Analytical techniques

• When a molecule absorbs infrared radiation, it makes the covalent bond vibrate more in a stretching or bending motion
• Factors affecting the amount of vibration of a bond:

  • Bond strength
  • Bond length
  • Mass of each atom in the bond
• Infrared spectroscopy works by:

  • Every bond having a unique vibration frequency in the infrared region of the EM spectrum
  • Bonds absorbing radiation with the same frequency as their vibration frequency
  • Infrared radiation from a sample missing the absorbed frequencies, which can be used to identify the compound’s functional group
• Peaks on an infrared spectrum represent the absorbance of energy from the infrared radiation
• To identify a substance as a carboxylic acid, two peaks must be present:

  • Very broad peak at 2500 - 3300 cm-1 for the O-H group
  • Sharp peak at 1680 - 1750 cm-1 for the C=O group
• Inside a mass spectrometer:

  • Organic compound is vaporized and passed through the spectrometer
  • Some molecules lose an electron and form molecular ions
  • Excess energy from ionization makes the bonds vibrate more, causing them to weaken and fragment
• The symbol of a molecular ion is M+
• Fragmentation inside a mass spectrometer is not predictable because it can happen anywhere in the molecule
• In mass spectrometry, a molecular ion is represented by the peak with the highest mass/charge ratio
• The molecular mass of the molecular ions is equal to the relative molecular mass of the compound
• The molecular ion peaks of two isomers of the same compound will be the same
• M/z value of CH3+: 15
• M/z value of OH- from alcohol: 17
• M/z value of C2H5+: 29
• M/z value of C3H7+: 43
• M/z value of C4H9+: 57
• Advantages of using mass spectrometry:

  • Cheap
  • Small quantities of samples required
• The main disadvantage of using mass spectrometry is that the sample is completely destroyed