the microscale laboratory
2. Reser, L. F:Wfliamson, K L. Orponir Erpetimenfs.3rd ed., D.C. Heath and Company: Lexington,MA, 1975, pp 3 ~ 4 6 : 3. Mahrig, J. R.: Neckere, D.C. Lobomlory Ezptimenfs in Organic Chemlatq. 3rd ed., D. Van Nostrand Company: New York. 1979. pp 527533.
Small Scale Electrophoresis Helen B. Brooks Synaps Lincoln, NE 68510 David W. Brooks University of Nebraska Lincoln, NE 68588
Lunetta and Doktycz ( I )report a student experiment in which five 9-V batteries are connected in series to serve as the power supply for a small scale gel electrophoresis. Here we report a simplification of their procedure. Gel electrophoresis is a technique in which components of a mixture are senarated from one another on the basis of differencesin charge and attraction to a gel phase. Electronhoresis is traditionallv used to s e ~ a r a tmolecules e that are'too large to be separated by faster techniques such as liquid or gas chromatography. DNA fingerprinting, which is providing evidence of identity in many court cases, involves an electro~horeticse~aration.Sickle cell anemia is diagnosed hy running an el&trophoresis of hemoglobin. At the DH tested. normal hemoalobin and sickle cell hemoglobin are attracted to opposite~electrodes. During electrophoresis, ions with the same charge may be separated because they interact differently with a gel. These interactions may be ion-ion, hydrogen bonding, or London dispersion interactions. Buffers are selected to optimize separations. Buffers may change the charge of the species in solution. In this experiment, food coloring is separated. All of the dye components are negatively charged from pH 3 up. The experiment is run in very dilute basic buffer to speed the separation. Separation does take place in water-softened tap water of pH 7.5. [Tap water varies considerably in pH depending on the locality.] Caution: Warn students not to short the electrodes bv conncctmg dwertly opposne w~resof the hattery A danzerous amount of heat la released! Also, raurlon axamst burns during the preparation of the gel.
Schematic of electrophoresis apparatus. A28
Journal of Chemical Education
Connect five 9-V batteries in series (to produce 45V). Solder or connect the black wire of one connector to the red wire of the adjoining connector. Continue until five battery connectors are in series with one open red end and one open black end. Tape all exposed wires to prevent short circuits. Do NOT connect the last black end to the first red end. Insert the batteries in the connectors. Tape the five batteries together. . Mix 1mL vinegar with 2 mL household a m m o ~ aDilute this mixture to 300 mL with tap water. Add 1teaspoon of unflavored gelatin to 50 mL of the buffer. (Or add 1envelope gelatin to 150 mL buffer.) Allow the gelatin to swell for 5 min. Heat to near boiling. This solution bumps even when a toothpick is inserted as a boiling chip. Watch carefully. Remove from the heat. Stir occasionally until all gelatin is dissolved. Cool for 5 min. Cut the top from a straight long stemmed plastic transfer pipet. Fill the bulb of the pipet half full with the gelatin mixture. Wiggle the pipet to begin flow. Allow flow through the pipet until all bubbles clear the pipet stem. Halt the flow by bending the tip up to the level of the gelatin in the bulb. Hold an ice cube on the tubing near the tip to chill the gelatin. When the tube feels firm, slowly lower the tip to determine whether the gelatin is still flowing. (Continue cooling if it does flow.) Once the gelatin stops flowing, straighten the tip and chill the rest of the tube. Some gelatin should remain in the bulb to keep the gel in place during electrophoresis. Set the tube in a buret clamp on a ring stand. Allow the gel to harden at room temperature. (This mixture is more concentrated than gelatin recipes that require refrigeration.) Clean up any gelatin and water that spilled during preparation of the gel. Check the bottom tip of the pipet. If the gelatin does not fill completely to the bottom, cut the tip with a sharp scissors. Mix one drop of red and one drop of blue food coloring on a flat surface. (Yellow food coloring may be included, but it is difficult to discern through the translucent pipet unless examined very closely. Green food coloring is a mixture of blue and yellow food colorings. Schilling blue and red food colorings list components on their labels.) Immerse the tip of the pipet into the food coloring mixture for 1min. Remove and wipe the outer portion of the plastic pipet tip with tissue paper to remove excess. Do not touch the gel. Allow the s a m ~ l to e diffuse into the eel for 2 min. Clamp the gel pipet with a buret clamp. Place 0.5 cm (114 in.) of buffer in a container (100-mLbeaker or a small Pctri dish work well). Connect the positive (red wire) to a wire immersed in buffer at the top of the pipet. Tape the connection to prevent touching the black connector. Connect the black wire to a wire immersed in the container of buffer. Clamp the wire to the side of the buffer container if necessary to keep it in place. Lower the pipet carefully into the buffer until the tip is covered (See the figure.) Do not disturb the tip. Do not stir the solution. Record observations at 40-min intervals. (Disconnect batteries to stop; reconnect at a later time as necessary The blue dye moves faster than the red dye.) If this experiment is performed as a demonstration, pour several pipets and perform the electrophoresis for different time intervals. Store in a refrigerator to minimize diffusion after the electrophoresis stops. Atter two days, diffusion will be apparent.
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Soak pipets in hot water. Dip several times. Rinse. Reuse. Solutions may be disposed in the sink. Because only household chemicals are used in this experiment, it may be assigned to students as a special pmject. Be sure to note the d e t y precautions including the battery fire hazard. A few plastic pipets, a clamping or holding device, and the battery connectors are the only special parts required. Students mav run several oioets fordifferent amounts of time and store the pipets in'aiefrigerator for display to the class. Students may even do the wiring by twisting wires and taping or by using standard twist type connectors so that soldering is not required. (Solder is not critical to making satisfadory connections.) If vou are willine to deal with the D T O ~ of~ manv ~ S sets of bkteries, all oFyour students can perform the experiment. Literature Cited 1. Lunetta, V. J.; Doktya. M. J. J C d l
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nhs. l a .21,314
Discovery-Based Microscale Catalytic Decarbonylationof Aldehydes Mary-Ann Pearsall, Alan M. ~ o s a n , ' Jennifer S. ~onrad,'Carrie A. endr ricks on,^ AnnMarie L. ~acchia.'and Daniel J. schantz2 Drew University Madison. NJ 07940 Increasing attention is being focused on the benefits of experiments-which encourag~studentsto work and behave as practicing chemists in discovering, - not simply . . verifying, chemical phenomena (1). We describe a short, inquiry-driven microscale exploration of the heterogeneously catalyzed decarbonylation of aldehydes that combines the advantages of microscale work with spectroscopic analysis, sharpens observational and analytical skills, and encourages hypothesis development, testing, and confirmation. Because this reaction is not discussed in standard texts at the sophomore level, students aooroach the exoeriment from a research Derspective anb'must formula'te and test their own ideas.' Aldehyde decarbonylation is an important synthetic (2) and biological process (3).I t can be contrasted usefully to common formylation processes (0x0 and Koch reactions). Both the deformylation of steroids ( 4 ) and the conversion of formvlmethvl to methvl . mouos - - (5) are valuable svnthetic ðodoiogies. Aliohatic and aromatic aldehydes as well as awl halides are readily decarbonylated by bhef heatingover iupported palladium catalysts; i.e., palladium on carbon (eq 1 ) (6, 7). PdK WHO-RH+CO (1) heat Other compounds, particularly homogeneous rhodium comolexes. can be used stoichiometricallv (8) or. sometimes a t higher temperatures, catalyticaily (9). For our oumoses these are less effective. 'fhe catalytic decarbonylation of high boiling aldehydes is performed on a microscale with good rates being exhib-
ited by those aldehydes that boil above approximately 150 "C3 ARer an initial micro determination of the boiling point of the aldehvde (10) a 2-e or 2-mL samole is refluxed in the presence of\% &I&) of; % PdIC." careful observation reveals that the reflux temperature decreases with time suggesting the formation of a more volatile, lower boiling comoound. Gas evolution can sometimes also be discerned. h e apparatus is fitted with a micro take-off head and the volatiles are distilled, collected, and analyzed. High boiling products can be vacuum distilled (10) or,-alternatively,t h i reaction mixture can be micro-filtered and analyzed directly. Typical yields are 40-90%. GC and IR analysis quickly confirm that the starting aldehyde is a minor constituent of the reaction mixture with the-major component exhibiting a shorter GC retention time.' Palladium catalyzed aldehyde decarbonylation provides the corresponding alkane, alkene, or substituted aromatic (eq 1).Spectroscopic data reveal that in certain cases significant hydrogen transfer can occur. We find that both 2phenyl and 3-phenylpropionaldehyde provide ethylbenzene. but the reaction of the linear aldehvde is less seleiive and 5-10 % styrene also is observe;. Students can confirm the latter oroduct bv discovering that cinnamaldehyde provides &edomin&tly styrene >long with -5-10 % ethylbenzene. Some insight into the reaction mechanism is eained bv analysis of the conversion of methyl-trans-cinnamaldehvde which orovides both (E)and (Z)1-ohenvl-l-orooene (7")and by noting that the deca~bon~la'tion "of 3:ph~nylbutyraldehyde proceeds cleanly (-5 % of 2-phenylpropene is produced) without any skeletal rearrangement. This experiment stimulates students to speculate ahout the role of the catalvst and the nature of the oroducts. They are encourage2 to obtain additional infoknation. NMR soectra are particularlv informative. At their request, iamples of appropriate kkanes and alkenes can be provided for cornoarison of spectral and GC data. We have found it productive to have students work in pairs or small groups with each group assigned a different aldehyde. In this way the product, degree of conversion, extent of hydrogen transfer and the yield can be explored
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'Author to whom correspondence should be addressed, 2Underaraduatesummer research associates. 3For example, heptanal (bp 153 "C),octanal (bp 171 "C), nonanal (bp 190-192 "C),trans-2-nonenal(bp 210 'C), decanai (bp 207-209 %), cyclohexanecarboxaldehyde (bp 162 "C),BNorbornene-2- carboxaldehyde (bp 190 "C), phenylethanal (bp 195 %), Pphenylpmplonaldehyde (bp 210 "C),3-phenylpropionaldehyde(bp223 "C), 3-phenvl-P~ropenal icinnamaldehvde) Ibp 248 %I. a-methvl-transcinnamilde'hyde (bp 260 "C), the isomeric tolualdehydes (ollho bp 199 "C, meta bp 199 "C, para bp 205 "C), (+/-)-3-phenylbutyraldehyde (bp 220 ' C ) , 1,4-Benzodioxan-6-carboxaldehyde(bp 230 "C, mp 50-52 "C), 3.4-(Methylenedioxy)benzaldehyde (piperonal) (bp 264 "C, mp 35-37 'C) and 4-Acetoxybenzaldehyde (bp 260 "C). 4A30-min reflux usually suffices.The most effectivecatalysts are (1-10%) palladium on activated carbon. Pd/AI,O, is less effective and Pd on 4-8 mesh carbon, while easy to handle, IS not satisfactory. The reaction rate is not substantially increased by improving the mole 1 :1000. ratio of ~a1ladium:substratebevond a~~roximatelv 5GC conditions vary with sibstrate but typicaliy are isothermal at 150 "C using 6' 10% SP-2100 on 100/120 Supelcopolt. Volume 72 Number 2 February 1995
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