Cleavage of Acetone Molecular Ion

The El mass spectrum of acetone is rather simple it shows three inportant peaks at m/z 58, 43, and 15. According to the formula CsHeO, the peak at m/z 58 corresponds to the molecular ion. The base peak at m/z 43 is related to this signal by a difference of 15 u, a neutral loss that can almost certainly be assigned to loss of a methyl radical, CHs . The peak at m/z 15 may then be expected to correspond to the ionic counterpart of the methyl radical, i.e., to the CHa carbenium ion (Fig. 6.3). In order to rationalize this mass spectrum in terms of ion chemistry, we consider the steps of electron ionization and subsequent fragmentation in detail. 

The acetone molecule has two lone pairs at the oxygen that will at least formally be the preferred ones to remove an electron (Chap. 2.2.2). The excited molecular ion may then cleave off a methyl radical by simply shifting one electron (single-barbed arrow or fishhook ) from the CO-CH3 bond to the radical site at 

Note The term a-cleavage for this widespread radical-site initiated process with charge retention can be misleading, because the bond cleaved is not directly attached to the radical site, but to the next neighboring atom. 

Including the ionization process, the lone pairs, and single-barbed arrows for each moving electron in the scheme is not necessary but preseaits a valuable aid. Alternatively, the a-cleavage may be indicated in a simplified manner  

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