1 / 25

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.

topaz
Télécharger la présentation

CHEM 344

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related