IR SPECTROSCOPY

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IR SPECTROSCOPY
R W Grime Ripon Grammar School
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• Light is one form of electromagnetic radiation.
• Light is only a very small part of the
  electromagnetic spectrum.
• Electromagnetic waves consist of electric and
  magnetic fields which are perpendicular to each
  other and to the direction of travel of the wave.
  
• The electric and magnetic fields vibrate at the
  same frequency as each other.

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• THE ELECTROMAGNETIC SPECTRUM

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• Atoms, molecules and ions can absorb (or emit)
  electromagnetic radiation of specific
  frequencies, and this can be used to identify
  them.

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• INFRA-RED SPECTROSCOPY

• All bonds vibrate at a characteristic frequency.
• There are different types of vibration.

Symmetric stretch
Assymmetric stretch
Bending

• The frequency depends on the mass of the atoms in
  the bond, the bond strength, and the type of
  vibration.

• The frequencies at which they vibrate are in the
  infra-red region of the electromagnetic spectrum.

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• INFRA-RED SPECTROSCOPY

• If IR light is passed through the compound, it
  will absorb some or all of the light at the
  frequencies at which its bonds vibrate.

• Wavenumbers (cm-1) are used as a measure of the
  wavelength or frequency of the absorption.
• Wavenumber
  1
• 
  wavelength (cm)

• IR light absorbed is in the range 4000 400 cm-1.

• Above 1500 cm-1 is used to identify functional
  groups.

• Below 1500 cm-1 is used for fingerprinting.

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• BELOW 1500 cm-1 Fingerprinting

• Complicated and contains many signals picking
  out functional group signals difficult.

• This part of the spectrum is unique for every
  compound, and so can be used as a "fingerprint".

• This region can also be used to check if a
  compound is pure.

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cyclohexane
CH
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cyclohexene
CH
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butanal
CH
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ethanoic acid
OH
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ethanol
OH
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butanal
CO
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propanone
CO
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ethanoic acid
CO
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methyl ethanoate
CO
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Exercise 1

• Match the following eight compounds to the
  following eight IR spectra.

hex-2-ene pentane methylpropan-1-ol 2-methylpentan
-3-one
butanal butanoic acid propyl ethanoate nitrobenzen
e
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propyl ethanoate
C-O
CO
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2-methylpentan-3-one
CO
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methylpropan-1-ol
O-H
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nitrobenzene
C-H
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pentane
C-H
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butanal
C-H
CO
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butanoic acid
O-H
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hex-2-ene
C-H
C-H
CC
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F
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G
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P
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Q
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R
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S
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T
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U