The mass apectra of three different saturated hydrocarbons are displayed below. Two are isomeric hexanes and the third is cyclohexane. Comments regarding the fragmentation patterns are presented in the box to the right of each spectrum. Ions are sometimes characterized by loss of a specific neutral fragment from the molecular ion. For example, a M-15 ion is identified as loss of a methyl group. Odd-electron ions, including the molecular ion, are colored orange when marked. Even-electron ions are colored magenta. The "Toggle Examples" button at the bottom will display a different set of spectra in which the influence of a particular functional group may be examined. Repeated clicking of this button will cycle through fifteeen spectra. In each example the molecular ion is designated by M •+.
The odd-electron fragment ions at m/z = 86 and 58 are the result of a McLafferty rearrangement, involving the larger alkyl chain, and a subsequent loss of ethene (the "double-McLafferty" rearrangement).Alpha-cleavage leads to the m/z = 99, 71 and 43 ions. The charge is apparently distributed over both fragments.
Alpha-cleavage gives ions at m/z=57 & 85 Da. The McLafferty rearrangement on the acid side generates a m/z=116 ion. Subsequent rearrangement on the alcohol side generates m/z=60 and 56 ions. The m/z=103 ion is probably C4H9CO2H2(+).
The molecular ion (m/z=114 Da) is not observed under electron impact ionization conditions. The highest mass ion (m/z=85) is due to an alpha-cleavage of ethyl; the other alpha-cleavage generates m/z=59. The rearrangement cleavage shown here generates the m/z=56 ion.
The loss of a methyl radical generates the base peak at m/z=95 Da. The m/z=81 & 67 ions are smaller homologues of this ion (14 mass units less). Cyclohexene compounds undergo a retro-Diel-Alder rearrangement to give diene and alkene fragments. The charge may reside on either fragment, with the larger usually predominating. In this case both ions are relatively strong (m/z=54 & 56).