![]() Other functional groups have predictable fragmentation patterns as well. Often the base peak is from an ‘oxonium’ ion. When alcohols are subjected to electron ionization MS, the molecular ion is highly unstable and thus a parent peak is often not detected. The base peak in this spectrum is again an acylium ion. The mass spectrum of 2-hexanone shows a 'McLafferty fragment' at m/z = 58, while the propene fragment is not observed because it is a neutral species (remember, only cationic fragments are observed in MS). This, as you might expect, is the result of formation of a methyl cation, in addition to an acyl radical (which is neutral and not detected).Ī common fragmentation pattern for larger carbonyl compounds is called the McLafferty rearrangement: Alpha cleavage results in the formation of an acylium ion (which accounts for the base peak at m/z = 43) and a methyl radical, which is neutral and therefore not detected.Īfter the parent peak and the base peak, the next largest peak, at a relative abundance of 23%, is at m/z = 15. This is the result of cleavage at the ‘alpha’ position - in other words, at the carbon-carbon bond adjacent to the carbonyl. We saw, for example, that the base peak in the mass spectrum of acetone is m/z = 43. Detailed analysis of the typical fragmentation patterns of different functional groups is beyond the scope of this text, but it is worthwhile to see a few representative examples, even if we don’t attempt to understand the exact process by which the fragmentation occurs. ![]() Much of the utility in electron-ionization MS comes from the fact that the radical cations generated in the electron-bombardment process tend to fragment in predictable ways.
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