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CHEM 344

CHEM 344. Organic Chemistry Lab. Spectroscopy of Organic Compounds Lecture 3 – Infrared and Mass Spec. September 9 th and 10 th 2008. 1. 1. Infrared radiation excites vibrational energy levels of a molecule Bond stretching, bending, twisting, rocking etc.

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CHEM 344

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  1. CHEM 344 Organic Chemistry Lab Spectroscopy of Organic Compounds Lecture 3 –Infrared and Mass Spec September 9th and 10th 2008 1 1

  2. Infrared radiation excites vibrational energy levels of a molecule Bond stretching, bending, twisting, rocking etc. IR spectroscopy most useful for functional group assignment IR bands reported in wavenumbers (cm-1)

  3. 100 Transmittance (%) N – H O – H C – H (sp > sp2 > sp3) C≡C C≡N C=C C=N C=O Fingerprint region C-C C-O C-X C-N Alkynes Nitriles Alkenes Imines Carbonyls Not used in CHEM 344 0 4000 3000 2000 1600 400 Wavenumber (cm-1) IR correlation tables in lab manual, textbook

  4. 3281 cm-1 N-H sp3 C-H 2900-3000 cm-1 2o amine R2NH

  5. 3291 cm-1 sym N-H 3369 cm-1 asym N-H sp3 C-H 2900-3000 cm-1 1o amine RNH2 Sym and asym N-H bands typical of 1o amine RNH2

  6. Terminal alkyne spC-H 3315 cm-1 2120 cm-1 C≡C sp3C-H

  7. Nitrile 2260 cm-1 C≡N

  8. C=O 1698 cm-1 3639 cm-1 O-H C-O band in fingerprint region

  9. C=O 1685 cm-1 1619 cm-1 C=C C=O and C-C in same region (1700 – 1600 cm-1) – which is which? sp2 sp3C-H C=O usually most intense band in IR spectrum

  10. 2800 - 2600 cm-1 Broad band typical of RCO2H 1716 cm-1

  11. Mass Spectrometry Uses high energy electron beam (70 eV), sample in gas phase Ionization potential for most organic molecules 8-15 eV Gives info on molecular weight of compound by m/z Gives info on connectivity of molecule by fragmentation pattern

  12. [M].+ (also called the parent peak) gives the mass of the molecule MolecularIon Molecule Molecular Fragments Fragments give info on connectivity (i.e. structure) of the molecule

  13. Mass Spectrum of methanol CH3OH 31 Base peak m/z = mass/charge ratio 32 [M]+. In CHEM 344, we 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

  14. Cation Radical A + B Radicals are not detected by MS Molecular Ion Cation radical A + B Radical Cation Cation stability more important than radical stability

  15. Mass Spectrum of methanol CH3OH 31 [CH2OH]+ Base peak 32 [CH3OH]+. [M].+ [CHO]+ 29 [CH3]+ 15

  16. Mass Spectrum of 2,2,4-trimethylpentane 57 3o carbocation 43 2o carbocation No [M].+ detected

  17. Mass Spectrum of 1-butanol 31 56 43 via loss of H2O 29 No [M].+ detected

  18. Mass Spectrum of 2-methyl-2-butanol 59 73 55 43 70

  19. Fragmentation pattern of 2-methyl-2-butanol

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

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

  22. McLafferty rearrangement of 1-octanone γ β α 1-pentene Carbonyl compound must possess gamma H-atoms to undergo McLafferty rearrangement Process always eliminates a neutral alkene, hence McLafferty product is a radical cation McLafferty rearrangement of a carboxylic acid ethene

  23. Mass Spectrum of Benzamide 77 105 [M].+ 121 viaa-cleavage viaa-cleavage 44

  24. Mass Spectrum of Toluene 92 91 Loss of H. Benzyl cation???

  25. Toluene radical cation m/z = 92 Tropylium cation m/z = 91 m/z = 91 common to all alkyl benzenes

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