms of radical mechanism now, do you think that both of these fragments, both the MTZ 15 and the emcee of one are going to be off equal abundance. Do you think it's gonna be like a 50 50 ratio, guys? Not at all. It turns out that one of them is gonna be very, very common. Okay, the emcee of 15 is very, very common, and the other one of h of M Z one is almost not observed it all. So why is that, guys? What has to do with the fact that a positive charge in a carbon is gonna be more stable than a positive charge on a hydrogen? So this has to do with in terms of predicting fragments, We would always predict the fragment that's going to give the more stable carbon cat I all right now, by the way, just to point out our metal carbo Catalans Very stable. Not at all. This is not the most stable carbon cat I ever, but it's better than ah, hydrogen. Okay, you may also recall that some of your professors or some in some of your homework we're supposed to avoid metal car broke a giant and primary carbo cattle because they're not the best, but guys keep in mind this is happening for very short periods of time. This is happening on the level of like nanoseconds or even less so. What that means is that it is possible to get these carbon Acadians because there's so it's such a fast process that by the time it hits the detector, it's already gone. Okay, so just just I know that it doesn't really jive with what we've learned about carve academies before, But keep in mind this all very extreme process that we're putting it under and it's over within a very, very short period of time. So, guys, now that we understand how to kind of predict which of the fragments is gonna be more abundant, let's talk about common splitting fragments. That air scene on different molecules and what I'm going to be doing is I'm gonna be showing you, not the common radicals. I'm sorry. Not the common carp Acadians, but the common radicals because it turns out that remember that we said that if you lose ah, hydrogen, that's called an M minus one. Okay, well, there are other. There are other radicals that air formed that you see very commonly, and these would be nicknames according to the in molecular weight that you're losing. So, for example, if a CH three gets a radical and just chops right off of your molecule, this is what we call an M minus 15 peak. Because that means that whatever your M is, I don't know what your M could be due to whatever size it is. Let's say it's a huge molecule. It's very likely that if there's a methyl group present, you're going to get an M minus 15 m minus 15. Meaning whatever your molecular weight is minus, subtract 15 from that and there's likely gonna be a peak there. Okay, we could say the same thing of O. H. For example, O. H. Remember that we said that it's very easy to take the radical off of the oh, so it's also very common to get Oh, um, minus 17 often. O h is present, By the way, the way we get 17 is that oxygen is 16 and hydrogen is one, all right, and you're going to see that like these other splits that I'm showing, you just have to do with basic arithmetic adding up the atoms and these are very common. So, for example, we talked about a methyl group. What about an ethyl group? And Ethel Group would be minus 29. So the M minus 29 is also very common because of the fact that it's very easy to lose an ethyl group. So, guys, these aren't here for you to memorize as much as just be familiar with. I want you to come away from this video saying, Hey, I have a pretty good idea of what types of fragments, arm or common and which types are less common. Alright. And you can just keep going. We could lose a meth Oxy group of chlorine and the foxy and a broken these air very, very common. These the ones you should just be on the lookout for. Okay, Now there is one that kind of stands out that I just need to point out really quick, which is water. Okay, this h 20 why does it stand out? Because notice that it's the only one that I didn't draw with a radical Did I make a mistake that I forget to put the radical there? No, It turns out guys that due to a very interesting mechanism that can occur, this is not a simple mechanism. There's a more complicated mechanism that involves several arrows, but it turns out that water can be lost all on its own without a radical, and this one would be do would be, um, on minus 18. So it's important for you guys to know that if you if you haven't alcohol present on your compound, it could either be lost as an M minus 17 which means that they always just gets chopped off and that's it. Or the alcohol could be lost as a water because on its way out, it's gonna grab one of the hydrogen and go with it. So just keep in mind you don't have to draw that mechanism right now, but just keep in mind that you would actually see both an M minus 17 and an M minus if alcohol is present. Awesome guys. So I'm just gonna end off with the fragmentation of butane. This is the actual mass spectrum on. I want to show you guys how it relates to these common splitting fragments that we're talking about. So just you guys know the MTZ ratio of my molecular ion should be 58 according to the molecular weight of butane. But guys, look, when we look at our mass spectrum and I'm gonna take myself out of the screen in a second, But I just want to point out how tall is my 58 Tiny? Remember that I told you that your molecular ion isn't always your base peak. In this case, the base peak is actually one of the fragments, so we're gonna look into that. But just keep in mind that your M your molecular ion is tiny because it's very rare to just knock off an electron here. Usually it's gonna fragment because the fragments are more stable. So I'll go ahead and take myself out of the screen now and we'll keep talking. So, guys, notice that what is my base peak? My base peak has a MTZ of 43. How does that make sense? Why do you think my base peak would have an empty sea of 43? We'll do you notice any common splits that could make that totally. Guys, the reason that my base peak is 43 is because that's the peak that forms after I lose one of my method groups. So after I chopped through this molecule and lose a methyl group, what I'm going to get is this cat island, okay? And this couple Catalan, happens to be more likely than the one that forms without it. Fragmenting might say, Johnny, why they're both primary. In that case, I'm gonna bring myself back really quick. It's a Johnny, but they both look primary. So what's the big deal? We'll just think about it. Guys, this is a very high energy process. So all this is saying is that it's very likely that as you pass an electron beam through this molecule that you're gonna break off a method. It's not saying that it's more stable. It's just way more likely that it's just gonna either the right side is gonna fall off or the left side is gonna fall off. It's very difficult to keep this molecule all in one piece. Okay, By the way, this is going to be a common trend where the bigger than molecule is the smaller than molecular ion is gonna be. And the reason is because it's very difficult to keep it together. Okay, If you have a huge, huge molecule and you run an electron beam through it, you're gonna fragment that thing, you're gonna shatter it into pieces, Okay? Whereas if it's a small molecule, it's more likely to stay together. Okay, so I just want to point out how that's a very common, um, fragment. That's the base peak. Now let's look at the next one. I'll take myself out of screen again. So another really common peak, Another really big one. That was 43. Another really big one happens at 29. Why do you think something happens? A. 29? Well, guys, that happens to be the fragment that forms when you lose an ethyl group. So when you lose an ethyl group, now you get a cat eye in. That looks like this again. Is that cat and way more stable? Probably not a lot more stable, but it's just very likely that it forms because it's very difficult to keep this thing all in one piece. When you're running that many electrons through it like railroading it, it's gonna break apart, okay, And guys, there's a whole lot of other peaks here that I'm not going to explain to you. And that's because it's not important. You don't need to know how. The 50 how the 27 form to the 26th. You could just imagine that means that I lost two hydrogen or two metals and a hydrogen. Whatever. The biggest point here is that I want you to figure out, and I want you to be able to understand how toe determine when you would have a common fragment like Oh, this makes sense that I would have a very high peak here because that's losing a metal or that's losing a water or that's losing in ATHOC. See something like that. Cool, awesome, guys. So we're done with splitting fragments. Let's go ahead and do some practice s so that we can solidify everything that we learned on this page.

really quick. I wanna make a clarification before you guys start typing this into the question box. Ah, few of you guys might be saying, Johnny, I get why 43 is a big peak and I get why 29 is a big peak. It has to do with those fragments that we spoke about. But how would you predict ahead of time that 43 is gonna be bigger than 29? And the answers that question is, you wouldn't, um it takes very complicated math and physics. Thio model What happens inside one of these machines? And really, the only way to know the size of these exact peaks is to run it through the mass spectrometer. So the point of this exercise wasn't that I expect you to know that 43 is going to be the biggest. I just want you to know that 43 is going to be big. And also 29 should be Bigas. Well, because both of these have very common fragments that are going to radicals that are gonna come off and then make cat ions of this size. Okay, now, there may be some practice questions in your textbook where it's very obvious that a certain fragment should form. For example, if you conform a tertiary Carvel cat ion, then you know maybe that has a very high chance of being common fragments. Then you should be able to predict that. But in this case, all of these carbon Catalans were of equal, similar, similar stability. So there's no way that you could predict that 43 was gonna be taller than 29 except just to know that both of them are going to be highly present inside of your mass spectrum. Awesome. So let's go ahead and flip the page.