Www2.Chemistry.msu.Edu Faculty Reusch VirtTxtJml Spectrp

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

These three examples are hydrocarbons having no functional groups. --------------------------------------------------------------------------- Hexane shows the same fragmentation pattern as other unbranched alkanes. Thus, alkyl carbocations at m/z=15, 29, 43 and 57 Da provide the dominant peaks in the spectrum. The m/z=57 butyl cation (M-29) is the base peak, and the m/z=43 and 29 ions are also abundant.

Chain branching clearly influences the fragmentation of this isomeric hexane. The molecular ion at m/z=86 is weaker than that for hexane itself and the M-15 ion at m/z=71 is stronger. The m/z=57 ion is almost absent (try to find a simple cleavage that gives a butyl group). An isopropyl cation (m/z=43) is very strong, and the corresponding propene radical-cation at m/z=42 (colored orange), produced by loss of propane, gives the base peak.

By having the six carbons of hexane closed to a ring, the fragmentation is profoundly changed. To begin with, the molecular ion at m/z=84 is much stronger than the corresponding ions in the previous acyclic compounds. The base peak at m/z=56 is produced by loss of ethene, so it is an odd-electron ion (colored orange). The alkenyl cations at m/z=41 & 27 are stronger than the corresponding alkyl cations (m/z=43 & 29). The loss of methyl (m/z=69), and a corresponding small m/z=15 ion obviously require some hydrogen rearrangements.

Toggle Examples

Return

Page 1 of 4Mass Spectrometry

3/19/2012http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/MassSpec/mspec....

Common Fragment Ions and Neutral Fragments

Common Small Ionsm/z composition

15 Da CH3

17 OH

18 H2O

19 H3O, F

26 C2H2, CN

27 C2H3

28 C2H4, CO, H2CN

29 C2H5, CHO

30 CH2NH2

31 CH3O

33 SH, CH2F

34 H2S

35(37) Cl

36(38) HCl

39 C3H3

41 C3H5, C2H3N

42 C3H6, C2H2O, C2H4N

43 C3H7, CH3CO

44 C2H4O

46 NO2

56 C4H8

57 C4H9

60 CH4CO2

79(81) Br

80(82) HBr

91 C7H7

Common Neutral Fragmentsmass loss composition 1 Da H

15 CH3

17 OH

18 H2O

19 F

20 HF

27 C2H3, HCN

28 C2H4, CO

30 CH2O

31 CH3O

32 CH4O, S

33 CH3 + H2O, HS

33 H2S

35(37) Cl

36(38) HCl

42 C3H6, C2H2O, C2H4N

43 C3H7, CH3CO

44 CO2O, CONH2

45 C2H5O

55 C4H7

57 C4H9

59 C2H3O2

60 C2H4O2

64 SO2

79(81) Br

80(82) HBr



End of this supplementary topic

Rearangement Mechanisms in Fragmentation

127 I

128 HI

127 I

128 HI

Return

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).

Return



Documents

Www2.Chemistry.msu.Edu Faculty Reusch VirtTxtJml Spectrp