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 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? What types of collision cells etc. involving the formation of acyclic $\ce{C6H5+}$ is proposed. Do I need HDMI-to-VGA or VGA-to-HDMI adapter? Clutch Prep is not sponsored or endorsed by any college or university. Let's look at the mass spectrum of 2-methylbutane. Did I forget to put the radical there? So, they're more difficult to knock off and some of them are going to be held very loosely so that means 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 cation that's formed or what's going to is a major radical cations that form after ionization has taken place. c) Fragmentation of M+ is not favored. Practice: Draw the most likely ion fragment of the molecule. 28                  Well do you notice any common splits that could make that? 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? Following are examples of compounds listed by functional group, which demonstrate patterns which can be seen in mass spectra of compounds ionized by electron impact ionization. Now, you might be wondering, Johnny does it matter that it's a ring? What is the source of the phenyl radicals? Unbranched side chains result in a more prevalent peak at m/z 92 than do branched groups. Can Mathematica be used to edit MP3 tags? So, basically we're going to discuss the step of ionization and everything that follows after that. The peak observed in most aromatic compounds at m/z 65 results from the elimination of an acetylene molecule from the tropylium ion. It also isn't clear that observed the "phenyl" radical cation is actually cyclic! So I'll go ahead and take myself out of the screen now and we'll keep talking so guys noticed that what is my base peak? So, you want to 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. Benzene rings with highly branched substituted groups produce fragments larger than m/z 91 by intervals of 14 units. (Face sets). So, then you should be able to predict that, but in this case all of these carbocations were of equal, similar stability, so there's no way that you could predict that 43 was going to be taller than 29 except just to know that both of them are going to be highly present inside of your mass spectrum. 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 mean 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-1, OK? The effects of methyl n-alkyl ketones and n-alkylbenzenes on hepatic cytochrome P450s in vivo and in vitro were investigated. How to decline a postdoc offer a few days after accepting it? It only takes a minute to sign up. Alkyl substituted benzene rings result in a prominent peak at m/z 91 (Figure 2.12). Totally guys, the reason that my base peak is 43 is because that's the peak that forms after I lose one of methyl groups so after I chop through this molecule and lose a methyl group what I'm going to get is this cation, OK? The MZ of 15 is very very common and the other one of the H of MZ 1 is almost not observed at all so why is that guys? It could be another isomer of $\ce{C6H5^{+.}}$. So, in terms of ionization, we wouldn't ionize the actual bonds of the ring, we would ionize maybe one of the hydrogen's or something that's attached to the benzene ring, for similar reasons double bonds come next. 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's going to break apart, OK? By the way, this is going to be a common trend where the bigger the molecule is the smaller than molecular ion it's going to be and the reason this could be very difficult to keep it together, OK? So, guys here are peas made a very simple trend that I'd look 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 ionize that lone pair, okay? Practice: What would be the value of the base peak (m/z)? e) none of the above. Let's look at the mass spectrum of 2-methylbutane. Well guys that happens because a lot of times your radical cation is going to fragment into more stable ions, OK? 2-methylbutane is an isomer of pentane - isomers are molecules with the same molecular formula, but a different spatial arrangement of the atoms. Look first at the very strong peak at m/z = 43. Only positive charges so that means that the M to Z 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 is 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? We said what types of Ions are detected by mass spectrometry? Side chains with more than two carbon atoms create a peak at m/z 92 (Figure 6.12). 18 - Reactions of Aromatics: EAS and Beyond, Ch. Substituted benzene rings also first undergo α cleavage followed by hydrogen rearrangement producing a grouping of peaks at m/z 77 from C 6 H 5 +, m/z 78 from C 6 H 6 +, and m/z 79 from C 6 H 7 +. NIST / TRC Web Thermo Tables, professional edition (thermophysical and thermochemical data) My base peak has a M to Z of 43, how does that make sense? The radical cation that is produced when an electron is knocked out of a neutral closed-shell molecule in EIMS initially possesses a lot of energy. Now do you think that both of these fragments both the M to 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? By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. Lighting network what is the difference between wallet balance, local balance and remote balance? Awesome guy 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 58 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 want to point out how tall is my 58? A several billion dollars project to stop people from sneezing, besides full hazmat suits? With benzilic acid as the ion source, it is possible that the small peak contains an oxygen atom, although searching for possible formulas with appropriate "exact" masses makes me think this isn't true (see below). Two peaks at m/z values 43 and 57 will appear in the mass spectrum. site design / logo © 2020 Stack Exchange Inc; user contributions licensed under cc by-sa. 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 to ionize with every step, so the next one would actually be aromatics, okay? rev 2020.11.12.37996, The best answers are voted up and rise to the top, Chemistry Stack Exchange works best with JavaScript enabled, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site, Learn more about Stack Overflow the company, Learn more about hiring developers or posting ads with us. Teacher asking my 5 year old daughter to take a boy student to toilet. Which is the case when a molecular ion in the mass spectrum is also the base peak? Batch conversion of Waters mass spectrometry file formats, Exceptions to the Nitrogen Rule in mass spectrometry, Heats of formation of neutral molecules and homolytic vs heterolytic bond dissociation in mass spectrometry. Can we recycle garbage with the principles of mass spectrometry? Use MathJax to format equations. Figure 6.12. The presence of an aromatic ring in a compound results in a prominent molecular ion. The largest of these peaks will result in a highly substituted cation and a large radical, like a simpler branched alkane. 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 an alkane. Awesome guys so we're done with splitting fragments let's go ahead and do some practice so that we can solidify everything that we learn on this page. So, aromatics, these are the general category of benzene and benzene like molecules, okay? Video explaining Mass Spect: Fragmentation for Organic Chemistry. 15 - Analytical Techniques: IR, NMR, Mass Spect, Ch. The mass spectrum of ethyl methanoate is shown below. Fragmentation of an Aromatic.Spectra from the NIST/EPA/NIH Mass Spectral Library. How to break the cycle of taking on more debt to pay the rates for debt I already have? That's definitely a plausible pathway, thanks ! 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 an 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 going to be held very tightly by molecules. You may also recall that some of your professors are still 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 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 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, OK? How can a chess game with clock take 5 hours? b) Fragmentation of M+ is favored. 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, OK? Perhaps a tetrahedrane-type structure is even possible? Concept #3: Clarification on the Base Peak. 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, OK? The mass spectrum of toluene (methyl benzene) is shown below. Fragmentation and Interpretation of Spectra 2.1 Introduction Before discussing fragmentation and interpretation, it is important to understand the many ways mass spectra are utilized. The same exact molecule we know this has an MZ equal to 16, OK? Why do you think my base peak would have an M to Z of 43?


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