the microscale laboratory
edited by ARDENP. ZIPV SUNY-Cortland c0--.Nyl-5
Sodium Hypochlorite Oxidation of 9-Fluorenol to 9-Fluorenone: A Reaction Monitored by Thin Layer Chromatography C. Susana ones' and Kim Albizati UC San Diego La Jolla CA 92093-0303. A common practical question facing organic chemists when carrying out a chemical reaction is "When is the reaction complete? The determination of this very important reaction parameter is generally not addressed in undergraduate organic chemistry courses in a practical sense. Normally, a reaction time is given to the student as part of a cookbook procedure. Undoubtedly, there are many reactions slow enough to explore reaction times within the time frame of undergraduate chemistry labs. However, there is a simpler way to introduce this idea while simultaneously providing a problem-solving experience for students that is rooted in real-world situations: Use incremental reagent addition to a rapid chemical reaction. Akey reagent is added in increments, and the reaction is assayed after each addition. The end of the reaction is signalled by the disappearance of starting material a s ascertained by TLC, IR, NMR, GC, or any technique one wishes to highlight. In uur intn8ducto~organic chemistry lab course we have used tho oxidittion of 9-fluorenol to 9-fluorenonehv hleach (sodium hypoehlorite) with monitoring by TLC a s a teaching device with a problem-solving aspect. The students add NaOCl in small amounts to the starting material and determine for themselves when they have added enough reagent to complete the reaction. They cannot easily calculate the required stoichiometric amount of aqueous NaOCl due to the variable composition and decomposition rates of commercial bleach. The oxidation of 9-fluorenol by this method is superio? from a n environmental point of view to the chromic acid on amberlite method published previously ( I ) .
Oxidation of fluorenolby sodium hypochlorite ments of bleach are added until the reaction is judged to be over by TLC analysis. A total of 1.2 mL of 5.25% NaOCl should be enough, but solutions of NaOCl decompose rapidly, according to the equation
Once the conversion is complete and the reaction sample shows no more starting material by TLC analysis, proceed (Continuedon page A2721
Experimental Procedure Dissolve 50 mg of 9-fluorenol in 3 mL of acetone using a 10-mL round-bottom flask. Use 3 to 5 FL of this sample as a control for thin layer chromatography (TLCj. Add to the flask 0.12 mL of glacial acetic acid, and start the oxidation by adding 0.4 mL of a 5.25% sodium hypochlorite solution (commercial bleach). After 5 min analyze the reaction progress by assaying the mixture for the presence of the starting material, 9-fluoreuol. TLC a s indicated in Mayo et al. (11,could be used to monitor the progress of this oxidation r e a ~ t i o nIf, ~the reaction is not finished (i.e., if startingmaterial is still present a s determined by TLC) 0.4-mL incre-
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Tne Jse of SodlJm nypocnlor to mlnm zes na7aroo~swaste, is econom~ca ano Jses a reagent tnal s easy ano safe to nanole W lh Easlman Kodak I Loresccnt sil ca ge pales w In 30". acetoneno% hexane as the eluant, 9-florenone has a retention factorof R, = 0.80, and 9-fluorenol has I?, = 0.56.
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Volume 71
Number 11 November 1994
A271
the microscale laboratory with an extraction of the 9-fluorenone. Extract this crude mixture two times, each with 2 mL hexane. Then wash the hexane extract with 1mL of 5% sodium bicarbonate solution to eliminate any acetic acid 2 mL water
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Dry the hexane extract with anhydrous sodium sulfate for a few minutes, and transfer the hexane extract to a previously tared vial. Rinse the sodium sulfate with 1mL of hexane. and add this to the hexane extract. Evaoorate the hexane'in a sand bath in a hood. This oxidation procedure is more efficient than the chmmic acid-amherlite procedure, which is a heterogeneous solid-liauid reaction that oroceeds ooorlv - due to the requirement of very efficient agitation.
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Recrystaiiization
The dried crude oroduct has a meltinc point range of 7879 "C and is sufliciently pure for cha;akerizatioi by IR, NMR. and ketone derivatiration. IIowevcr, if a higher - -DUrity is desired, recrystallization from 1 m~ of hexane, as specified in Mayo et al. ( I ) , is recommended. Acknowledgment
Our thanks for proofreading and advice to Barbara Sawrey and Antonio Ochoa, whose work and enthusiasm are stimulating, and to teaching assistants and colleagues at UCSD, who it was a pleasure to work with. Literature Cited 1.Mayo. D. W:PiLe,R. M.; Butchsr, S. S. M I c ~ c ~ cOrgoniehbomtary. le 2nd ed.;Wiley, 1989.
A Microscale Rotary Evaporator Daivd F. Maynard
formaSlate Unlvers Iy San Bernara~no, CA 92407
Ca
tanks, and manifolds. To solve the ahove difficulties, we have developed a simple roto-vap-type apparatus using a 10-mL round-bottom flask and capped Hickman still a& tached to a water aspirator. A typical experimental procedure involves transferring product with solvent and washings to a preweighed 10-mL RB flask. (We have found that the heavy walled "V reaction vials are not as efficient for solvent removal.) The flask is then attached to a Hickman still stoppered with a rubber septum and threaded compression cap. The entire apparatus is connected to a water aspirator and trap through a thick-walled vacuum hose. With the water aspirator on, the student shakes the apparatus while warming the flask in the ~ a l mof the hand. This action inhibits bumping and expedites solvent evaporation. The Hickman still also acts as a solash euard. Heat transfer is verv effective, and the student can easily determine when the solvent is removed by noting the temperature of the RB flask. Once the flask remains at ambient temperature, the apparatus can be secured with a clamp for 1-2 min to allow for the last traces of solvent to be removed. The vacuum is released by unscrewing the Teflon septum. The aspirator is turned off, and weighing and analysis (IR, GC, RI, NMR, etc.) of the product is carried out. The above procedure has greatly improved the yields and purity of the products synthesized in the microscale lab. This apparatus allows for the rapid removal of low boiling solvents, such as diethyl ether (bp = 35 OC) and methylene chloride (hp = 40 'C). Even tetrahydrofuran (bp = 65 "C) can be removed, but this appears to be the boiling point limit for solvents that can be efficaciously removed within an undergraduate laboratory period using this procedure. Overall, yields are more accurate, and analyses of products are free of solvent impurities. Extra heating equipment is not required, thus reserving hood space for the dispensing of chemicals. Because students remain at their benches, their time is more effectively used for cleaning glassware, organizing equipment, and updating notebook records.
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The rapid removal of excess solvent from products is of critical importance in the microscale laboratory. L A Due to the small quantity of com~ o u n dsynthesized, incomolete sol- Threaded compression cap kent removal may result in-unrealis- Teflon-lined rubber septum +[/. tic reaction yields (>loo%) and extraneous peaks in the chromatographic analysis. Recommended means for solvent removal currently include the use of a stream of dry air or nitrogen, heating the reaction vial in a warm sand bath, or using a ruhber septum with a syringe needle a& tached to a vacuum. However, these methods often result in the removal Threaded compression cap of product along with solvent, resulting in low yields and insufficient 10-mL RE flask product for further chemical analysis. In all of the ahove methods, students have difficulties determining when the solvent is completely removed. These procedures also require hood space, heating apparatus, nitrogen Construction of the rotary evaporator
To water as~irator 4 with trap
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A272
Journal of Chemical Education
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m with hands or h(lt water
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