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16 Organic Chemistry Solutions Manual If you also said roughly where these types of carbon atom appeared in the spectrum, you should be pleased as that enhances your answer. Here are the actual numbers for interest only: A (adamantane): 37.8 and 28.4 p.p.m. B (quinone): 187.0 and 136.4 p.p.m. C (naphthalene): 133.4, 127.8, and 125.7 p.p.m. D (EDTA): 173.8, 60.9, and 54.5 p.p.m. E (triethanolamine): 59.5 and 57.1 p.p.m. Problem 11 How would mass spectra help you distinguish these structures? 0 0 Purpose of the problem To show you that fragmentation in the mass spectrum is sometimes useful. Suggested solution The three compounds are isomers, C₅H₁₀O, and have the same molecular ion at 86. Reminder. Only positively charged Fragmentation of ketones usually occurs next to the C=O group to give first an acylium ion and fragments are detected in the mass then, by loss of CO, a carbocation. The fragment ions are different in the three cases. The first spectrum. compound might lose the methyl or propyl side chains and give two series of fragment ions. Two of the fragments happen to have the same mass (43) so there will be peaks at 86, 71, 43, and 15 as well as others. 0 a b a b 0 Pr Me 43 71 86 43 15 The second ketone is symmetrical and we need show only one fragmentation pattern. There will be peaks at 86, 57, and 29 and the two fragments have different masses from those above. CO 0 86 57 29 The third ketone is branched but will give the same basic pattern of fragments as the first compound: 86, 71, 43, 15. How can we tell the difference? The first compound has a long enough chain to do the McLafferty rearrangement losing a molecule (ethylene or ethene) and giving a fragment with an even mass. H So, to summarize, the first ketone shows peaks at 86, 71, 43. and the second at 86, 57, and 29; and the third at 86, 71, 43, and 15 but with no peak