Chapter 9: Spectroscopic Identification of Organic Compounds

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Chapter 9 Spectroscopic Identification of
Organic Compounds

• CH 9-1 Review of IR and Hydrogen Deficiency
• Infrared (IR) Spectroscopy used to determine
  the functional groups present in an organic
  compound.

• Nuclear Magnetic Resonance (NMR) Spectrometry
  helps to identify the C-H framework of an organic
  compound.

• Mass Spectrometry (MS) used to determine the
  molecular mass of organic compounds, and to
  identify certain structural features of the
  compound.

• Information from all three instrumental
  techniques is usually required to help identify
  the structure of an unknown organic compound.

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Review (Chap 2) Infrared (IR) spectroscopy

• IR is an instrumental method used to identify
  some of the functional groups present in an
  organic compound.

• Basic Theory A molecule is subjected to
  electromagnetic radiation (EMR) at a frequency in
  the IR region of the electromagnetic spectrum.

• When the IR frequency matches the vibrational
  frequency of a specific functional group, we
  observe an energy absorbance band in the IR
  spectrum.

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Example IR spectrum of organic compound
The absorption bands represent IR energy absorbed
by the covalent bonds of different functional
groups, read as frequency units off the x-axis
(4000 - 600 cm-1).
Most of these bands are due to C-C and C-H bonds
and can be ignored.
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The absorption bands of different functional
groups absorb at different frequencies and
intensities. Keep it simple!
(3300 if terminal)
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Tool for Unknown IdentificationHydrogen
Deficiency
Hydrogen Deficiency measure of unsaturation,
or deficiency in the Hs you would expect vs
Cs.
A deficiency in the number of hydrogens
indicates the presence of pi bonds or rings.
HD (Max Hs Actual Hs)
(Max Hs 2n 2 where n carbons)
HD for double bond 2
HD for ring 2 (cycloalkane)
HD for triple bond 4
HD for benzene ring 8
Combinations are possible!