Infrared light has a wavelength of 770nm to 1000nm, which corresponds to a wavenumber range of 12,900 to 10 cm
-1
. Infrared light with wavenumbers from 400 to 4000 cm
-1
will be used to identify various identify various organic compounds.
When a beam of infrared radiation is passed through a substance, the radiation can be either absorbed or transmitted, depending on its frequency and the structure of the molecules it encounters.
Energy is absorbed when its frequency matches the natural frequency of the bond motion. The energy absorbed by the molecules may bring about increased vibration, stretching, bending or rotating. The IR spectrophotometer measures the frequency of the energy causing the bond motion. Each type of bond absorbs specific wavenumbers of infrared light depending on its structure, nature of atoms bonded and effects of surrounding atoms.
In this experiment, various samples will be placed into a beam of infrared light. Some of the light will be absorbed. The absorption of infrared light will be indicated by a valley (called a peak) in a graph called a spectrum . You will compare the wavenumbers of the peaks to the table of known values to determine the identity of the unknown samples.
Liquid samples can be prepared for IR analysis by simply placing an undiluted sample between a pair of transparent crystal windows (called salt plates ). The windows are held together and placed in the spectrophotometer for data collection. The salt plates are made of potassium or sodium bromide, which are ionic compounds and do not have peaks in the IR spectra. Before running the sample, a background should be collected. This will make the computer subtract any peaks from the surrounding atmosphere when you collect the data for your sample.
Students will be able to demonstrate the procedure for collecting an infrared spectrum and will obtain spectra of a liquid sample. Students will compare peak values to the table enclosed and will attempt to identify the unknown compounds.
KBr Salt plates*
Disposable pipettes
Kimwipes
Rubber gloves
Plate holder
Waste beaker
Liquid Samples
Acetone
*NOTE: Salt plates will react with water and care should be taken to ensure that no water or moist materials come in contact with them. Always clean the plates with ACETONE only before and after use. Wear gloves when handling the plates, and wipe sample with a kimwipe to keep them clean. *****
Infrared Spectroscopy of Organic Liquids Page 1


Always wear safety goggles in the lab.

Always wear gloves when handling the samples and salt plates.

Work safely to ensure your safety and the safety of those around you.

Dispose of waste as instructed by the teacher.
Part I: Collecting the Background Spectrum
1.
Make sure there is nothing inside the collection chamber of your spectrophotometer.
2.
Click on “Collect 
Background”. a.
Enter a title of “bg”. b.
Click on the enter arrow (

).
3.
Wait for the scanning to complete. A background spectrum should appear. It will be automatically saved and used to set a ratio for your sample(s).
4.
Click on “View 
Clear Spectrum”; answer NO to the save option.
5.
Move on to Part II: Collecting a Liquid Sample Spectrum .
Part II: Collecting a Liquid Sample Spectrum
6.
Separate the salt plates and put them onto a kimwipe on the table.
7.
Use a disposable pipette to place 2 or 3 drops of the sample onto one of the salt plates.
8.
Put the second salt plate on top (like a sandwich). Make sure the liquid is spread between the plates. Try to avoid having air bubbles between the plates. Add more liquid if needed.
9.
Put the salt plates into the holder.
10.
Put the holder into the collection chamber. It will not entirely close.
11.
Click on “Collect 
Sample”. a.
Enter the name of your sample (include group name or initials) for example:
Mark, Jane, Chris : L1 or MJC: L1 b.
Click on the enter arrow (

).
12.
When the scanning is done, the spectrum should appear on the screen. Move on to Part III:
Processing and Printing an IR Spectrum for the next part of the process.
Part III: Processing and Printing an IR Spectrum
Processing the Spectrum
1.
When the data collection is complete, the spectrum should appear on the screen.
2.
If the peaks appear too small, remove the salt plates and add more sample. Collect data again.
3.
If the peaks appear too large, remove the salt plates; wipe off some with a Kimwipe. Collect again.
4.
Click on “Process 
Correct Baseline”.
5.
Click on “Process 
Smooth”.
6.
Click on “Analyze 
Find Peaks”. a.
Set the “threshold line” by moving it to the desired level and clicking (you want 4-5 points). b.
Click “OK”.
7.
Click on “View  Show Report” if your report (data table) is missing.
Infrared Spectroscopy of Organic Liquids Page 2

Printing the Spectrum
8.
Click on “File 
Print”. a.
Change the orientation to Landscape. Then click “OK”.
9.
After printing is complete, make sure that the spectrum and the report have printed.
10.
Click “View  Clear Report”. Answer NO to the save option.
11.
Click “View 
Clear Spectrum”. Answer NO to the save option.
12.
Move on to Part IV: Cleaning Up .
Part IV: Cleaning Up USE ACETONE ONLY
Between Samples
1.
Remove the holder from the collection chamber.
2.
Rinse the salt plates with acetone into the waste beaker.
3.
Gently wipe the salt plates with a Kimwipe.
4.
If you are going to do another sample, repeat the steps starting with Part II. If you’re done, follow the directions below.
When Lab is Complete
5.
Place the salt plates (cleaned with acetone) back into the metal container.
6.
Put all materials into the plastic bin.
7.
Plastic pipettes, rubber gloves and kimwipes should be thrown in the trash.
8.
Leave the waste beaker to be disposed of by the instructor.
Part V: Analyzing the IR Spectra
1.
List the wavenumbers of the major peaks from the spectrum on pages 5-6.
2.
Identify the functional groups (as many as possible) represented by the peaks using the Peak
Table on page 4.
3.
Identify each unknown by comparing the functional groups found with the possible samples shown on page 4.
4.
Give a brief explanation of why you picked that molecule as the identity of the sample.
5.
Answer the questions.
Infrared Spectroscopy of Organic Liquids Page 3

:
Sample wavenumbers of infrared light absorbed by the bonds in organic compounds.
Functional Group Structure Bond Wavenumber (cm -1 )
Alkanes
CH
3
-CH
2(n)
-CH
3
(single bonds)
C-H stretch 2950 – 2850 (m or s)
Alkenes
Aromatic Rings
Alcohol/Phenol
Carboxylic Acid
(organic acid)
Aldehyde
Ketone
Ester
CH
2
=CH
2
(double bonds)
C-H stretch
C=C stretch
C-H stretch
C-H bending
C=C bending
O-H stretch
3100 – 3010 (m)
1680 – 1620 (vs)
~ 3030 (vs)
860 – 680 (s)
1700 – 1500 (m,m)
3550 - 3200 (broad, s) R-OH
O
O-H stretch
C=O stretch
3000 – 2500 (broad, vs)
1780 – 1710 (s)
R OH
O
C=O stretch 1740 – 1690 (s)
R H
O
C=O stretch 1750 – 1680 (s)
R R
O
C=O stretch
C-O stretch
R O
R
1
Possible structures for Liquid Unknowns
OH
1750 – 1735 (s)
1300 – 1000 (s)
O
CH
3 CH
3
H
3
C
H
3
C CH
3
H
3
C hexane isopropyl alcohol methyl ethyl ketone
CH
3 O
O
H
3
C ethyl acetate
CH
3 toluene
Infrared Spectroscopy of Organic Liquids Page 4

Sample/Unknown #: _______________
Major Peaks (cm -1 ) – Peak Positions
Identity of Sample:
Why do you think this is the identity of the sample?
Sample/Unknown #: _______________
Major Peaks (cm -1 ) - Peak Positions
Identity of Sample:
Why do you think this is the identity of the sample?
Sample/Unknown #: _______________
Major Peaks (cm -1 ) – Peak Positions
Identity of Sample:
Why do you think this is the identity of the sample?
Infrared Spectroscopy of Organic Liquids
Functional Group
Functional Group
Functional Group
Page 5

Sample/Unknown #: _______________
Major Peaks (cm -1 )- Peak Positions
Identity of Sample:
Why do you think this is the identity of the sample?
Functional Group
Sample/Unknown #: _______________
Major Peaks (cm -1 )- Peak Positions
Identity of Sample:
Why do you think this is the identity of the sample?
Functional Group
1.
Why must water be kept away from the experiment?
2.
Since salt plates are used in this experiment, what can you determine about the interaction of salt with infrared radiation?
3.
Why is it necessary to obtain a background spectrum?
Infrared Spectroscopy of Organic Liquids Page 6