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Friday, October 7, 2011

eulogy

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Abstract


Two unknowns were attempted to be identified by a series of solubility tests, followed by a series of functional group tests. Derivatives of the compounds were to be formed to confirm the identity of the substances. The solid did not have a melting point, rather, it sublimed. The solid and liquid were tested for solubility in water, 5% NaOH, 5% NaHCO, 5% HCL, and HSO4. The liquid was found to have a boiling point of 184°C. The liquid was tested using the beilstein test for halogens, the ferric chloride test for phenols, ,4-DNP for carbonyls, Br/CCl4 for multiple bonds, and the Tollens test for aldehydes. No derivative could be formed with the liquid because the sample evaporated. It was concluded that the liquid was benzaldehyde, which has a BP of 17°C.





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Introduction


Qualitative analysis of unknown compounds is based on functional group chemistry. Several different classification tests can be performed on a substance to determine the functional groups that are present, or absent for that matter. For example, the Beilstein test, tests for halogens. A copper wire is heated and then coated with the sample and then heated again. If a green flame is present, this is a positive result. The green flame is the result of the production of a volatile copper halide ().


The most important tests that should be performed initially are the solubility tests. These tests give important information about the functional groups present. For example, compounds that contain 4 or less carbons, and contain oxygen, nitrogen, or sulfur are usually soluble in water (). A flow chart on page 44 of the lab text outlines the procedure to follow for the solubility tests. Identity of unknown compound can be concluded by logical elimination of certain functional groups, and by using the results of each classification test to narrow the possibilities.





Results and Discussion


The tests and results for the solid unknown #6 are located in table 1.


Table 1
































Solubility tests


According to the chart on p. 44 of the lab text, the solid probably does not have a low molecular weight, and is not a carboxylic acid or amine because it was not soluble in water. Furthermore, it was also indicated that it is not a carboxylic acid because it was not soluble in NaOH. NaOH insolubility also suggests it is not a phenol. The solid was not soluble in HCl, which suggests it is not a base, such as an amine. The solid was soluble in HSO4, which indicates that the solid is a neutral compound, or inert.


Possibilities based on solubility tests


· Alkenes


· Alcohols


· Ketones, aldehydes


· Nitro compound


· Esters, ethers


· Alkyl halides


· Aromatic compound


Classification tests


The solid was first tested for multiple bonds to eliminate or indicate an alkene. The tests is based on the reactivity of a double bond with the addition of bromine


The solution of Br/CCl4 and the solid did not become colorless, which indicates that the compound does not contain a multiple bond between carbons.


The solid was then tested for an aldehyde or ketone with the ,4-DNP test. The color of the precipitate formed is used as a guide to the type of functional group present. A yellow precipitate usually indicates an unconjugated ketone or aldehyde (). A red precipitate indicates conjugation. The solid tested produced a yellow-orange precipitate. From the results of the saturation test, which suggested no multiple bonds, it can be concluded that the solid may be an unconjugated aldehyde or ketone.


The Tollens test was used to differentiate between an aldehyde and a ketone because of the highly positive ,4-DNP test. The Tollens test is based on the fact that aldehydes normally produce a precipitate of silver metal by reduction of ammoniacal silver nitrate


The solid did produce a heavy silver precipitate on the sides of the glass test tube. Because ketones do not normally react in this test, one can conclude the solid is an aldehyde.


Derivative formation


It was concluded from the classification tests that the solid is an aldehyde, however no derivatives were formed. ,4-DNP and semicarbazone were attempted to be produced. The procedure for ,4-DNP was repeated twice with extra precision. Upon consultation with Mrs. Collins, the lab manager, it was concluded that the second batch of ,4-DNP was contaminated or produced incorrectly. The second batch was compared to the first and they were clearly different. There was not enough time to repeat the procedure a third time with new ,4-DNP.


The solid did appear to have a melting point. The melting point was attempted of the Fisher-Johns apparatus and the Mel-Temp apparatus twice. The solid appeared to sublime, which occurs when a solid goes directly to the vapor phase. This may have occurred because the vapor pressure of some solids depends on the temperature of the surroundings.


According to the appendix of derivatives on p. A of the lab text, the most likely possibility for the compound is pentanal. This was concluded because it was the only aldehyde that did not for semicarbazone. Also, according to the HO solubility test, the solid most likely has more than 4 carbons. Pentanal is not conjugated and has no double bonds, which was suggested by the ,4-DNP and the saturation test. The structure for pentanal is


Table


Test Positive result Result Comment


Solubility tests


The liquid was soluble in water, which suggests the compound contains 4 or less carbons, and may contain O, N, or S. If a compound is soluble in water, the Ph of the liquid should be considered. The liquid did not change the color of the red or blue litmus paper, so the compound is neutral. The compound was soluble in both NaOH and NaCO, which indicates a phenol, or carboxylic acid.


Possibilities based on solubility tests


· Carboxylic acid


· Phenol


· Low MW neutral compound


Classification tests


The liquid was tested for a phenol with the ferric chloride test. Red, Blue, or purple colors are produce in this test from the formation of complex phenols with the Fe(III) ion (). The liquid did not produce any colors. The liquid was then tested for the presence of a halogen with the Beilstein test. The Beilstein test is positive with the production of a green flame. The color is produced from the formation of a copper halide (). This test was also negative.


Because the litmus test indicates a neutral compound, and the ferric chloride test eliminated the possibility for a phenol, the liquid was tested for an aldehyde or ketone. The solubility of the unknown in water may have been due to the presence of a double bonded O. The ,4-DNP test was positive for the presence of a carbonyl. Furthermore, the Tollens test was positive for an aldehyde, giving the same result as the unknown solid. The saturation test, Br/CCl4, was positive for multiple bonds.


No derivatives could be made with the liquid because the sample appeared to have evaporated. Based on the BP of 184°C, and the above data, the unknown was concluded to be benzaldehyde, which has a BP of 17°C











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