BRANDEISUNIVERSITY
CHEMISTRY 29A
PRELAB WRITEUP AND EXPERIMENTAL REPORT SHEET
NAME Richard Fan______T.A.'S NAME David Gay______
DATE 7/25/06______EXPERIMENT# ___5___TITLE ___Aldehydes and Ketones______
PURPOSE _Identifying an unknown aldehyde and unknown ketone by using a 2,4-dinitrophenylhydrazone
(DNP) test, Schiff’s test, iodoform test, boiling point, and infrared (IR) spectroscopy.
PRELIMINARY WRITEUP:Materials:
10mL 2,4-dinitrophenylhydrazine6mL iodine solution
unknown aldehyde, ketone4mL 1,2-dimethoxyethane
water, ethanolHirsch funnel, flask,
2mL Schiff’s reagent4mL NaOH
reaction tubes, pipettes, pH paper
2,4-dinitrophenylhydrazone (DNP) test /
Schiff’s test
Iodoform test
EXPERIMENTAL PROCEDURES*
(Include in preliminary writeup.)
DNP test
-Fill 2 large test tubes each with 5mL 2,4-dinitrophenylhydrazine, add 0.05g of unknown compound, one to each tube.-Mix well, let both sit in warm water for 10 minutes, then in room temperature for 10min, then ice for 10min.
-Dry each using Hirsch funnel. When crystals are just moist, wash in-funnel with water until the pH is neutral (check with pH paper).
-Recrystallize in ethanol, get melting points.
Schiff’s test (for aldehydes)
-Add 2 drops of unknown compound to 1mL Schiff’s reagent.
-Test a known aldehyde (benzaldehyde), a known ketone (acetone), and unknowns.
Iodoform test (for methyl ketones)
-Check for water solubility.
-If soluble-Mix 4 drops of unknown with 2mL water
-Add 2mL 3M NaOH, then slowly add 3mL iodine solution, perform for known methyl ketone, known non-methyl ketone, and unknowns.
-If insoluble-mix 4 drops of unknown with 2mL 1,2-dimethoxyethane
-Add 2mL 3M NaOH, then slowly add 3mL iodine solution.
Boiling point
-Add 1 pipetteful of each unknown into respective reaction tubes, position thermometer just above liquid and heat, record temperature when it becomes constant.Infrared spectroscopy – perform for unknowns. / MODIFICATIONS AND OBSERVATIONS*
(Obtained from the lab experience)
DNP test
4 drops of unknown #156 were used, producing a thick yellow orange mixture, and 2 drops of unknown #7 were used, producing a bright red thick mixture. After drying in the Hirsch funnel, the crude products were moist and pasty. These crude products were dissolved in 1-2mL of ethanol and recrystallized. Unknown #7 was difficult to dissolve, and much of the solid remained after adding ethanol and applying heat. The resulting final products yielded melting points of 102º-104ºC and 213ºC for #156 and #7, respectively.Schiff’s test
Unknown #156 produced a peach color, very similar to the original Schiff’s reagent. Unknown #7 produced a deep purple colored liquid with what appeared to be some insoluble particles. In comparison, Schiff’s test of acetone produced a peach color, and testing of benzaldehyde produced a very deep purple colored liquid.Iodoform test
Both of the unknown compounds resulted in insolubility in water. Upon combining the unknown, NaOH, and 1,2-dimethoxyethane, #156 produced a 2-layered mixture composed of a clear light yellow top layer and a similar bottom layer. #7 also produced a 2-layered mixture composed of a clear yellow top layer and a cloudy brown bottom layer. Upon adding the iodine solution, #156 first turned yellow and then into a cloudy brown solution with some precipitate formation. #7 first turned clear and brown, and then into a clear yellow liquid. For comparison, the known methyl ketone acetone produced a cloudy yellow liquid with some precipitate formation, and the known non-methyl ketone cyclohexanol produced a clear yellow liquid.Boiling point
The boiling points for #156 and #7 were observed to be 130ºC and 161ºC, respectively.Infrared spectroscopy – see attached.
RESULTS, DISCUSSION AND CONCLUSIONS*
The tests that were used to determine the identities of the unknown aldehyde and unknown ketone were the 2,4-DNP derivative, Schiff’s test, Iodoform test, boiling point, and IR spectroscopy. Each of these tests contributes a piece of information that can help identify and rule out different identities.
The first test creates a stable imine derivative that is a solid, which is then useful for finding melting points of compounds that are natively liquid. After 2,4-dinitrophenylhydrazine bonds to the aldehyde or ketone at the double bonded oxygen position, the melting point of the 2,4-dinitrophenylhydrazone that is formed can be compared to known values of DNP derivatives in an organic chemistry appendix and then matched to the respective aldehyde or ketone.
The next test, Schiff’s test, identifies whether or not the unknown compound is an aldehyde. The carbonyl group of the aldehyde is reduced to an alcohol and replaces a hydrogen at three positions on the Schiff’s reagent. The resulting compound is purple in color. Unknown #7 produced a deep purple color, indicating that it is very likely an aldehyde, while unknown #156, producing a very light color, was ruled out as an aldehyde. To double check the purpose of the test, the known aldehyde benzaldehyde was tested, and indeed produced a purple color.
Now that #7 has been established as a likely aldehyde and #156 as a likely ketone, we can aim the Iodoform test at the nature of the suspected ketone. This test determines whether or not the suspected ketone is a methyl ketone. It does this by trihalogenating the methyl group of the ketone with iodine. This group then leaves and is protonated to produce iodoform (CHI3). This compound exists as a crystalline solid (it’s melting point is approximately 120ºC), and should be visible in the test result as a precipitate. Testing of the known methyl ketone, acetone, produced a yellow liquid with precipitation. While testing of the suspected ketone did not produce a yellow liquid, it did produce a precipitate, indicating that unknown #156 is a likely methyl ketone. The brown color of the liquid in this case may be due to excess reagent, which does have a brown color.
The boiling points of the two unknown compounds were a stronger indicator of their identities. While different compounds may all come close to the experimental boiling points, the unknown identities can simply be then narrowed down by using the information from the previous tests.
The final test was the result of IR spectroscopy. While this test can provide many of the characteristics of the compound’s components, it is not as effective as some of the other tests in determining the unknown identities. This is because the differentiating characteristic to look for is the ketone group versus the aldehyde group. While the unknown can be distinguished based on this, the actual spectroscopy results of the unknowns show that both have a large peak near the wavelength of both ketone and aldehyde groups.
The most accurate indicator is the NMR results provided for the two unknown compounds. For aldehydes, there is a characteristic shift near 180-200ppm, which is present on one set of NMR results (labeled G) but not the other. In addition, the set lacking the aldehyde shift (labeled V) shows the characteristic shift for a ketone group around 35-45ppm.
These tests together indicated that unknown #7 is likely an aldehyde and that unknown #156 is a likely ketone. Their respective boiling points and DNP derivative melting points further suggest that unknown #7 is 2-furaldehyde, and that unknown #156 is 5-hexen-2-one.
While there are additional tests that could have been run on the unknown compounds, the ones used in this experiment were the most useful and effective. Since the unknown compounds were already provided as isolated compounds, bisulfite addition was unnecessary. Cyanide is a similar reaction, but is somewhat hazardous due to the toxicity of cyanide. Tollens’ reagent can produce a reaction and results similar to that of Schiff’s reagent, but the preparation of the Tollens’ reagent Ag(NH3)2OH is again somewhat hazardous due to the nature of the necessary silver nitrate.
ANSWERS TO END-OF-CHAPTER QUESTIONS*
9. Fluorenone has a ketone carbonyl C=O group, which should show a peak around 1705-1725cm-1. The IR spectrum shows a large peak around 1720cm-1, which is that of the carbonyl group.
10. In 2-butanone, there are three different hydrogen groups, as shown by the three shifts. From the structure of 2-butanone, we know that there should be one methyl group close to the C=O, one CH2 group close to the C=O, and another methyl group on the end of the structure away from the C=O. These groups should show shifts around 2.1-2.3ppm, 2.2-2.6, and 0.9, respectively. The NMR does show these shifts, indicating that the groups present (in order of decreasing shift) are the CH2 group, the methyl group close to the C=O, and the methyl group away from the C=O.
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