Chapter 11 Structure Determination of Organic Compounds
Non-Instrumental methods <ul><li>Molecular formula=n(empirical formula) </li></ul><ul><li>Finding the Hydrogen Deficiency ...
Qualitative analyses  <ul><li>Chemical tests can be used to indicate the presence of functional groups </li></ul><ul><ul><...
GC-MS GC El mass spectrometer Data Acquisition and Analysis Computer <ul><ul><li>The HP 5972 is an electron ionization mas...
Mass Spectrometry Sample eluting from GC Pressure changes to ~10- 5  torr (1 ng/M sensitivity) Electron Impact (EI ) sourc...
Mass Spectrometry <ul><li>Mass spectroscopy is a very powerful technique that can detect upto a nano gram (even femtograms...
Interpreting Mass Spectra <ul><li>M + e -  -> M .+  + 2e - </li></ul><ul><li>The strongest peak is set to 100 on an intens...
IR spectrometer for solids Sample Laser Path Mirrors
FT-IR Spectrometer-For liquids <ul><li>Liquid samples can be applied neat (undiluted) onto the NaCl salt plates </li></ul>...
FT-IR  NaCl Salt Plate holder Laser Beam
Interpreting FT-ir <ul><li>When molecules absorb energy from an energy source (sun, electromagnetic radiation etc.) the bo...
IR spectral absorptions <ul><li>Hydrogen bonding broadens absorptions  </li></ul><ul><ul><li>COOH stretch broad band 2800-...
UV- Vis Cuvette Clear side  should face  beam Sample  should be  dilute (~10 -5  M)
UV-Visible spectroscopy <ul><li>Very dilute samples ~10  -5  mole/L can be analyzed </li></ul><ul><li>A=  cl  Absorbance=...
Nuclear magnetic resonance The sample is placed  between powerful magnets and a magnetic field sweeps  through the sample ...
NMR <ul><li>NMR is a very useful technique for organic compounds.  </li></ul><ul><li>When a magnetic pulse is applied the ...
NMR Spectra <ul><li>The protons that are next to electronegative atoms are deshielded (electron density around the proton ...
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Spectroscopy Org12

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Spectroscopy Org12

  1. 1. Chapter 11 Structure Determination of Organic Compounds
  2. 2. Non-Instrumental methods <ul><li>Molecular formula=n(empirical formula) </li></ul><ul><li>Finding the Hydrogen Deficiency index </li></ul><ul><ul><li>Non-cyclic alkanes are saturated C n H 2n+2 </li></ul></ul><ul><ul><li>2 valence electrons are needed to form a ring/cyclic compound </li></ul></ul><ul><ul><li>alkenes have 2 hydrogens less/double bond (unsaturation) </li></ul></ul><ul><ul><li>C n H 2n would indicate the presence of a double bond (carbonyl, alkene) OR ring (not both) </li></ul></ul><ul><ul><li>For each 2 Hydrogens less a ring or double bond is possible </li></ul></ul>Chapter 15 G. Sathianathan, PSU
  3. 3. Qualitative analyses <ul><li>Chemical tests can be used to indicate the presence of functional groups </li></ul><ul><ul><li>Alkenes: decolorization of Br 2 / CCl 4 or KMnO 4 </li></ul></ul><ul><ul><li>Alkynes: Terminal alkynes afford white precipitate with Ag(NH 3 ) 2+ OH - </li></ul></ul><ul><ul><li>Alcohols: CrO 3 solution orange to green for I and II </li></ul></ul><ul><ul><ul><li>HCl/ZnCl 2 gives white precipitate for III >II>I </li></ul></ul></ul><ul><ul><li>Aldehydes: Produce a silver mirror with Ag 2 O </li></ul></ul><ul><ul><li>Carbonyls: React to give colored crystals with DNPH </li></ul></ul><ul><ul><li>Acids: CO 2 evolution with NaHCO 3 , Na 2 CO 3 </li></ul></ul>
  4. 4. GC-MS GC El mass spectrometer Data Acquisition and Analysis Computer <ul><ul><li>The HP 5972 is an electron ionization mass spectrometer that can detect one ng or less of sample </li></ul></ul>
  5. 5. Mass Spectrometry Sample eluting from GC Pressure changes to ~10- 5 torr (1 ng/M sensitivity) Electron Impact (EI ) source: W filament sends electron beams that ionize molecules (M --> M + ) High Voltage Quadrapule rod The molecules are separated according to their mass to charge (m/Z) ratio. Detector. Counts the ions as they elute by m/z. Electron multipier amplifies it To data systems
  6. 6. Mass Spectrometry <ul><li>Mass spectroscopy is a very powerful technique that can detect upto a nano gram (even femtograms) of sample </li></ul><ul><li>The injected sample is first vaporized and an interface depresses the pressure to ~10 5 torr </li></ul><ul><li>70 electron volt or more powerful beams ionize the molecules </li></ul><ul><li>They are then sorted by mass/charge </li></ul>
  7. 7. Interpreting Mass Spectra <ul><li>M + e - -> M .+ + 2e - </li></ul><ul><li>The strongest peak is set to 100 on an intensity scale </li></ul><ul><li>m/z = m when z=1. The molecular ion peak gives the molecular weight of the sample </li></ul><ul><li>MS of organic compounds usually contain an [M+1] .+ peak that corresponds to the 13 C isotope </li></ul><ul><ul><li>The fragmentation pattern gives information of the framework and functional groups of the molecules. ex. 44 = CO 2 indicates acid or ester </li></ul></ul>
  8. 8. IR spectrometer for solids Sample Laser Path Mirrors
  9. 9. FT-IR Spectrometer-For liquids <ul><li>Liquid samples can be applied neat (undiluted) onto the NaCl salt plates </li></ul><ul><li>Place 2 drops on a NaCl plate placed in the holder. Place second salt plate on top and smear liquid by rotating the metal cover over the plates </li></ul><ul><li>Solids can be dissolved in a few drops of CH 2 Cl 2 and run . </li></ul>
  10. 10. FT-IR NaCl Salt Plate holder Laser Beam
  11. 11. Interpreting FT-ir <ul><li>When molecules absorb energy from an energy source (sun, electromagnetic radiation etc.) the bonds vibrate. </li></ul><ul><li>The bond stretching, bending motion absorbs energy that is characteristic of the functional group that is vibrating </li></ul><ul><li>The absorption or transmittance spectral absorbtions are measured in wavenumbers (cm -1 =1/  m x 10 4 ) </li></ul>
  12. 12. IR spectral absorptions <ul><li>Hydrogen bonding broadens absorptions </li></ul><ul><ul><li>COOH stretch broad band 2800-3400 cm -1 </li></ul></ul><ul><ul><li>N-H stretch 3000-3800 cm -1 </li></ul></ul><ul><li>The finger print region gives characteristic peaks for functional groups </li></ul><ul><ul><li>C-C, C-O, C-N 1300-800 cm -1 </li></ul></ul><ul><ul><li>C=C, C=N, C=O, N=O 1900-1500 cm -1 </li></ul></ul>
  13. 13. UV- Vis Cuvette Clear side should face beam Sample should be dilute (~10 -5 M)
  14. 14. UV-Visible spectroscopy <ul><li>Very dilute samples ~10 -5 mole/L can be analyzed </li></ul><ul><li>A=  cl Absorbance=ext. coeff x Concentration x pathlength </li></ul><ul><ul><ul><li>Extinction coefficient ~10,000-20,000 </li></ul></ul></ul><ul><ul><ul><li>Pathlength = 1 cm </li></ul></ul></ul><ul><li> max indicates the type of unsaturation </li></ul><ul><li>Conjugated double bonds increase the  max </li></ul>
  15. 15. Nuclear magnetic resonance The sample is placed between powerful magnets and a magnetic field sweeps through the sample NMR tube Magnets Sample Spinner
  16. 16. NMR <ul><li>NMR is a very useful technique for organic compounds. </li></ul><ul><li>When a magnetic pulse is applied the nuclei spin orientation is in the field direction. The spins revert during relaxation and the energy released is measured </li></ul><ul><li>Each proton that is in a different chemical environment absorbs at a different frequency (radio frequency) </li></ul>
  17. 17. NMR Spectra <ul><li>The protons that are next to electronegative atoms are deshielded (electron density around the proton is decreased) and appear downfield (closer to 10 on a scale of  1-10 ppm) </li></ul><ul><li>The nuclei spins interact -spin spin coupling: Splitting of a proton absorption by the neighboring protons. </li></ul><ul><ul><li>The multiplicity of the peak is determined by the number of protons on adjacent ataoms </li></ul></ul><ul><ul><li>The multiplicity is n+1, n being the number of neighboring protons that are identical </li></ul></ul>

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