EI-MassSpectra of Assorted Organic Compounds

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 •+.



End of this supplementary topic

Common Fragment Ions and Neutral Fragments

Common Small Ions
m/z composition
  15 Da CH3
  17OH
  18H2O
  19H3O, F
  26C2H2, CN
  27C2H3
  28C2H4, CO, H2CN
  29C2H5, CHO
  30CH2NH2
  31CH3O
  33SH, CH2F
  34H2S
  35(37)Cl
  36(38)HCl
  39C3H3
  41C3H5, C2H3N
  42C3H6, C2H2O, C2H4N
  43C3H7, CH3CO
  44C2H4O
  46NO2
  56C4H8
  57C4H9
  60CH4CO2
  79(81)Br
  80(82)HBr
  91C7H7
  127I
  128HI
Common Neutral Fragments
mass losscomposition
  1 Da H
  15CH3
  17OH
  18H2O
  19F
  20HF
  27C2H3, HCN
  28C2H4, CO
  30CH2O
  31CH3O
  32CH4O, S
  33CH3 + H2O, HS
  33H2S
  35(37)Cl
  36(38)HCl
  42C3H6, C2H2O, C2H4N
  43C3H7, CH3CO
  44CO2O, CONH2
  45C2H5O
  55C4H7
  57C4H9
  59C2H3O2
  60C2H4O2
  64SO2
  79(81)Br
  80(82)HBr
  127I
  128HI


End of this supplementary topic

Rearangement Mechanisms in Fragmentation

4-nonanone

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.

butylpentanoate

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(+).

5-methyl-5-hexen-3-ol


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.

4,4-dimethylcyclohexene


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).