1. Lecture 3 Mass Spectrometry and Infrared Spectroscopy

2. The Fragmentation Pattern of Alkyl Halides Electron sink Simple heterolytic cleavage Cl is more electronegative than C Cl has a positive formal charge

3. The Fragmentation Pattern of Alkyl Halides  -cleavage ● ●+ ● CH3

4. 63 65 m/z

5. The Fragmentation Pattern of Ethers

6. Simple heterolytic cleavage

7.  -cleavage

8. The Fragmentation of Alcohols

9. Elimination of H 2 O    The elimination undergoes a six-member ring transition state

10. Common Fragmentation Behavior in Alkyl Halides, Ethers, and Alcohols 1. A bond between carbon and a more electronegative atom—heterolytic cleavage 2. A bond between carbon and an atom of similar electronegativity break homolytically, e.g.  -cleavage 3. The bonds most likely to break are those which lead to formation of the most stable cation

11. Stability of Carbocations Tertiary > Secondary > Primary hyperconjugation

12. Carbocations conjugated with  -bonds or lone paired electrons are stable Aromaticity

13. Fragmentation Pattern of Ketones An intense molecular ion peak

14. McLafferty rearrangement may occur A six-member ring transition state ● + ●

15. Molecular Spectroscopy Study of interactions between electromagetic radiation and matter Electreomagnetic Radiation E = h = hc/ Wavenumber = 1/  (cm -1 ) ~ h is called Planck’s constant

16. Infrared radiation excites covalent bond’s vibration E

17. The covalent bonds in molecules are constantly vibrating Each stretching and bending vibration of a bond occurs with a characteristic frequency Molecule absorbs infrared radiation at its characteristic vibrational frequency Stretching Vibration Bending Vibration

18. How an IR Spectrometer Works IR radiation Detector Computer IR Spectrum Sample wavenumber % transmittance

19. The spikes are absorption bands An Infrared Spectrum 4000 – 625 cm -1 2.5 – 16  m

20. The fingerprint region The functional group region 1400

21. It takes more energy to stretch a bond than to bend it

22. The greater the change in dipole moment, the more intense the absorption  = 0 C=C stretch is IR inactive r (C=C) = 1.330 År (C=C) = 1.332 Å 1.34 Debye 1.51 Debye  = 0.17 Debye C=C stretch is IR active