Fragmentation of M

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Fragmentation of M

• Draw structure of M if possible and then draw
  possible heterolytic cleavages (curved arrows) or
  homolytic cleavages (fish hooks)
• Alternatively, show homolytic or heterolytic
  cleavage cleavage of the molecule and then remove
  an electron from one of the fragments.
• Fragmentation to give neutral molecule and a
  radical cation must involve some kind of
  rearrangement.

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General Rules
Probablity of bond cleavage depends on stability
of fragments produced. (3º gt 2º gt 1º gt CH3)
(oxonium gt carbocation) (allylic, benzylic gt no
resonance)

1. R on cycloalkanes a cleave. Cyclohexenes do retro
   Diels-Alder.
2. R on aromatic rings ß-cleave to benzylic
3. Bonds ß to heteroatoms cleave
4. Loss of small stable molecules, H2O, NH3, C2H4,
   HCN, CO, H2S,etc often occur

1. Increase branching ? decrease M
2. Add carbons ? decrease M
3. Cleave more likely at higher º C
4. Unsubst cyclic have strong M (especially
   aromatic)
5. Alkenes cleave to allylic species

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Alkanes

• Straight chain alkanes homologous series of
  ions smooth curve max at 57 or 71. M-15 absent.
  picket fence
• Discontinuity branching. Absence of homolog ?
  no single cleavage possible
• Cycloalkanes. Larger M. More even mass fragments
  (M-C2H4)

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Alkenes

• Cleave next to double bond to form allylic
  fragments. (Double bonds can shift in M.)
• Homologous series is -2 (27, 41, 55, 69..)
• Cyclohexenes do retro Diels Alder give even
  mass fragment (M-C2H4, M-C3H6 etc.)

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