Abstract:
A qualitative analysis experiment was conducted to determine the identity of an unknown aldehyde or ketone. The tests include a Tollens test, a Schiff test, an iodoform test, and a derivative melting point test. The data of the first three tests was inconclusive. The final derivative melting point test was utilized to successfully determine that the unknown was the ketone, Propiophenone.
Introduction:
Qualitative analysis is a method or series of methods used to determine the identity of an unknown compound (1). It is imperative that two main rules are always followed when conducting a qualitative analysis (1). The first rule is that the instructions must be followed exactly. The second rule is that a known positive test and a known negative test should always be performed for a comparative visual analysis (1). If these rules are broken, the analysis will give an inconclusive result (1). Several different fields depend on qualitative analysis to aid in their ability to identify organic chemicals. For example, atomic absorption spectrometry is used by toxicologists to check for cadmium and lead and other chemicals in potable water (2). Also, gas chromatographic-mass spectrometry is used by biologists and agricultural scientists to check soil for volatile organic compounds (VOC’s) (3). In addition to these, qualitative analysis is useful for determining medical diagnoses, developing industrial materials, and for pharmaceutical and forensic research (4). This experiment focuses on determining if an unknown chemical is an aldehyde or a ketone. This determination is made based on the results of a Tollens Test, a Schiff test, an iodoform test and a derivative melting point analysis. An aldehyde is a Carbonyl group that is attached to a carbon atom and a hydrogen atom (5). Aldehydes are useful as perfumes, flavoring agents, dyes and in plastic development (5). One of the most important industrial aldehydes is formaldehyde (5). A Ketone is a Carbonyl group that is attached to two carbon atoms (6).
Ketones are utilized in industrial solvents, dry erase marker ink, perfumes, and plastic development (6). The monosaccharide fructose is a ketone which combines with glucose to form what we know as table sugar (sucrose) (6). The first test performed was a Tollens Silver Mirror test. This test included creating a known positive and a known negative reaction with a Tollens reagent test solution (1). The unknown was then tested and compared to the known positive and known negative solutions to determine the unknown identity (1). The Tollens reagent is prepared in the laboratory because it will become explosive upon standing (1). The first part of the preparation follows equation number 1. This creates a brown precipitate of silver oxide (AG2O) (1). This precipitate is then treated with ammonia until all of the brown AG2O has reacted and the solution is clear (1). This reaction results in a silver solution that contains silver ions. These ions will oxidize as presented in equation 2 (1). A positive on this test is either a silver mirror forming on a test tube or a heavy black precipitate (1). A negative indicator for this test is a clear solution (1). Equation 1:
2 AgNO3 (aq) + 2 NaOH (aq) → Ag2O (s) + 2 NaNO3 (aq) + H2O (l) Equation 2:
The next test performed was a Schiff test for an aldehyde. The Schiff reagent will indicate positive for an aldehyde with a purple fuchsia color. The negative will result in a solution with a yellow hue. The unknown is then compared to the known positive and the known negative to determine if it is an aldehyde (1). The chemical equations for this test are complex and are not yet understood (1). The next test was an Iodoform test. This test is to determine if a molecule is a methyl ketone (1). This test is carried out by oxidizing a methyl ketone with I2 as seen in equation 3 (1). Elemental iodine is produced in equation 4 by reacting hypochlorite (OCL-) with iodide to oxidize the iodide (1). A known positive and a known negative are prepared and the unknown is compared to them to determine if it is a methyl ketone (1). A positive on this test is indicated by the formation of a cloudy light yellow precipitate in the solution (1). Equation 3:
Equation 4:
2I-(aq) + OCL- (aq) + H2O (l) I2 + CL- (aq) + 2OH- (aq)
The final test performed was a semicarbazide derivative melting point analysis. This test includes forming a derivative of the unknown with a semicarbazide to form a derivative called semicarbazone (1). After the derivative is formed, the melting point is determined (1). This information was then compared to known literature values of semicarbazone derivatives to determine the identity of the unknown (1). Semicarbazone is derived by reacting an aldehyde or ketone with semicarbazide as seen in equation 5 (1). Semicarbazide hydrochloride is then reacted with sodium acetate to obtain semicarbazone as in equation 6 (1). Equation 5:
Equation 6:
Experimental procedure
To perform the Tollens test, 3 test tubes were labeled as “known positive”, “known negative” and “unknown” (1). Two ml of 5% AgNO3 and 1ml of 10% NaOH was added to each test tube. This created a brown Precipitate of Ag2O (1). The precipitate was dissolved by adding ammonia (NH4OH) dropwise until the solution was clear (1). This required 9 drops of ammonia. After each solution was clear, one drop of o-tolualdehyde was added to the “known positive” test tube (1). Next, one drop of 2-butenone was added to the test tube labeled “known negative” (1). Finally, two drops of the unknown was added to the tube labeled “unknown” (1). The unknown was then compared to the “known positive” and the “known negative” to determine if an aldehyde was present (1). A mirroring on the test tube or heavy dark precipitate in the solution was considered a positive test, while a mostly clear solution was considered as a negative test (1).
The Schiff test was performed utilizing a Schiff reagent (1). To perform the Schiff test, again, three test tubes were labeled as “known positive”, “known negative” and “unknown” (1). One ml of Schiff solution was then added to each test tube (1). Following this, one drop of o-tolualdehyde was added to the “known positive” test tube, one drop of 2-butenone was added to the test tube labeled “known negative”, and one drop of the unknown was added to the tube labeled “unknown” (1). The unknown solution was then compared to the “known positive” and the “known negative” to determine if an aldehyde
was present (1). A positive for the Schiff test was a heavy purple or fuchsia solution coloring, while a negative was a yellow solution.
The Iodoform test was accomplished using with an Iodoform reagent (1). To perform the Iodoform test, three test tubes were labeled as in the previous two tests (1). Two ml of Iodoform solution was then added to each test tube (1). After the Iodoform solution was in all three test tubes, they were all heated in an 80◦C water bath for 30 s (1). One drop of 2-butenone was added to the “known positive” test tube, one drop of o-tolualdehyde was added to the test tube labeled “known negative” and two drops of the unknown was added to the tube labeled “unknown” (1). The unknown solution was then compared to the other two test tubes to determine if a Methyl Ketone was present (1). A positive result for methyl ketone was a cloudy light yellow precipitate, while a negative was a translucent yellow solution (1).
The final test was to determine a melting point using a semicarbazone derivative (1). Three ml of 95% ethanol and Three ml of distilled water was mixed together and placed in the ice bath to create a rinsing solution (1). After this, One ml of 95% ethanol was placed into a test tube and 5 drops of the unknown solution was added to it (1). Next, .15 g of anhydrous sodium acetate and .15 g of semicarbazide hydrochloride were placed in a test tube and 2 ml of distilled water was added (1). This contents of this test tube was added to the test tube containing the unknown mixture (1). This test tube was placed in the ice bath to cool for 10 minutes (1). It was shaken for 5- 10 seconds every minute that it was in the ice bath. After precipitate formed in the reaction tube, it was poured into a Büchner funnel, vacuum filtered and rinsed with 25 ml of distilled water to obtain the initial semicarbazone precipitate (1). The semicarbazone precipitate was then cooled to room temp and reheated in a 2 ml of ethanol/2 ml water mixture until the precipitate dissolved (1). Again the test tube containing the dissolved precipitate was placed in the ice bath, recrystallized, and filtered (1). These crystals were rinsed with the previously prepared ethanol water mixture (1). The derivative was allowed to dry and was tested for a melting point (1). This melting point was compared to literature values to determine the identity of the unknown (1). Results:
The unknown was determined to be propiophenone. This was based mostly on the given boiling point range data and the data derived from derivative melting point test. The results of the Tollens test, the Schiff test, and the iodoform test were ambiguous and inconclusive. One Tollen’s test was positive for aldehyde while the Schiff test was negative for aldehyde. In addition to this, the iodoform test was negative for a methyl ketone, which indicates an aldehyde since those were the only options available from which to choose. The results of the three tests can be seen in chart 1. CHART 1:
KNOWN POSITIVEKNOWN NEGATIVEUNKNOWNCOMMENTS
TOLLENS TEST MIRRORING ON TEST TUBE WITH A SMALL INSOLUBLE LAYER ON TOP OF SOLUTIONCLEAR WITH VERY LITTLE DARK PRECIPITATEHEAVY DARK PRECIPITATE WITH SMALL INSOLUBLE LAYER ON TOP OF SOLUTIONPOSITIVE FOR AN ALDEHYDE SCHIFF TESTPURPLE SOLUTION WITH A SMALL AMOUNT OF PRECIPITATEYELLOW SOLUTIONYELLOW SOLUTIONNEGATIVE FOR AN ALDEHYDE IODOFORM TEST CLOUDY SOLUTION CONSITING OF A LIGHT YELLOW PRECIPITATETRANSLUCENT YELLOW SOLUTIONTRANSLUCENT YELLOW SOLUTIONNEGATIVE FOR A METHYL KETONE
The Tollens test on the unknown produced a heavy dark precipitate. The known positive produced a mirrored glass with a small insoluble layer on top. The known negative was clear with a small amount of dark precipitate. The unknown developed a large amount of dark precipitate which can indicate a positive for an aldehyde. When the unknown was compared to the known positive and known negative it was determined that the unknown test indicated the presence of an aldehyde. The result of the Schiff test on the unknown was a translucent yellow solution. The known positive reaction yielded a deep purple solution while the known negative gave a yellow solution. When the unknown solution was compared to the known positive and known negative it was discovered that the Schiff test indicated negative for an aldehyde. The iodoform test on the unknown resulted in a translucent yellow solution that was most like the known negative solution. The known
positive developed a light yellow precipitate in a cloudy solution while the known negative was a translucent yellow. When the unknown was compared to the known positive and the known negative it was determined that this test was negative for a methyl ketone (positive for an aldehyde). The derivative melting test yielded a melting point of 172.4◦C. This information, paired with the given boiling point range of 218-230 for the unknown, indicated that the unknown was phenylpropanol, m-methoxybenzide, propiophenone, or butyrophenone. The data for the four possible unknown chemicals can be seen in chart 2 and 3. The aldehyde with a lower melting point than the unknown was disqualified due to the principle that an impure compound will have a lower melting point than a pure compound (5). This means that only the compounds with the higher melting points were still qualified to be the unknown compound. The melting point of 170.3◦C – 172.4◦C is closest to the melting point of propiophenone. Therefore, based on the result of the derivative meting point test, the unknown compound is propiophenone. CHART 2:
POSSIBLE ALDEHYDESBOILING POINT ◦CSEMICARBAZONE MP ◦C
3 PHENYLPROPANOL224127
m-METHOXYBENZALDEHYDE230205
CHART 3:
POSSIBLE KETONESBOILING POINT ◦CSEMICARBAZONE MP ◦C
PROPIOPHENONE218174
BUTYROPHENONE230187
Based on the outcomes of two of the four tests indicating an aldehyde and two tests indicating a methyl ketone, it cannot be determine with a significant level of certainty if the unknown is an aldehyde or a ketone. If the unknown were a methyl ketone it would mean that the Tollens test and the Schiff test gave a false positive for aldehyde. If the unknown were an aldehyde, it would mean the iodoform test gave a false positive and that the experimental melting point is around 33◦C below the literature value of 205 for m-methoxybenzaldehyde. Based on the given boiling point data and the melting point evidence it was determined that the unknown chemical was
propiophenone. This determination indicates that the Tollens test gave a false positive for aldehyde. In addition to this, the Iodoform test resulted in a false negative reading, also indicating that the unknown was an aldehyde.
The false negative on the iodoform test can attributed to the insolubility of the propiophenone in the iodofrom test reagent (8). Propiophenone is a methyl ethyl ketone, which will not dissolve well in the iodoform reagent (9). Any chemical that will not dissolve well in iodoform test reagent can yield a false negative (1). The false positive on the Tollens test could be attributed to two possible reasons. First, the experiment did not call for the reagent to be filtered. However, the inclusion of this step after the addition of the ammonia could lead to fewer false positives (10). The second reason is that propiophenone has a tendency to be oxidized by strong oxidizing agents (11). Either one of these or a combination of both could have led to the false positive on the Tollens test. In addition to the procedure that was performed, some changes could be made to increase the researcher’s certainty of the identity of the unknown. First, several trials of each test could be administered to ensure that they are being conducted properly. This could also show a trend in outcomes if the results could be repeated. Another option could be the addition of spectrometry or other reagent tests to help narrow down the identity of the unknown. Conclusion:
The initial data from the Tollens test, the Schiff Test and the iodoform test did not narrow down the identity of the unknown. This initial data was contradictory and actually indicated for an aldehyde while the unknown was actually a ketone. However, the final test of measuring a derivative melting point successfully pointed to the correct chemical.
This experiment was useful in showing how false negatives and false positives can affect the determination of an unknown chemical. The failure of two of the four tests shows that it is imperative that the researcher understands the reasons that a test can fail. The researcher should also understand the properties of the chemicals being tested and how they can
interact with the test reagents. The selection of several different methods of finding the unknown is vital in revealing its final identity. Improvements to the procedure could include attempting several trials of each test and also addition of more tests, such as spectroscopy, could be added to decrease the uncertainty even more.
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