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Textbook and Chapter: Carey and Giuliano 8th Ed. (2010), Chapter 13
Keywords: alpha cleavage, heterolytic cleavage, homolytic cleavage, mass spec
Description: Goes over alpha-cleavage and the McLafferty rearrangement in mass spectrometry. (Alpha cleavage is more important to know than the McLafferty rearrangement)
Total Problems: 3
The mass spec of 4-nonanone shows peaks at m/z = 58, 71, 86, 99.
Using curved arrows or hooks, show how each of these fragments can form via alpha cleavage or the McLafferty rearrangement. (and draw the structure of the indicated species in the appropriate box).
Note that I drew the McLafferty rearrangements using arrows (2 electrons moving at once). Some textbooks use hooks instead, but the results are the same.
In most ungraduate organic chemistry courses, being able to draw an alpha cleavage is much more important than a being able to draw a McLafferty rearrangement (which tends to only show up on bonus problems).
The mass spec of methyl ethyl ether shows peaks at m/z = 45 and 59.
Use hooks to show the alpha cleavages that result in these two fragments.
MS ionization will knock off an electron from the heteroatom (atom that's not C or H), in this case, the oxygen, leaving behind a positively charge compound. This is the molecular ion (M+).
Oxygen usually undergoes homolytic cleavage- the bond splits and each atom gets one electron. Since only one electorn is involed, we use hooks instead of the usual curved arrows.
If your professor is into mass spec cleavage mechanisms, I suggest you practice this mechanism, called an alpha cleavage (see image below).
