Infrared Spectroscopy & Qualitative Tests Lab Report

CHEM 2243 Introduction to Infrared Spectroscopy and Qualitative Tests

Purpose: In this virtual lab experiment, you will explore the use of infrared spectroscopy and various qualitative tests to distinguish between four organic compounds: acetophenone, benzaldehyde, benzoic acid, and benzyl alcohol.

Infrared spectroscopy is useful for distinguishing classes of organic compounds (ie. functional groups).

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We can identify functional groups using infrared (IR) spectroscopy. IR radiation is in the same range of frequencies (4000-600 cm-1) as the stretching and bending vibrations of bonds in organic molecules. When the frequency of the radiation exactly matches the frequency of the bond, the molecule absorbs energy, resulting in an absorption peak in the IR spectrum. Most functional groups show absorption bands in the region of 4000-1400 cm-1. See the handouts: Characteristic Infrared Absorption Peaks and IR Tutorial UCLA for help identifying the peaks.

Prior to modern spectroscopy and spectrometry techniques, one way that chemists used to identify classes of organic compounds was through qualitative tests. These simple chemical tests can be used to indicate the presence (or absence) of certain functional groups. You will use the Beyond Labz virtual platform to record the IR spectra, and to perform the following qualitative tests on the four organic molecules that you are investigating in this virtual lab experiment. (acetophenone, benzaldehyde, benzoic acid, and benzyl alcohol).

Functional group qualitative tests:

Permanganate Test: A purple permanganate solution is placed in a test tube and unknowns are added directly to the permanganate solution. For solid unknowns, the solid is first dissolved in acetone. If the purple color of the permanganate persists, the test is negative. If the solution changes from purple to brown (MnO2), the molecule is oxidizable and the test is positive and the unknown contains one or more of the following: alkene, primary or secondary alcohol, or an aldehyde.

Jones Test: The unknown is mixed with acetone to produce a saturated solution, and then added to the Jones reagent (chromic acid). The persistence of the bright orange color indicates a negative test. A color change to blue or green is a positive test. A rapid change to a light blue color indicates a primary or secondary alcohol. If the color change takes longer than one or two seconds, the unknown contains an aldehyde.

Tollens Test: Concentrated aqueous NH3, 5% aqueous AgNO3, and 10% aqueous NaOH are added to a test tube followed by a minimal amount of the unknown. The formation of a silver mirror on the test tube wall is a positive test for aldehydes and reducing sugars.

2,4-Dinitrophenylhydrazine Test (DNPH): Liquid unknown or a solution containing the solid unknown and ethanol is added to the orange dinitrophenylhydrazine solution. The formation of a precipitate ranging in color from red to yellow is a positive test for aldehydes and ketones. Allylic and benzylic alcohols may also give a positive test. Absence of a precipitate is a negative test.

Iodoform Test: The unknown and a small amount of 10% aqueous NaOH are heated to 60˚C. KI/I2 is then added to the solution followed by the addition of NaOH until the solution is again colorless. The appearance of a light-yellow precipitate is a positive test for methyl ketones. Alcohols which may be oxidized to produce methyl ketones or acetaldehyde may also give positive tests. If the solution remains clear, the test is negative.

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Virtual Lab Instructions (video instructions also available). Help is available by clicking on the bell on the stockroom counter.

  1. Open the organic chemistry lab on the Beyond Labz platform. Select “Qualitative Analysis” at the top left of the screen to open the Qualitative Analysis virtual lab.

*Note: The order that you test the four compounds in the virtual lab does not matter. Likewise, the order that you perform the qualitative tests also does not matter. The instructions are simply written in the order that the compounds appear by functional group on the chalkboard, and then by the order that the reagents appear on the benchtop. What matters is that for each compound you record the IR spectrum and the results (+ or -) of each test.

**Note 2: Although you will be practicing recording the IR spectra in the virtual lab, you are provided with the spectra at the end of this handout with the key peaks clearly labeled.

  1. Select “Alcohols” from the chalkboard, then click and drag (or double-click) the reagent bottle for the benzyl alcohol to add it to the flask.
  2. Drag the flask to the cork ring support on the lab bench.
  3. Record the IR spectrum by clicking on the IR spectrometer and dragging the salt plate icon to the flask. This will display the IR spectrum on the screen. You can type the name of the compound on the spectrum and click save to save it to your lab notebook. Click ok to close the spectrum.
  4. To get a sample of the compound to test, click and drag from the flask to the clamp (you can also double-click on the flask). This will put a sample in a test tube on the ring stand. Notice the new window that appears on the top left of the chalkboard. As you perform each test, a video of the test will display here.
  5. Perform the permanganate test by clicking and dragging from the bottle labeled “KMnO4”. The video that displays is an actual video of the test. The test tube on benchtop will show a simulated result. Record the result of the test (+ or -) in your data table.
  6. Discard the test tube by dragging it to the waste container.
  7. Take a fresh sample of the compound by again clicking and dragging from the flask to the clamp (or double-clicking on the flask).
  8. Perform the Jones test by clicking and dragging from the bottle labeled “H2CrO4”. The video that displays is an actual video of the test. The test tube on benchtop will show a simulated result. Record the result of the test (+ or -) in your data table.
  9. Discard the test tube by dragging it to the waste container.
  10. Take a fresh sample of the compound by again clicking and dragging from the flask to the clamp (or double-clicking on the flask).
  11. Perform the Tollens test by clicking and dragging from the bottle labeled “AgNO3”. The video that displays is an actual video of the test. The test tube on benchtop will show a simulated result. Record the result of the test (+ or -) in your data table.
  12. Discard the test tube by dragging it to the waste container.
  13. Take a fresh sample of the compound by again clicking and dragging from the flask to the clamp (or double-clicking on the flask).
  14. Perform the 2,4-Dinitrophenylhydrazine test by clicking and dragging from the bottle labeled “DNPH”. The video that displays is an actual video of the test. The test tube on benchtop will show a simulated result. Record the result of the test (+ or -) in your data table.
  15. Discard the test tube by dragging it to the waste container.
  16. Take a fresh sample of the compound by again clicking and dragging from the flask to the clamp (or double-clicking on the flask).
  17. Perform the Iodoform test by clicking and dragging from the bottle labeled “I2/KI”. The video that displays is an actual video of the test. The test tube on benchtop will show a simulated result. Record the result of the test (+ or -) in your data table.
  18. Discard the test tube by dragging it to the waste container.
  19. Be sure that you have recorded the results of all 5 chemical tests and have recorded the IR spectrum before moving on to the next compound.
  20. Reset the qualitative analysis lab by clicking on the waste container to “clear lab”.
  21. Select “Aldehydes” from the chalkboard, then click and drag (or double-click) the reagent bottle for the benzaldehyde to add it to the flask.
  22. Repeat steps 3-21 with the benzaldehyde sample.
  23. Select “Ketones” from the chalkboard, then click and drag (or double-click) the reagent bottle for the acetophenone to add it to the flask.
  24. Repeat steps 3-21 with the acetophenone sample.
  25. Select “Acids” from the chalkboard, then click and drag (or double-click) the reagent bottle for the benzoic acid to add it to the flask.
  26. Repeat steps 3-21 with the benzoic acid sample.
  27. Your virtual experiment is now complete!

Questions:

Draw the structures of each of the four compounds (acetophenone, benzaldehyde, benzoic acid, and benzyl alcohol) you will be studying in this experiment and list the functional groups that they contain including benzene rings. (2 points)

Results:

Copy and fill in with your results the following data tables. (4 points)

Table 1: Qualitative Tests Results (+ or -)

 

Permanganate test

Jones test

Tollens test

DNPH test

Iodoform test

benzyl alcohol

     

benzaldehyde

     

acetophenone

     

benzoic acid

     

Table 2: Qualitative Tests Observations (1 point)

Test

Positive Test Description

Permanganate test

 

Jones test

 

Tollens test

 

DNPH test

 

Iodoform test

 

Table 3: IR Peak Assignments (refer to spectra saved in the virtual lab and the spectra provided at the end of this handout) (4 points)

Compound

Absorbance Peak(s) (cm-1)

Bond(s)/Functional groups

Benzyl alcohol

~3325

 
 

~3088-3031

 
 

~2932-2875

 
 

~1500

 
 

Benzaldehyde

~3085-3030

 
 

2820 & 2738

 
 

1703

 
 

~1600

 
 

Acetophenone

~3065-3005

 
 

~2965-2865

 
 

~1685

 
 

~1600

 
 

Benzoic acid

~3200-2550

 
 

~1690

 
 

~1600

 

Questions (2 pts ea; 8 pts total):

1. How would you distinguish benzaldehyde and acetophenone by the results of their qualitative tests? (1.5 points)

2. How would you distinguish benzaldehyde and acetophenone by their infrared spectra? (1.5 points)

3. How would you distinguish benzoic acid and benzyl alcohol by the results of their qualitative tests? (1.5 points)

4. How would you distinguish benzoic acid and benzyl alcohol by their infrared spectra? (1.5 points)

5. Explain how you would distinguish between the two compounds shown below using IR and qualitative tests? (3 points)

IR Spectra

Benzyl alcohol:

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Benzaldehyde:

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Acetophenone:

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Benzoic acid:

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