Electrolyte conductivity

1
Characterisation of organic compounds using IR
spectroscopy

• By the end of this session, you should be
  able to
• Explain the theoretical basis of IR spectroscopy
• Interpret IR spectra
• Discuss the strenghts and limitations of IR

• Bibliography
• WADE, L. G. Jr (2006) Organic Chemistry (6th
  Edition), Prentice Hall, New Jersey, USA, Ch 12
• Video An Introduction to Infrared Spectroscopy
  (1996), Sheffield University Learning Media Unit
• http//www.cem.msu.edu/reusch/VirtualText/Spectrp
  y/InfraRed/infrared.htm (accessed 1/2/2006)

2
How do we know the synthesis has been successful?

• Has the product the desired structure?
• Is it sufficiently pure?
• We need techniques that work with a tiny amount
  of product and do not destroy the sample
• Spectroscopic techniques meet these requirements

Absorption spectroscopy is the measurement of the
amount of light absorbed by a compound as a
function of the wavelenght of the light
3
Fundamentals of spectroscopy

• Visible, infrared and ultraviolet light,
  microwaves and radiowaves are ELECTROMAGNETIC
  RADIATION
• They all travel at the speed of light, but differ
  in frequency and wavelenght
• Frequency (n) number of complete wave cycles
  that pass a fixed point in a second (Hz)

• Wavelenght (l) distance between any two crests
  or troughs (cm)

4
The electromagnetic spectrum
http//imagers.gsfc.nasa.gov/ems/waves3.html
(23/01/06) http//loke.as.arizona.edu/ckulesa/c
amp/spectroscopy_intro.html (23/01/06)
5
Infrared spectroscopy

• Infrared photons can cause groups of atoms to
  vibrate with respect to the bonds that connect
  them
• The types of vibrations available to a molecule
  is determined by the
• - Number of atoms
• - Type of atoms
• - Type of bonding between atoms

IR spectroscopy is a powerful technique in
characterising pure inorganic and organic
compounds
6
Infrared spectroscopy

• The position of an infrared band is specified by
  its wavelenght (l). The units in IR spectra are
  wavenumbers ( ), cm-1
• Two main types of vibration

- There are two stretching (symmetric,
asymmetric) and three bending modes (rocking,
scissoring, wagging, twisting). To see them in
action, go to http//www.shu.ac.uk/schools/sci/ch
em/tutorials/molspec/irspec1.htm (accessed
23/02/05)
7
The IR spectrum

• An infrared spectrometer measures the frequencies
  of infrared light absorbed by a compound
• The different bonds present in an organic
  compound will have characteristic IR absorptions.
  There are tables with the characteristic
  absorptions of functional groups.
• It is very unlikely that two compounds (unless
  enantiomers) will have the same frequencies for
  all their various complex vibrations
• Samples for IR can be solid or liquids
• Solid are mixed with KBr to make a disc, or
  mixed with nujol (an inorganic oil) and deposited
  over NaCl plates
• Liquids can be analysed directly as a thin film
  in NaCl plates

8
Strenghts of IR

• An IR spectrum
• Provides an indication of the functional groups
  in the compound
• Shows the absence of other functional groups that
  would give strong absorptions if they were
  present
• The absence of a signal is proof that the
  functional group is absent.
• Can confirm the identity of a compound by
  comparison with a known sample

9
Limitations of IR

• IR does not provide much information about the C
  skeleton or alkyl groups
• Some signals may be ambiguous
• You can rarely determine a structure based only
  on the IR spectrum