11.3 Spectroscopic identification of organic compounds

Degree of unsaturation or index of hydrogen deficiency (IHD)

  • Used to determine number of rings or double bonds from a molecular formula
    • Double bond = 1 IHD
    • Triple bond = 2 IHD
    • Ring = 1 IHD
    • Aromatic = 4 IHD (3 double bond + 1 ring)
  • For the generic formula CcHhNnOoXx:
    • IHD = (0.5)(2c + 2 – h – x + n)

Electromagnetic Spectrum

  • Various regions of EMS are basis of different types of spectroscopy

EM Spectrum

  • X-Rays – Give information about bond distances and bond angles in a structure. Basis of X-ray crystallography
  • Visible and UV light – Gives information about electronic energy levels and form basis of UV-vis spectroscopy
  • Infrared Radiation – Provides information of functional groups present. Basis of IR spectroscopy
  • Microwaves – Gives information of bond length
  • Radiowaves – leads to information of connectivity of atoms and their chemical environments. Basis of Nuclear magnetic resonance (NMR)


  1. Infrared Spectroscopy
    1. Used in identifying functional groups. Basis of this is the spring model where every covalent bond is considered a spring. The frequency that these springs vibrate are called the wavenumber (1/λ)
    2. Identify each group by starting with the lowest peak, and working upwards
    3. Refer to Section 26 of Data booklet for wavenumber of different functional groups.
  2. 1H NMR Spectroscopy
    1. Gives information on different chemical environments of hydrogen atoms in a molecule. Basis is that nuclei of every H atom have two possible spin states and exist like magnets. The position of this spin relative to the standard spin (tetramethylsilane, TMS) is termed chemical shift
    2. Gives information on relative number of hydrogens per environment in a ratio. (Area under curve is number of hydrogens)
    3. Refer to Section 27 of Data booklet for chemical shift data
  3. Mass Spectroscopy
    1. Gives further information of functional groups When gaseous molecule is ionized, a molecular ion M+ is formed. The fragmentation pattern observed in a mass spectrum is each mass after a functional group is removed
    2. Refer to Section 28 of Data booklet for fragment data

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