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

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

4. 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 and formula of compound (m/z, isotopes) Gives info on connectivity of molecule (fragmentation pattern) Like shooting an egg with a shotgun...... and being able to predict the shape of the fragments!

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

6. 32 31 29 15 Base peak [M] +. m/z = mass/charge ratio We will 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

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

8. Molecular Ion Cation radical CationRadical CationRadical Radicals and neutral compounds are NOT detected by MS + +

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

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

11. ✔ ✔ ✔✔ ✔

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

13. Branched alkanes fragment at the branch points ✔ ✔

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

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

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

17. ✔ ✔

18. Mass Spectrum of 1-butanol 56 31 43 29 via loss of H 2 O

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

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

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

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

23. Mass Spectrum of Ethylbenzene 91 106

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

25. 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 H 2 O (-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