In my Organic Chemistry II class, we were asked to blog on our muddiest or most unclear point thus far in the semester. One thing that has been discussed so far that I do not understand, is the mass spectrum of cyclic alkanes. It was explained to me in class that, when talking about methylcyclopentane, the fragment with a m/z=56 is more stable and therefore more concentrated than the m/z=69 fragment. I do not understand what makes this a more stable or more favorable fragmentation. After looking into this subject, I was not able to find anything that explained it more clearly. The most informative source that I found on this subject further confused me because, from how I read it, it goes against what I was taught in lecture. The source talks about the fragmentation of cyclic alkanes, methylcyclohexane specifically, and says "Note that the loss of the methyl side chain is perhaps the most important fragmentation event, and the M - 15 ion of m/z 83 gives the most intense signal in the spectrum." To my followers, if you can clarify my issue or provide information that you think might be of help, it would be greatly appreciated. OK, no really, someone help!
-The High School Chemist
Good morning!
ReplyDeleteI see where you might have found this apparent contradiction for methylcyclopentane (the molecular example discussed in class): http://www.massbank.jp/jsp/Dispatcher.jsp?type=disp&id=JP012175&site=10
One thing to keep in mind is that different ionization methods can lead to subtle differences in the intensity of the peaks observed. In the example linked above, the m/z of 83 is actually 1 unit less than the molecular mass and is likely due to the loss of an H atom from the molecular ion. The peak observed at m/z =85 is due to this method being chemical ionization (CI-MS), which means a weak acid is used to create a "stable" ion.
Dear Mr. High School Chemist,
ReplyDeleteI sincerely wish I could help you, however, you are far above my head! : ) I do see your point, though. That's pretty confusing. The only thing I understand about fragmentation, is that the most stable carbocation will have the tallest peak. Sorry I couldn't be of any help. Good job on the blog, though, and I'm sure you'll figure it out.
-Just as confused as you ; )
High School Chemist...you have a lot of time on your hand to find something that few in your class would be able to help you with other than your lecturer. However, you create a good point and I too find this point somewhat muddy. While your blog is great and understandable, I cant really help with the topic. But at the same time i think your blog could have one thing added to it; the source.If you could just add the source, either the actual website or the book that was used to find this information, it would help the reader with following your post and mayeb even help where you became lost.
ReplyDeleteThe Source -http://www.chemistry.ccsu.edu/glagovich/teaching/316/ms/alkanes-cyclic.html
ReplyDeleteWell High School Chemist I think you get the award for the most difficult question so far. I'm in the same boat as everyone else in the class so sorry can't help you with an answer. Since I'm not sure which fragment is the most stable with this compound, I can't help you with the base peak.
ReplyDelete