A Mini-Qualitative Organic Analysis Session A. Johan H. Labuschagne, Johannes S. Malherbe, and Cornelius J. Meyer University of Stellenbosch, 7600, South Africa Qualitative organic analysis experiments are standard practice (1-6) in most organic programs. We have developed a thoroughly tested mini-scheme that works for the selected unknowns and enables u s to introduce our nonmajor chemistry students to a variety of functional group tests i n a single laboratory session. To facilitate progress during the three-hour session, flow charts1 for water soluble and water insoluble compounds are made available. Once the student has determined the latter property, the relevant flow chart will allow identification based on chemical reactivity. The order of tests i s not rigid and can be rearranged on the chart if the reactivity of each functional group is always taken into consideration. Our students are instructed to practice either individually or in pairs on the known compounds provided. Students who directly attempt to identify their unknowns, often, due to a lack of knowledge, misinterpret their results when they try to match their tests to the flow charts. We have found that students who do their practice runs i n pairs are more conscientious and often have enthusiastic discussions. They, thereafter, seem to tackle their individual unknowns with more confidence, rather than to view i t a s a daunting "unknown" prospect. The traff~cin the laboratory also is less and a savings on chemicals is achieved.
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I t i s to be stressed that the success of a n unambiguous identification requires that the chemical procedures are r u n i n the specified order, because the flow charts are based on a urocess of elimination. Methodology for reagents not prepared according to standard procedures (1,5, 61, is described fully i n the experimental section. Test 1: Water Solubility Five drops of the unknown are added to 2 cm3 of water. shake viporouslv and allow to settle. ~h~ unknown organic compounld is considered to be soluble if it dissolves~ompletely, ~h~ mixture is homogeneous. Test 2: Iron(lll) Chloride Test Five drops of a 1% aqueous FeC13 solution is added to 1 cm3 of the unknown. In the case of phenol, a n immediate purple coloration is a positive test. All of the other possible unknowns in this series give a negative test.
Experimental Students a r e warned to treat all chemicals as potentially toxic. All tests are conducted a t room temperat u r e unless otherwise stated. All t h e unknowns a r e liquids,' and each student receives two different numbered unknowns, one of which will be water soluble (flow chart A) and the other water insoluble (flow chart
B)? Alist4 of all possible unknowns is included in the flow charts. The attention of the students is drawn to the fact that the 1-alkyne used is, i n fact, 2-propyn-1-01 (HC=C-CH2OH) which is both a 1-alkyne and a primary alcohol.
~ethanol/~thanol/ PropanonejZ-Propyn-l+l
2-~eth~l-d-propanol Confirm !vith (K2Cdr. &SO, 14)
12, NaOH ( 5 )
'we have found the flow-chart approach the most enicient procedure for students with limited analytical experience. Z ~ l lthe possible unknowns are supplied in pure form, with the exception of phenol, which is a 1% solution in water. 3The more observant student, having done the preliminary tests and recordings thoroughly, will be able to use the density of some of the unknowns as 2-Propyn-l+l Propanone Met Jan01 an additional aid, e.g., I-bromobutane is denser than water. 4All the possible unknowns are listed in flow charts Esteridcation con& with Hydrdzone A and 6. This saves the student some time in not (10) unsaturation formation (6) havina to consult a seoarate list of "oossibles"for the tests (a), (9) best Gt. Students a150 act~velycompare the~runknown results wlth those of the relevant test cornpounds in thew notes Flgure 1 Flowchart Afor water soluble unknowns
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Ethanol
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Esteridcation (10)
Test 3: Lucas Test
Test 6: Formation of 2,4-Dinitrophenyihydrazones
A thorough investigation of this test recently has appeared (7). Add 10 drops of the unknown to 25 drops of Lucas reagent without mixing. I n our experience 2methyl-2-propanol produces a n immediate cloudiness that is a positive test. All of the other compounds in this series give a negative test.
Five drops of the unknown is added to 25 drops of the 2.4-dinitrophenylhydrazine reagent and mixed thoroughly. In a positive test, propanone produces a fine, yellowishorange precipitate within 3 min. Benzaldehyde immediately produces a bright orange hydrazone, and if acetaldehyde is included a s a n unknown in flow chart A, a yellow precipitate is immediately formed in a positive test. None of the other unknowns give this test.
Test 4: Oxidation with Dichromate
Carefully add 2 drops concentrated HzS04to 25 drops of a 5% aqueous KzCrz07 solution. Add 5 drops of the unknown to this reagent. Mix carefully after each drop is added. Ethanol reacts immediately to yield a positive bluish-green solution (4). Benzaldehyde reacts positively, but is insoluble and tends to form a brownish-green layer on the reagent. Propanone and 2-methyl-2-propanol show no sign of reacting even after standing a t room temperature for two hours. The unchanged orange color of the dichromate solution is a negative test (4). Methanol and phenol also give positive tests in this series.
Test 7: Test for the l-Aikyne with the Siiver-Ammonia Complex Ion [Ag(NH&)N03
The silver-ammonia complex is prepared a s follows just prior to running the test. Add a -1 M ammonia solution slowly with mixing to 1em3 of a 5% aqueous &NO3 solution until the initial brown precipitate of AgzO just dissolves. Add this complex dropwise to 10 drops of the unknown until a permanent white precipitate is formed in a positive test. The sequence of the flow chart must be followed for the successful identification of 2-propyn-1-01, which, in addition to the above, gives positive oxidation and unsaturation tests. Test 8: Bromine Test for Unsaturation
Test 5: lodoform Test
Amixture of 10 drops 5% aqueous NaOH solution and 10 drops of the unknown is carefully heated to boiling point. Iodine reagent is then added dropwise until a permanent pale yellow precipitate of iodoform is obtained in a positive test. No precipitate is formed in a negative test. The only visual result is a dilution of the color of the iodine reagent. Propanone and ethanol are the only unknowns giving positive tests. In our experience ethanol is slow to react in the cold (I, 4) and we have found our students are less prone to producing a negative result if the test is run a s described above.
The Brz, C C 4 reagent is added dropwise to 10 drops of the unknown. In a positive test (cyclohexene and 2-propyn1-01] the reddish-brown color of the reagent immediately disappears as the addition reaction takes place. If excess reagent is added, the solution takes on the diluted color of the reagent a s in the case of a negative test. Test 9: The Baeyer Unsaturation Test
Add 5 drops of the unknown to 1em3 of a 0.5% aqueous KMn04 solution. Cyclohexene and 2-propyn-1-01 give a n immediate brown precipitate (the solution is initially a brown color, whereafter the brown precipitate is deposited) of MnOz in a positive test. Test 10: Esterification of Methanol and Ethanol
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Hydraaone formatron (6) Olddation (4)
ConfiA with unsaturation test (8)
Methanol Add -0.1 g (a spatula point) of salicylic acid followed by 2 drops of concentrated HzS04to 10 drops of the unknown. Mix thoroughly and carefully heat to the boilina point. Cool the mixture and carefullv observe theEharacteristic odor of wintergreen oil (methyl salicylate, after pouring the mixture onto ice in a ckcihle and carefulfy waft?ng the vapors toward your nose. Ethanol Add 15 drops glacial acetic acid and 2 drops of concentrated HzSO4 to 10 drops of the unknown. Mix thoroughly and carefully heat to boiling point. Cool the mixture and pour onto water containing ice in a crucible. Carefully waft the vapors toward your nose and observe the odor of the ester (ethyl acetate). Test 11: Toliens Test
~liminatiohtest
(13)
Figure 2. Flowchart B for water insoluble unknowns
The Tollens reagent, Ag(NH&OH is prepared as follows. Add 2 drops of a 5% NaOH solution to 2 cm3of a 5% &NO3 solution in a clean test tube. Add -1 M ammonia solution dropwise until the initial precipitate formed just dissolves. Add 5 drops of the unknown to the Tollens reagent and heat in a waterbath a t 60 "C for up to 10 min. A silver deposit on the walls of the test tube is a positive test. Volume 71 Number 12 December 1994
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Test 12: Nal, Acetone Test
It is essential to use a dry test tube for a successful test. Add 2 drops of the unknown to 25 drops of the NaI, acetone reaeent. 1-Bromobutane ~ v e as ~recivitatewithin 3 min at room temperature a n d k t h i n l mih in a waterbath at 50 "C. Students often heat their test mixtures for low penods causing the acetone to evaporate and thus precipitating the NaI in a false positive test.
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Cool and acidify with dilute HN03 (litmus paper). Test for the halide with2 drops of a 5% AgN03 soluiion A creamywhite precipitate is as a suspension in a positive test, ~i~~~~~~~~ cited
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3.
4. ~
Test 13: Elimination of Halides with Alcoholic KOH
5.
KOH Add drops of the unknown to 25 drops of a solution in ethanol and carefully heat to the boiling point.
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Gtiswold, J. R.; Rauner, R. J. Ckm 68, 41M20. ~R A J~ ckem~ E ~ U~C1890,67,516517. ~ , ~ i f ew, K J cham. ~ d 196845,416418. w Siluert. 0. J. J.Cham. Educ 1981. M. 871472. ~ ~ ~ , ~ ~ , ~ Hart, H.; Craine, L. E. Ogonic Ckmidry. A Shod Coum:Lobomtory Manual, 8th ed.: Houghton Miflin Company: Boston, 1991. Williamsm, K L. Mocmsmb and Micmsmle Organic E x p t i m n f s : D. C. Heath and Company: Lex%@m., 1989. % ~ n ~ a a , ~ . ~ . ; l ( l e d f oJr d them. , ~ . ~ due 1 ~ ~ 1 , 6 8 , 7 w 7 0 6 .
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