Recognition of Molecular Ion

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Recognition of Molecular Ion

• Can use CI to verify (usually not convenient)
• Nitrogen rule
• No N present M is even
• N present M is odd
• M-15 present? M-18? M-31? M-29?
• M-3 ? M-14 peaks indicate not

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Nitrogen Rule
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Spectrum of an Amine
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Spectrum of an Alcohol
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M present?
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Recognition of Molecular Ion

• Can use CI to verify (usually not convenient)
• Nitrogen rule
• No N present M is even
• N present M is odd
• M-15 present? M-18? M-31? M-29?
• M-3 ? M-14 peaks indicate not

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Deducing molecular formula

• For small molecules dont need HRMS
• Use m1 to get of Cs
• Use Nitrogen rule
• M-18 peak indicates oxygen
• Doublet M peaks indicate Cl (31) and Br (11)
• Peak at 127 I
• Peak at 77 is phenyl group (C6H5)
• M2 peak 4-5 indicates S
• Deduce molecular formula!

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Molecular Formula?
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Molecular Formula?
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Molecular Formula?

• m/z abund
• 20.2
• 1.2

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Index of Hydrogen Deficiency

• Units of Unsaturation r db
• CnHmXxNyOz
• r db n m/2 x/2 y/2 1

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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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