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Recognition of Molecular Ion

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M-3 M-14 peaks indicate not. Nitrogen Rule. Spectrum of an Amine. Spectrum of an Alcohol. M present? Recognition of Molecular Ion ... – PowerPoint PPT presentation

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Title: Recognition of Molecular Ion


1
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

2
Nitrogen Rule
3
Spectrum of an Amine
4
Spectrum of an Alcohol
5
M present?
6
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

7
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
  • Pek at 77 is phenyl group (C6H5)
  • M2 peak 4-5 indicates S
  • Deduce molecular formula!

8
Molecular Formula?
9
Molecular Formula?
10
Molecular Formula?
  • m/z abund
  • 20.2
  • 1.2

11
Index of Hydrogen Deficiency
  • Units of Unsaturation r db
  • CnHmXxNyOz
  • r db n m/2 x/2 y/2 1

12
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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14
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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