3.06 Organic Analysis

• What is mass spectrometry used for?

  To find the relative molecular mass of organic compounds
• What does the x axis show on a mass spectrum? What does this effectively show and why?

  • Shows m/z value (mass/ionic charge)
  • Since most ions are 1+, this effectively show Mr
• Why are there multiple peaks when molecules are put into a mass spectrometer?

  • The molecular ion is shown as a peak, but it will also fragment into smaller molecules, so these peaks are shown as well
  • Also, due to isotopes of atoms, different peaks may be seen
• What is high resolution mass spectrometry?

  Mass spectrometers which give Mr to 3-4 decimal places are called high resolution.

  • Low resolution mass spectrometry is to the nearest whole number
• What does high resolution mass spectrometry allow you to do?

  Distinguish between compounds that have the same Mr to the nearest whole number, but are made up of diff. atoms and therefore have different values of Mr to 3 d.p
• Why do atoms and chemical bonds absorb infared radiation?

  They are constantly vibrating - they can absorb infared radiation that is the same frequency as their frequency of vibration
• What effect does a stronger bond have on the frequency of vibration?

  Vibrates faster with higher frequency
• What effect do heavier atoms have on the frequency of vibration?

  Vibrate slower, with a lower frequency
• How does IR Spectroscopy work?

  • Each bond has a unique vibration frequency in the infared region of the electromagnetic spectrum
  • Bonds absorb radiation that has the same frequency as their frequency of vibration
  • Infared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound's functional group
• What happens inside an IR spectrometer?

  • Beam of infared radiation with a range of frequencies is passed through the sample
  • Radiation that emerges is missing frequencies that have been absorbed by the bonds in the sample
  • Graph is plotted - intensity against frequency of radiation
• What do the troughs on an infared spectrum show?

  The frequencies where radiation has been absorbed - match the table to find out what they represent (table given in exam)
• What is the fingerprint region?

  • Area of the infared spectrum below the wavenumber of 1500cm-1
  • Many peaks, caused by complex vibrations of the whole molecule. Unique to every compound, so can be used to identify compounds.
• How is the fingerprint region used to identify compounds?

  The fingerprint region's pattern is matched to a database on a computer to identify the compound accurately.