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.
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-)...
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