F322: Modern Analytic Techniques

Infra-Red Spectroscopy

  • Allows us to identify a functional group in a molecule.
  • All molecules absorb infra-red radiation; making them either stretch or bend (vibrating).
  • Different bonds absorb different wavelengths.
  • At AS only three peaks are needed to be recognised:
    -At 3230 - 3500 (big trough for O-H bond). [alcohols]
    -At 1680 - 1750 (peak for C=O bond). [aldehydes and ketones]
    -At both (two peaks). [carboxylic acids] 
  • Must state when answering:
    -The wavelength of any peaks.
    -The functional group causing the peak.
    -Any missing peaks that are significant (i.e. missing O-H trough).
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Infra-Red Spectroscopy (E.g.)

  • Graph shows O-H stretch/trough.
  • And also C=O stretch.
  • (Others surpass AS level). 
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Mass Spectrometry

  • Can be used to:
    -Determine abundance of each isotope of an element.
    -Identify unknown compounds.
    -Gain further knowledge about the structure and chemical properties of a molecule. 
  • Mass spectra of elements:
    -Determines the mass of the isotopes present and abundance, using a m/z ratio (m being mass, z being charge).
    -If there are say three peaks, the number of peaks repressents the three isotopes, the height equals the percentage abundance.
  • Mass spectra of molecules:
    -Used to ionise organic molecules and breaks the ions up into fragments; the positive ion formed is the M+ ion; which is used to identify the unknown molecule.
    E.g. CH3OF + e- -> [CH3OH]+ + 2e-   (m/z = 32)
    -As the molecule is ionised, excess energy is transferred to make it vibrate.
    -This causes bonds to weaken and break, causing it to fragment.
    E.g. C2H5OH+ -> CH3 + CH2OH+
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Mass Spectrometry (Cont.)

  • These fragment ions are broken up even smaller, forming a comparable fingerprint.
  • The highest significant m/z value is the M+, the other peaks are fragmentation.
  • The small peak to the right of the M+ is due to C-13 ion.
  • The rest is manipulating the graph to determine what fragments have occured, so determining whether the molecule is an alkane or alcohol etc.
  • N.B. Draw the displayed formula for the M+ and try to split at certain points.
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