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INSTRUMENTAL ANALYSIS
LAB.
Mr Haydar A.M.S
BSc, MSc
Faculty of Science
Chemistry Department
Email: [email protected]
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Infrared Spectroscopy: Analyse the functional groups
of benzoic acid
Lab 7
Aim of the experiment
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To analyze the functional groups of benzoic acid using infrared spectroscopy
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Introduction
IR spectroscopy deals with the interaction of infrared radiation with matter.
It is a light with a longer wavelength and lower frequency than visible light.
Typical IR wavelengths range from 8x10-5 cm to 1x10-2 cm, and this corresponds to energies of around 1-10 kcal.
This energy is sufficient to make atoms vibrate, but not enough to cause electronic transitions.
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IR Spectroscopy
Symmetrical stretching Antisymmetrical stretching Scissoring
Rocking Wagging Twisting
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IR Spectroscopy
Heavier atoms vibrate more slowly than lighter ones,
so a C-D bond will vibrate at a lower frequency than a C-H bond.
Thus, stronger bonds generally vibrate faster than weaker bonds.
So O-H bonds which are stronger than C-H bonds vibrate at higher frequencies.
IR Spectrum
• A plot of % transmittance (%T) vsvibrational frequency in wavenumbers (cm-1)
c
c
1
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λ = wavelength
υ = frequency
c = speed of light in a vacuum
1 wavenumber
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40004000--30003000
cmcm--11
30003000--20002000
cmcm--11
20002000--15001500
cmcm--11
15001500--10001000
cmcm--11
OO--H H
NN--HH
CC--HH
CCCC
CCNN
C=CC=C
C=OC=O
CC--OO
CC--FF
CC--ClCl
deformationsdeformations
40004000--30003000
cmcm--11
30003000--20002000
cmcm--11
20002000--15001500
cmcm--11
15001500--10001000
cmcm--11
OO--H H
NN--HH
CC--HH
CCCC
CCNN
C=CC=C
C=OC=O
CC--OO
CC--FF
CC--ClCl
deformationsdeformations
Increasing energy
Increasing frequency
IR Spectrum
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IR Spectrum
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IR Spectrum
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Instrumentation
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Sample Analysis of Solid Compounds
KBr is used mostly in IR Spectroscopy techniques
In IR spectroscopy, samples are analyzed by grinding with powdered potassium bromide and pressing into a disc.
Potassium bromide is transparent from the near UV to long-wave IR wavelength (0.25-25 µm) and has no significant optical absorption lines in its high transmission region.
It is used widely as infrared optical windows and components for general spectroscopy because of its wide spectral range.
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Sample Analysis of Benzoic acid
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Procedure
• Transfer some KBr out of the desiccator into a mortar and grind it into a very fine powder using a clean pestle.
• Add about 1 to 2 % of your sample (Benzoic acid), mix and grind it well again.
• The mixture is now ready for placement in a stainless steel die that should have already been assembled as per the following figure.
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Procedure
KBr die – Side
View
KBr die – Cross Section
ram
upper pellet
lower pellet
KBr+sample
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Procedure
• Apply 8 to 10 tons of pressure for 3 to 5 minutes. Leave the vacuum on during this step also.
• Slowly bleed pressure from the die by slowly opening the bleed screw and remove the disk from the die
• Using tweezers transfer the pressed KBr disk to the appropriate disk holder for IR analysis. Avoid contact with your fingers as the disk may pick up moisture.