the microscale laboratory
Vitamin C Determination Using the lndophenol Method and Glucose Determination Using the Benedict's Procedure
Standard
Stock Conc.
Drops of Sample
Conditions
LED
red or green
Food Dyes -
0 4
cu2+
0 4
2 d household NH3
red
Vitamin C 80 ppma 0-5
1 d 0.1M Na citrate 1 d 0.1% 2,3 Cln-
green
0.03 M
indophenol Glumse C o p p e r Concentration (M) Figure 2. Standard curve for cu2+analysis using a microwell colorimeter. Food Dye
Portions ofblue food dye (6,5,4,3,2,1, and 0 drops) are placed into different wells and diluted to a volume of six drops by adding water. The resistance of each is measured and a plot of resistance versus volume of dye (drops)is generated. Analysis of cu2* in Solution
Six, five, four, three, two, one, and zero drops of a 0.03M standard CuS04 solution are placed into seven adjacent wells. The volume in each well is adjusted to six drops with water. Six droos of a CuZ+solution of unknown concentrainto another well. n o drops of household tion are ammonia are added to each well and the resistance of each well is read using a red, supra-bright LED. The nonlinear relationship between the CuZ+concentration and resistance shown in Figure 2 for the standard solutions is due to the characteristics of the CdS photocell and the logarithmic dependence of light transmitted on concentration. Once graphed, however,-the standard resistancdconcentration relationship is useful for determining the concentration of an unknown. Many spectroscopic experiments written for milliliter volumes can be proportionally reduced to the "drop" scale and run in a similar manner to the Cu2+analysis above. The table, for example, outlines the procedures for Vitamin C determination using the indophenol method (5)and glucose determination using the Benedict's procedure. Discussion
The microwell colorimeter is an inexpensive way to incorporate quantitative chemistry experiences into a high school or undergraduate chemistry curriculum. The only working components, i.e., the photocells and the LED'S, are cheaply and easily replaced when they fail. The low cost of the microwell colorimeter makes it possible to have several available in the laboratom so that students aren't required to wait fora s~nglespectrophotometer.Apotential source of error is the failure to "eveball" the oosition of the mierowell correctly, but studentsUwilllearn ;uickly to perform this alignment satisfactorily. A200
Journal of Chemical Education
0.4 g /I00 mL
0 4
5 d Benedict's soln. red 1 hr under heat lamp, 5 d transferred to new well
Crush a 500 mg tablet in 250mL Hz0. Dilute 10 mLta 250 mL.
A major benefit of the microwell mlorimeter lies in its pedagogical nature. Many undergraduate students treat the spectrophotometer as a mysterious "black box" that somehow should be capable of generating concentration data without the need for standards. The light transmission-resistance relationshio of the microwell colorimeter is easier for the students to inderstand, and they recognize the need for runnine standards because each instrument is unique in the data it generates. Literature Cited 1. B e 4 J. Chrmim1Explorofims:D.C . Heath and Co., Lexington, MA, 1993. 2. Waterman, E. L.: Thornpaan. S. Smoii Smie ChemisfryhboralwyManua1;AddiiiWesley Pub. Co.: Menlo Park, CA. 1993. 3. Beran, J. A. Chrmislryin ffiohbomfory;J. Wlley:New Yo&, 1993. 4. Wendlandt.W .W J. Ckem Edue. 1076,53,134. 5. Mass, L R.; Boibss, R. S. Chomiml Ptimipies in iho h b o m f o r y ,2nd 4.;Harper and k Publishers,New Yark 1981, p 289.
The Ritter Reaction: Trapping a Carbocation with a Nitrile R. David crouch' Department of Science and Mathematics Coker College Hartsville. SC 29550 Carbocations as reactive intermediates usually are demonstrated in oreanic chemistrv labs as unimolecular elimination (El)reakions (1,2)with the accompanyingskeletal rearraneements (3)or in solvolvtic kinetic experiments (4). As withumany instructional experiments, these reactions are illustrative in nature with little student discovew required. ORen, the experimental outcome can be pre&cted through similarities with examples given in the textbook or in lecture. In contrast, experiments that yield unpredictable products help students to develop analytical and problem-solvingskills. The advantages of discovely-based laboratory exercises have been discussed elsewhere (5-7). One such experiment is described here. Current address: Department of Chemistry, Dickinson College, Carlisle, PA 17013.
'
The Ritter reaction involves the in situ trapping of a carbocation intermediate by anitrile followed by hydrolysis in an aqueous workup to produce an amide (8-10).
-A+ Q = C - R
I
-
-A-N=sR
I
,
While the formation of a carbocation generally is recognized by students, the benzamide product usually is not predicted. However, the presence of an amide is clearly indicated by a strong carbonyl absorption in the infrared spectrum. Close examination of the structure of benzonitrile allows it to be seen as a potential nucleophile that could attack the carbocation urior to the formation of 2methylpropene via the more:familiar eliminat~onpath. IIvdrolys~salso can be viewed as nucleophilic attack on a carhoc-ation with amide formation occurring upon tautomerization. Xelds ol'pwified product range from 4060%. cumoanne favorahlv with the 6W virld reoorted in the iterakre students might be askLd to spe'culate on the side reactions that might diminish the yield.
(6.
Although amides are prepared more commonly by the reaction of amines with acid chlorides, the Ritter reaction is an easy method for the preparation of amides via stabilized carbocations. As one of the few synthetic routes to amides of N-tertiary alkyl amines (8), the subsequent hydrolysis ofthe amide to liberate an m i n e provides the best path to 3' alkyl amines (10). In this experiment, tert-butyl alcohol is treated with concentrated HzSOato form a 3" carbocation that is trapped by the nucleophilic nitrogen of benzonitrile (11).
A Representative Experimental Procedure In a 5-mL Erlenmeyer flask, 0.50 mL of benzonitrile (4.9 mmol, 1.00eq) and 0.50 mLof tert-butyl alcohol (5.3 mmol, 1.08 eq) are mixed thoroughly by swirling. The mixture is cooled in an ice-water bath to 0 C and 0.50 mL of concentrated HzS04is added carefully dropwise with swirling of the flask to ensure complete mixing. The reaction is removed from the cold bath and warmed to 40-50 'C in a sand bath. After 30 min, the cloudy, viscous mixture is transferred into a 25-mL beaker containing chipped ice and water. The white solid product that f o m is isolated by vacuum filtration. The crude product is dissolved in a minimum amount of boiling ethanol, cooled to room tem(Continued on next page)
Volume 71 Number 8 August 1994
A201
the microscale laboratory
perature, precipitated by the addition of distilled water, and isolated by vacuum filtration. The recovered mass of N-tert-butylbenzamide is 437 mg for a yield of 51%N-tertbutylbenzamide: mp = 131-133.5 "C; literature mp = 133135 "C (12). IR (CHC13,cm-') 3420,3050,1645,1505,1210. Literature Cited 1. Williamson, K. L. Mocmscole and Micrmmk Orgvnie Ezpriments; D.C. HeaUI: Lexington, MA, 1989; pp 171-180. 2. Mayo, D.W.: Plke, R. M.; BuWler, S. S.Micmsmlr Orgonichbordory; John Wlley: NewYork, 1986:pp 112-117. 3. Lehman, J.OpmliomiOganieChemislry;AllynandBamn: Boston, 1998;pp 1 6 s 177~ ~~
4. Rodig, 0. R.; Bell, C. E.: C1ark.A. K O w n i c Chemistry hbomtory: Saunders College Publishing:Phibdelphia, 1990; pp 203-209. 5. Kildah1,N.;Berka,L. H. J Cham.Educ. 1993,70,671. 6. Erwin,D. K. J Chem Educ. 1991,68,862. 7. Rieci, R. W.: Ditzler, M. A. J Chem. Educ. 1991,68.228. 8. f i m e n . L. I.: Cota. 0. J. OmanicRocfions 1969.17212425. . 9. Mvndy, B. ~ l k ; d .M. 0 . " Rpocfions ~ ~ and ~ Remen& in Oganie Chemistry; John Wiley: NewYork. 1988: n, 18CL181. 10. Mareh,J.Aduancd Oganie Chemistry. 4hed.; John W~ley:NelvYork,pp97G971. 11. Chtisto1.H.: Lsurmt.A.:Mouaseron.M.BuN. Soc. Chim. X). 1961.2313-2318.
.
PI
.
2. Landgrebe, J A . Thooryondhctice in IheOganieLnbomfory,4th ed.:BraaksiCole: Pacific Gmve, CA: 1993: p 31. 8. Williamson. K.L. Mocrosmle and Micmseda Org.nie Ezmn'mnts:DC Heah: Lerington, MA; 1989; p 15.
Manual Head Space Sampling for Gas Chromatographic Analysis Jeffrey A. Corltill and Kenneth W. Raymond Eastern Washington University Cheney, WA 99004 Gas chromatography (GC) is one of the most frequently used techniaues for the analvsis of volatile ~roductsderived from microscale organic chemistry experiments (18). Manual iniection of liauid samples is the most irre~roducible facecof GC anal& espekally for novice chromatorwil~hers. - . The usual method of iniectine a 1-2 JL liouid sample can frequently result in inappropriate injection volumes (either too large, resulting in massive solvent peaks that can obscure &ly anal& peaks, or too small, resulting in unreliable chromatograms). In addition, inadequate syringe cleaning can lead to inter-sample contamination. These factors can seriously affect the reproducibility and the accuracy of GC analyses. We have found that a manual head space sampling technique can alleviate or reduce most of the aforementioned problems of solvent injection. This head space method is particularly well-suited for use in microscale experiments where, frequently, only limited amounts of products are available. The syringe handling techniques of such head space sampling are much easier and present much less of an obstacle to obtaining meaningful chromatographic data for students. Tvoicallv. students lace one d m (40-100 ~ uL) of their mixmixt'ure f& analysis in a 1dram viai, seal the latter with a Teflon-lined caD and dace the vial.in an 60-80 OC oven for about 45-90 %'ThecLp i s removed and the needle of a syrinec (we use a 10-uL SGE rcolaceahln needle s e n g e with a gexible plungerj is positioned in the centerbf tLe vial. The oluneer is pumped twice and 3 5 uL of head mace vap& is &-awn ;ntoLthe syringe. The k p l e is inje'cted ra~idlvinto the GC iniector. The figure shows an example of; student chromato&am obtain; using this techniq;e. In our organic chemistry laboratory, manual head space sampling for GC has increased si@~cantly both the productivity of sample processing and the reliability of the quantitative analysis of mixtures of volatile organic compounds.
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A Simple But Accurate Pipet for Use in Microscale Organic Labs Gerald C. Swanson Department of Science Dayiona Beach Community College 1200 International Speedway Blvd. Dayiona Beach, FL 32114 Calibrated pipets can he made by students for use in dispensing small amounts of liquids in microscale organic chemistry experiments (13).These consist of disposable Pasteur pipets, which students calibrate during their first lab period. The calibration is usually limited to 0.5- and 1.0-mL volumes. The accuracy of the pipets is dependent on the quality of the student's calibration techniques. An inaccurately calibrated pipet results in incorrect volumes of liquids being dispensed throughout many of the subsequent experiments. Additionally, the tips of the pipets sometimes brake off, requiring the preparation of new calibrated pipets during later experiments. A simple and more accurate pipet can be constructed from l-mL serological pipets. Since an ordinary pipet bulb will not fit on this size pipet, an adapter can be made from a Pasteur pipet and a short piece of rubber tubing, as shown in the figure. Once made, this adapter can become part of the normal equipment stocked in the lab drawer. Literature Cited 1. Pauis, DL.; Lampman, G.M.; M z , G.S.; Engel. R.G.Inlmducfion faOrg.nieLnbomlow Technioues.A MiemseoleAoomoch. 3rd d.: Saunders:New Yark: 1990: o 44
Literature Cited
Adaptation of the serological pipet for microscale use A202
Journal of Chemical Education
1. Pavia. D. L.: Lampman, G. M.: M l , G. S.:Engel, R. G. Inkduction to OrganieLobom t w y TPchniwes. A Mtcmscok Appmoch. 3rd ed.; Saundera College Publishing: Philadelphia. PA, 1992: Experiments 8, 11, 14, 16, 18. 2. McRitFhiclkkly D.: Gab"elsm, B. Ogonie Chemistry in the Lnbomfory, 2nd ed.; KendalWunt: Dubuque. lA, 1985:Experimenta l6snd 28. 3. Maya, D. W.; Pike, R. M.; Butcher, S. S. MicrweleOganieLabwolory,2nd ed.:John Wtley: New York, 1989: Experiments 1.5.9. . C. Heath and CN Lexington, 4. Williamson, K. L.Miemsmla Oganie Ezprimnfs: D MA. 1987: Expetimenfs 11-13. 5. Peterson, T H.; Blyan, J. H.; Koevil. T A J. Chem Educ. 198% 70.A96.498. 6. Wigal, C . T;Hopkins, W T;Ronald, B. P. J Chem. Edue 1631,68,A229. 7. Lee. M. J C k m . Educ 1992.69.Al72A173. 8. Gmch, E. E. J. Chem Educ ISSO, 67.A232-A233.
