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CHEM 344

CHEM 344. Organic Chemistry Lab. Structural Determination of Organic Compounds Lecture 1 – Mass Spectrometry. January 20 th & 21 st 2009. MS. NMR. IR. Connectivity/Weight. Detailed connectivity. Functional groups. Structure Determination. UV-vis, EPR CD, AA, Raman.

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CHEM 344

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  1. CHEM 344 Organic Chemistry Lab Structural Determination of Organic Compounds Lecture 1 – Mass Spectrometry January 20th & 21st 2009

  2. MS NMR IR Connectivity/Weight Detailed connectivity Functional groups Structure Determination UV-vis, EPR CD, AA, Raman X-ray diffraction

  3. Mass Spectrometry Uses high energy electron beam (70 eV), sample in gas phase Ionization potential for most organic molecules 8-15 eV Like shooting an egg with a shotgun...... and being able to predict the shape of the fragments! Gives info on molecular weight and formula of compound (m/z, isotopes) Gives info on connectivity of molecule (fragmentation pattern)

  4. MolecularIon [M].+ [M].+ gives the mass (m) of the molecule Molecule Molecular Fragments Fragments give info on connectivity (i.e. structure) of the molecule

  5. 31 Base peak m/z = mass/charge ratio [M]+. 32 We will only consider singly charged species (i.e. z =1) The most intense peak is referred to as the base peak 29 All other peaks measured relative to base peak 15

  6. Where is the electron lost from? Alkanes – sigma bond Alkenes – pi bond Heteroatom compounds (O, N, S, etc.) – non-bonding lone pairs

  7. Cation Radical + Radicals and neutral compounds are NOT detected by MS Molecular Ion Cation radical + Radical Cation

  8. Mass Spectrum of Octane 43 57 29 85 71 [M].+ 114

  9. ✗ ✗ Both pathwways involve formation of a methyl radical or a methyl cation Neither are good…..

  10. ✔ ✔ ✔ ✔ ✔

  11. Mass Spectrum of 2-methylpentane 43 71 [M].+ 29 86

  12. ✔ ✔ Branched alkanes fragment at the branch points

  13. Mass Spectrum of 1-hexene 41 43 [M].+ 84

  14. ✔ Resonance-stabilized allyl cation – typical for alkenes

  15. Mass Spectrum of 1-butanol 56 31 43 29 No [M].+ detected

  16. ✔ ✔

  17. Mass Spectrum of 1-butanol 56 31 43 via loss of H2O 29

  18. Mass Spectrum of 2-octanone 58 43 viaa-cleavage viaa-cleavage [M].+ 113

  19. a-cleavage at a C=O group ✔ m/z = 43 Molecular ion m/z = 128 ✔ m/z = 113 Resonance stabilized acylium cation

  20. Mass Spectrum of 2-octanone 58 43 via McLafferty rearrangement viaa-cleavage viaa-cleavage [M].+ 113

  21. McLafferty rearrangement of 2-octanone Carbonyl compound must possess g H-atoms to undergo a McLafferty rearrangement Process always eliminates a neutral alkene, hence McLafferty product is a radical cation

  22. Mass Spectrum of Ethylbenzene 91 106

  23. m/z = 106 Benzylic carbocation Tropylium carbocation (aromatic) m/z = 91

  24. Summary Mass spectrometry – smashing electrons into a molecule Molecular ion – radical cation; breaks up predictably; stability Alkanes - “unzipping” (-14); branch points Alkenes - allyl cation (m/z = 41) Alcohols - eliminate H2O (-18); oxonium cation Carbonyls - alpha cleavage; McLafferty if gamma protons Alkyl benzene - tropylium cation (m/z = 91) Resonance is good Pages 418 - 435 Solomons & Fryhle UW Edition

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