1. 11 CHEM 344 Organic Chemistry Lab September 9 th and 10 th 2008 Spectroscopy of Organic Compounds Lecture 3 –Infrared and Mass Spec

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. 4000 3000 2000 1600 400 Wavenumber (cm -1 ) N – H O – H C – H (sp > sp 2 > sp 3 ) C≡CC≡NC≡CC≡N C=C C=N C=O Fingerprint region Transmittance ( %) 100 0 Alkynes Nitriles Alkenes Imines Carbonyls C-C C-O C-X C-N IR correlation tables in lab manual, textbook Not used in CHEM 344

4. N-H 3281 cm -1 2900-3000 cm -1 sp 3 C-H 2 o amine R 2 NH

5. asym N-H 3369 cm -1 3291 cm -1 sym N-H 2900-3000 cm -1 sp 3 C-H 1 o amine RNH 2 Sym and asym N-H bands typical of 1 o amine RNH 2

6. C≡CC≡C 2120 cm -1 sp 3 C-H sp C-H 3315 cm -1 Terminal alkyne

7. 2260 cm -1 Nitrile C≡NC≡N

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

9. 1685 cm -1 C=O 1619 cm -1 C=C sp 2 sp 3 C-H C=O usually most intense band in IR spectrum C=O and C-C in same region (1700 – 1600 cm -1 ) – which is which?

10. 1716 cm -1 2800 - 2600 cm -1 Broad band typical of RCO 2 H

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

13. Mass Spectrum of methanol CH 3 OH 32 31 29 15 Base peak [M] +. m/z = mass/charge ratio In CHEM 344, we only consider singly charged species (i.e. z=1) The most intense peak is referred to as the base peak All other peaks measured relative to base peak

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

15. Mass Spectrum of methanol CH 3 OH 32 31 29 15 [CH 2 OH] + Base peak [CHO] + [CH 3 ] + [CH 3 OH] +. [M].+

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

17. Mass Spectrum of 1-butanol 5631 43 29 via loss of H 2 O No [M].+ detected

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

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

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

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

22. McLafferty rearrangement of 1-octanone 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 α β γ McLafferty rearrangement of a carboxylic acid ethene

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

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

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