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 are 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 in organic 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 m/z equal to 16. But it turns out that there are two 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 two atoms, but that radical would choose to go to one 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/z 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 m/z 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 m/z 15 and the m/z of 1 are going to be of equal abundance? Do you think it's going to be like a 50/50 ratio? Guys, not at all. It turns out that one of them is going to be very, very common. The m/z 15 is very common, and the other one of m/z 1 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 choose 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 nanoseconds or even less. So what that means is that it is possible to get these carbocations because they are such a fast process that by the time it hits the detector, it's already gone. So, 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 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/z -1. 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/z -15 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/z -15. m/z 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 O, for example. O, 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/z -17 if an O 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 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 m/z 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 woul