A Brief Freshman Experience in QualitativeOrganic Analysis John R. Griswold and Richard A. Rauner Cedar Crest College, Allentown, PA 18104 I n colleee settilws with separate freshman chemistry courses foiscience majors and "onmajors, the second semester nonmaiors' course is ofren an introduction to urganic and biochemistry. With such large fields compressed into a single term, the laboratory sessions must he carefully chosen to achieve maximum benefit. Although several versions of brief qualitative organic exercises are available (1-5) treating varying combinations of compound types and their reactions, no single lab experience can encompass a complete analytical investigation of the oxygen-containing functional groups. We recognize that most modern textsin nonmajors'orgauic-biochemistry courses introduce functional groups in a common seouence, proceeding from unsaturation t o hydroxy compounds and then t i carbonyl compounds. A cohesive well-timed laboratory with qualitative identifications of alkenes, alcohols, ethers, phenols, aldehydes, and ketones is therefore a valuable enhancement to the lectures. We have found that this experimental session fulfills this goal, being a most useful "best fit" with the established preferred order of functional group discussion. We have excluded carboxylic acids and amines because they are usually addressed later than the functional groups mentioned ahove. Also, acids and their derivatives usually merit a separate laboratory experience (6), while amines often present additional toxicity problems. Because we confine this experimentation to a 3-h laboratory session, the strategy is a flow chart sequence of semimicroscale chemical tests (Figure) providing unknown identification by reactivity comparison with known substances. It utilizes hands-on wet techniques and requires interpretation of accumulated data through deductive reasoning. Water solubilitv, combustion properties, and chemical reactivity are the focus of the experience, and upon completion of the ~racticalphase students consult a prepared list of unknowns i ~ a b l elj to select the compound that is most consistent with their results. If the flow chart is followed, each listed compound can be correctly identified.
Experimental With the exception of the ignition test and iodoformtest, all tests are run at room temperature. This investigation has nine procedures, and the first three can be run in any order. Tests 6 9 have a prescribed order that avoids false positive and ambiguous results and prevents unnecessary procedures that do not apply when certain preceding observations are negative. Each student or pair of students selects an unknown (all are liquids), and the determinations are carried out at designated rewent stations in the laboratorv. Each station includes a "known" cwnpound that gives a clearl) positive result with the renwnt m que3tion. Reagents and "knowns" are in amnll drc,pper bottles to minimize chances uf cmtaminarim. -
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Test 1: Water Solubility Three milliliters of water are placed in an 18- X 150-mm test tube, 5 drops of the unknown are added, and the tube agitated for 1min. If someof the compound is still present as a separate phase, it is considered insoluble at room temperature. This is a revealing test, since most oxygen-containing monofunctional compounds of up to
418
Journal of Chemical Education
Table 1. Organlc Ted Compounds Compound Type
Suggested
Unknowns
"~nowns"
Used
Water Sol. Flame Notes
24hylhexanal - clean ' ptolualdehyde sooty clean Ketones acetone ?-butanone cyclohexanone - clean propiophenone - sooty acetophenone - sooty sooty Phenols phenol phenol guaiacol sooty alcohol^ ethanol 1-hexan01 - clean 2-butanol l-propanol clean ted-smyl alcohol 2- or 3-pentanal + clean clean ter+butanol clean a Alkenes cyclohexene 1-octene plnene - sooty dl-sbml ether clean Ethers not needed phenetole waty a Alternate = valeraidehyde. Phenol liquifien with 10% water or ethanol added. * ~ . P B ~ O Ogives I a positive iodoform tsst, while 3-pntsnol dms not. If iodoform reaction on methyl carbinolo is not discussed prim to lab. ii is best to omit one of these aicohols. OAlternates = cyciohexene. carene. =Alternate= anisole. Aldehydes not needed
+ + + +
C-4 (alcohols to C-5) are water soluble. Exceptions are rare, with most compounds of C-5 or greater being polyfunctional in order to show water solubility,
Test 2: Ignition In the fume hood several crucible covers are mounted on clay triangles using tripods. A few drops of liquid unknown are placed in a crucible cover and a microburner flame is applied directly to the sample, igniting it. A clean flame, sometimes bluish due to oxygen content, reveals a fairly simple aliphatic substance with a high degree of saturation. A smoky, sooty flame shows polyunsaturation, as in aromatics or alipbaties with multiple double bonds. Some cyclic unsaturated alipbaties and complex polyeyclic structures with a relatively high carbon-to-hydrogenratio may also burn sooty. Test 3: Mild Perrnanganate Oxidation (Baeyer TesO Three drops of unknown are added to 2 mL of water or aqueous ethanol, and 1 drop of 2% KMn04 is added with shaking. In a positive test, as with aldehydes, phenols, and alkenes, the purple color is discharged with subsequent precipitation of brown MnOa. Other compounds in this analysis give negative results, although ethanol in the solvent will gradually generate s color change. An optional follow-up test using bromine can he carried out at this point in the fume hood (Test #9). Alkenes readily absorb the red bromine solution when it is added dropwise, giving a n instant clear solution. Phenols and phenyl ethers substitute bromine with release of hydrogen bromide, visible with agitation as a "fog" that will turn moist hlue litmus red when it is placed over the lip of the test tube. Phenols are verified, however, with the ferric chloride test (Test #6). Results of the permanganate oxidation test are used with interpretations of subsequent tests.
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green, indicates a phenol, which forms a deeply colored complex withFe(II1). An unknown givinga positive test will also have tested positive with permanganate and burned with a sooty flame.
Table 2.
Resaentsa
Test Number
Reagent Preparation
3
2% Aqueous potassium parmanganate: 8 g KMnO, in 392 mL of water: let stand overnight.
and filter into a dark bottle.
Test 7: Ceric Nitrate Test for Alcohols If Tests 3, 4, and 6 were negative, this test is used to reveal an alcohol. Three drops of the unknown are added to 7 or 8 drops of the reagent, with mixing. The rapid appearance of a red color, due to complexstion of cerium (N) with the alcohol, is a positive result. Most alcohols up to C-10 give the red color when mixed with the yellow reagent. Over variable time, the red changes to clear with reduction of the eerie ion to Ce(III), but it is the initial red eolor that signifies an alcohol (8,9).
2.4Dinibo~henvlh~dr~~Ine: . 0 sso YB 9 6 g of 2.44nnrophenyihydraz ns in 48 mL conc s~if.rlcacla Wolh sllrr ng, add tho$ soIUt on to s soiut on of 64 mL of water and 224 mL 01 95% ethanol. Filter if cloudy.
5
lndaform reagent: Dissolve 25 g iadine and 50 g potassium iodide in 200 mL of water. Ferric chloride, 5% aqueous:b 33.3 g FeCl, .6H20is disroived in 467 mL water. The reagent should be freshly prepared. Csrk nitrate reagent: With heating, dissolve 40 g cericammonium nitrate in a solution of 13 mL conc. nitric acid In 88 mL Of water. Lucas reagent: With cooling, dissolve 272 g of anhydrous ZnCir in 177 mL conc. HCI. Bromine in carbon tehachlaide. 5%: 6 mL of oromins in 223 rn- CCi.. Use only in hood
6
7
8
If positive, the alcohol type (I0,2', or 3') is determined with the Lucasreagent (Test #a). Test 8: Lucas Test for Alcohol Type This analysis is run only if the previous test was positive. Nine drops of the unknown are added to 25 drops of the reagent (caution! reagent is corraaive and toxic), the contents are mixed and the test tube is allowed to stand. The appearance of a cloudy suspension due to formation of the alkyl halide is a positive test. The time of formation reveals the type of alcohol, the reaction going through a carhocatian intermediate that forms very rapidly with tertiary alcohols. Therefore, a tertiary alcohol will produce a cloudy suspension within one min, secondary alcohols within 7 min, and primary alcohols fail to react within this time period. The test is useful for alcohols up to about (2.6, which are initially soluble in the reagent but yield insoluble halides that appear as the cloudy separate phases. The zinc chloride in the reagent catalyzes carhocation formation.
RJ-OH
ZnC1, + HCI -R,C-CI + H,O
If a secondary alcohol is revealed, the icdoform test may be run to see if a methyl is adjacent to the carhinol carbon. Test 9: Bromine Addition to Double Bonds If this test had not been run following a positive permanganate reaction and if the permanganate test is the only positive one thus far, this analysis should confirm an alkene. In the fume hood, 3 drops of the unknown are mixed with 2 mL of carbon tetrachloride, and the hromine-CC4 reagent is added dropwise with shaking. The red bromine eolor disappears on contact with the unknown alkene due to formation of the colorless dihromide.
Phenolsand phenyl ethers will also react wirh bromine in aruhstitutiun process, evolving HHr, which is dererted with moisr litmus. A phcnol, however, should have heen indirat~dhy a deep color in Test 6, and a phenyl ether should have failed the permanganate test. Ethers With the exception of phenyl ethers, which substitute bromine with evolution of HBr, ethers give negative results with all the aforementioned reagents, aliphatic ones being particularly unreactive. Therefore, anyone with the misfortune to have an unknown inert through the entire analysis can conclude the compound is an elher
Using all evidence, students consult the table of possible unknowns, which also includes water solubility and flame test data, 420
Journal of Chemical Education
..
4
9
m s amo,mo
r ren here u Ioeneat ma 50 slroenlr aqueou~reagent works well for me slrnpls phenols In this exercise. A mare ~nsaivereagent uses anhydrous ferric chlorlde in chloroform 181.
and establish an identification. The table does not include structures, students being requiredto look them up in the available references prior to reporting results. The final report includes test results with interpretations, equations for the positive tests, identification, and structure.
Dlscusslon If procedures are carried o u t a s described and t h e flow chart (figure) is followed, all compounds listed in Table 1can be correctly identified without t h e assistance of boiling point data. Since all test reactions are on a very small scale, many using only 13- X 100-mm test tubes without heating, many students are served a t minimal cost, hazard, and waste generation. We advise our students t o treat all reaeents and unknowns a s toxic, with all appropriate safety a n a disposal procedures being in effect. Table 2 summarizes preparation of the reagents needed. One is left free t o suhstitute various unknowns on t h e list in Table 1,but, in order for this exercise to succeed, t h e compounds used should be relatively simple, monofunctional, and pretested t o insure satisfactory res;lts. This is a precise, t i m e ~ l i m i t e d e ~ ~ e r i etnhcaet nevePtheless employs many of t h e important analytical reactions for organic functional groups. We have found it t o be a timely, instructive, and visually pleasing laboratory exercise for those freshman students who are not likely t o encounter this topic elsewhere in their academic programs. Literature Clted 1. Shskhsshiri, 9.Z;Hsighf GOP.;Hmm,M. P.;Dirreen, G.E.Manual!nrLnbomtom fmuesrigoaonsin G ~ ~ chrmistm: . ~ ~ I stipps:charnpaipn, IL,1976: pp 287-251. 2. Langsjoen,A.;Everett.G. W.;Lieder. P.:Lsts,A.J.Erp~rim~ntsin C e n m l , orgonie, and Biological Chemistry; Hsrcourt. Brace, Jauanovich: Nev York. 1988:pp 198-207. 3. Boschmsnn, E.: Wells, N. Chemistry in Action: A Lobomtory Manual /or cenami, Organic, and Biological Chemistry, revised ed.: McGraw-Hill: New York,1985; pp
-".
?f,.?E7
4. Lee. J. C.:Bette1heirn.F.A. LaboroloriiMonuol To AccompanyIntroduefiontoOon~r-
