4.2.4 - analytical techniques

• When a molecule absorbs infrared radiation it makes the covalent bond vibrate more in a stretching or bending motion.
• The factors affecting amount of vibration of a bond are

  bond strength

  bond length

  mass of each atom in the bond
• IR spectroscopy works because every bond has a unique vibration frequency in the IR region.

  bonds absorb radiation that has the same frequency as their vibration.

  IR emerged from a sample is missing the frequencies that have been absorbed which can be used to identify functional groups
• The peaks on an IR spectrum represent the absence of energy from the infrared radiation
• Two peaks that must be present to identify a carboxylic acid is:

  very broad U shaped peak at 2500-3300 —> the OH group

  sharp peak at 1680-1750 —> C=O group
• Inside a mass spectrometer:

  organic compound is vaporised and passed through the spectrometer

  some molecules lose an electron and form molecular ions

  excess energy from the ionisation makes the bond vibrate more

  vibration causes bonds to weaken

  molecular ion splits by fragmentation
• The symbol for a molecular ion is M+
• Fragmentation is unpredictable because it can happen anywhere in the molecule.
• In a mass spectrum, the molecular ion is represented as 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 (Mr)
• The molecular ion peaks of two isomers of the same compound will be the same
• The m/z value of CH3+ is 15
• The m/z value of OH is 17
• The m/z value of C2H5+ is 29
• The m/z value of C3H7+ is 43
• The m/z value of C4H9+ is 57
• Two advantages of using mass spectrometry:

  cheap

  small quantities of samples needed
• The main disadvantage of using mass spectrometry is that the sample is completely destroyed