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Lecture 21

Lecture 21. Lecture Problem 7 Due Today Mass Spectrometry Continued UV/Vis Spectroscopy This week in lab: Synthetic Experiment #2 PreLab Due Ch 7: TLC Final Report Due Next week in lab: Ch 8 Final Report Due. Lab Guide Problems. Problem 11.32

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Lecture 21

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  1. Lecture 21 • Lecture Problem 7 Due Today • Mass Spectrometry Continued • UV/Vis Spectroscopy • This week in lab: • Synthetic Experiment #2 PreLab Due • Ch 7: TLC Final Report Due • Next week in lab: • Ch 8 Final Report Due

  2. Lab Guide Problems Problem 11.32 For each of the following compounds, indicate the m/z value expected for the molecular ion. Also, indicate any expected major peaks due to fragmentation. Ethanol Ethylbenzene

  3. Lab Guide Problems Problem 11.37 The most intense peak in the mass spectrum of 2,2-dimethylbutane is at m/z = 57. Propose a structure of this fragment ion (a.k.a. daughter ion).

  4. Additional Fragmentations in MS Loss of Ethylene (Ethene) & CO:

  5. Additional Fragmentations in MS Alpha-cleavage: Amines & Alcohols

  6. UV/Vis Spectroscopy Basic Idea: Expose compound to UV/Vis radiation UV: 200-400 nm Vis: 400-800 nm 2. Absorption of radiation causes electronic excitations at specific wavelengths (lmax) Main Use: Detects the presence of pi systems in a compound. Unlike NMR, IR, and MS, UV/Vis is NOT used to determine pieces (specific bonds) of a molecule.

  7. Electronic Transitions Types of Electronic Transitions: The length of the arrow is roughly proportional to the energy difference between two levels. The longer arrow means a higher Energy transition. Higher energy means lower wavelength of radiation.

  8. UV/Vis Spectroscopy Transitions observed by UV/Vis: p to p* (strong) n to p*(weak, often not observed) n to p* transition is lower energy (higher wavelength) p to p* transition is higher energy (lower wavelength)

  9. UV/Vis of Acetone p --> p* Shorter wavelength, stronger absorption n --> p* Longer wavelength, weaker absorption

  10. The more conjugation, the lower the energy, the higher the Wavelength (lmax value). p to p* transitions

  11. Quantitative Data from UV/Vis Extinction Coefficients or Molar Absorptivity (): A numerical value of the intensity of the absorptions. Calculate  by using the Beer-Lambert Law: A (Absorbance) =  b c Where b = path length of cell (usually 1.0 cm) c = concentration of the solute When reporting data, chemists only report lmax’s and ‘s from UV/Vis data.

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