15. Analytical Techniques:IR, NMR, Mass Spect
Mass Spect:Fragmentation
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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. Recall that the radical cation 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 cation is going to fragment into more stable ions. So it's going to basically fall apart and deconstruct itself, blow up in a sense, so that it can become more stable. 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 cation fragment usually determines the relative amounts of the fragments found in the sample. So that means that every radical cation or molecular ion can separate into a radical and a cation. To determine which side is going to get the cation, you would think about carbocation stability similar to the carbocation stability 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. The same exact molecule. We know that this has an mz equal to 16. But it turns out that there's 2 different ways that thing can fragment if it wants to. It could just stay as the radical cation, 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 2 atoms, but that radical would choose to go to 1 or the other. So it would either go to the H or it would either go to the carbon. Let's see what happens if the radical goes to the H. Well, you would draw an arrow that has basically a fishhook arrow because only one electron is moving and what that means is that you would get an H radical because the radical moved to the H and now my carbon is missing an electron, so you would get a cation fragment. Now notice that not both of these are going to be detected by the mass spectrometer. We said what types of ions are detected by mass spectrometry? Only positive charges. So that means that the m to c ratio that I'm going to see for this molecule is going to be 15. It's going to be 15 because we lost a hydrogen. This one is not observed in my mass spectrum because it's 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 goes to the carbon instead? Totally fair. This could also happen. Well, I'm still going to get a radical and a cation separately now because it fragments it. I'm going to get my radical that has basically it's a methyl radical and I'm going to get a positively charged H. So that means that now my because it's only detecting the positive charge is going to be equal to 1. 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 the radical mechanism. Now do you think that both of these fragments, both the mz15 and the Mz of 1 are going to be of equal abundance? Do you think it's going to be like a 5050 ratio? Guys, not at all. It turns out that one of them is going to be very, very common. The mz15 is very, very common and the other one of mz1 is almost not observed at all. So why is that guys? Well, it has to do with the fact that a positive charge on a carbon is going to be more stable than a positive charge on a hydrogen. So this has to do with in terms of predicting fragments, we would always the fragment that's going to give the more stable carbocation. Now by the way, just to point out, are methyl carbocations very stable? Not at all. This is not the most stable carbocation ever, but it's better than a hydrogen. You may also recall that some of your professors or in some of your homework were supposed to avoid methyl carbocations and primary carbocations because they're not the best. But guys, keep in mind that 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 carbocations because they're so it's such a fast process that by the time it hits the detector, it's already gone. So just I know that it doesn't really jive with what we've learned about carbocations before, but keep in mind, this is all a 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 going to be more abundant, let's talk about common splitting fragments that are seen on different molecules. And what I'm going to be doing is I'm going to be showing you not the common radicals I'm sorry, not the common carbocations, but the common radicals because it turns out that remember that we said that if you lose a hydrogen, that's called an M-one. Well, there are other radicals that are formed that you see very commonly and these would be nicknamed according to the molecular weight that you're losing. So for example, if a CH3 gets a radical and just chops right off of your molecule, this is what we call an M-fifteen 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-fifteen. M-fifteen 15 meaning whatever your molecular weight is, subtract 15 from that and there's likely going to be a peak there. We could say the same thing of Oh, for example. Oh, remember that we said that it's very easy to take the radical off of the O, so it's also very common to get an M-seventeen if an Oh is present. By the way, the way we get 17 is that oxygen is 16 and hydrogen is 1. 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? An ethyl group would be -29, so the n-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 are more common and which types are less common. And you can just keep going. We could lose a methoxy group, a chlorine, an ethoxy and a bromine. These are very, very common. These are the ones that you should just be on the lookout for. Now there is one that kind of stands out that I just need to point out really quick, which is water. This H2O, 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? Did 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. This is 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 N-18. So it's important for you guys to know that if you have an alcohol present on your compound, it could either be lost as an M-seventeen, which means that the Oh just gets chopped off and that's it Or the alcohol could be lost as a water because on its way out, it's going to grab one of the hydrogens 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-17 and an M-18 if an alcohol is present. Awesome guys. So I'm just going to end off with the fragmentation of butane. This is the actual mass spectrum and I want to show you guys how it relates to these common splitting fragments that we're talking about. So just so you guys know, the M to Z ratio of my molecular ion should be 50 8 according to the molecular weight of butane. But guys, look, when we look at our mass spectrum and I'm going to 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 going to 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 going to 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 of 40 3. How does that make sense? Why do you think my base peak would have an of 43? Well, 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 methyl groups. So after I chop through this molecule and lose a methyl group, what I'm going to get is this cation. And this carbocation happens to be more likely than the one that forms without fragmenting. You might say, Johnny, why? They're both primary. In that case, I'm going to bring myself back really quick. You might say, Johnny, but they both look primary, so what's the big deal? Well, 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 going to break off a methyl. It's not saying that it's more stable, it's just way more likely that it's just going to either the right side is going to fall off or the left side is going to fall off. It's very difficult to keep this molecule all in one piece. By the way, this is going to be a common trend where the bigger the molecule is, the smaller the molecular ion is going to be. And the reason is because it's very difficult to keep it together. If you have a huge, huge molecule and you run an electron beam through it, you're going to fragment that thing. You're going to shatter it into pieces. Whereas, if it's a small molecule, it's more likely to stay together. So I just want to point out how that's a very common fragment. That's the base peak. Now let's look at the next one. I'll take myself out of the 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 at 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 cation that looks like this. Again, is that cation 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 going to break apart. 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 27 formed or the 26. You can just imagine that means that I lost 2 hydrogens or 2 methyls 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 to 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 methyl or that's losing a water or that's losing an ethoxy, something like that. Cool? Awesome guys. So we're done with splitting fragments.
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