Purpose: Oxidize fluorene to fluorenone, separate the products by column chromatography, and determine a percent conversion of reactant to product..
Preparation: Read Mayo, et al., p. 139-142. Look up and record
the melting points, molar masses, and structures of fluorene and fluorenone.
Writing a balanced equation for this oxidation-reduction reaction is complex.
The oxidation of fluorene [symbolized by (C12H8)>CH2]
to fluorenone [symbolized by (C12H8)>C=O] involves
a 4 electron transfer:
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while the oxidizing agent, dichromate, undergoes a 6 electron reduction:
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Now it is necessary to join these two half reactions into a single reaction in which the electrons lost cancel with the electrons gained. Some cancellation will also occur with H+ and H2O. Using the above information write a complete equation for the reaction in your results section..
Using the procedure below, only part of the fluorene is oxidized and fluorenone is not formed quantitatively. Column chromatography is used to separate the product, fluorenone, from the remaining fluorene. Note that dichromate is a versatile oxidizer but it is also poisonous and all chromium compounds need to be disposed of separately.
Procedure: In the 5 mL conical vial place 50 mg of fluorene and 0.25 mL of glacial acetic acid. Attach the condenser (no hoses needed) and warm the mixture on the steam bath. To this solution pipet 0.5 mL of a premixed solution of 0.15 g (0.5 mmole) of sodium dichromate dihydrate (Na2Cr2O7.2 H2O) in 0.5 mL acetic acid. Heat the mixture at 80 oC for 15 min (a beaker containing hot steam bath water mixed with tap water can be set up quickly and maintained on a hot plate for this purpose). Cool the mixture, add 1.5 mL of water and agitate it for 2 min. Disconnect the condenser and transfer the mixture to a prepared Hirsch funnel (filter paper and suction). Briefly reattach the condenser and rinse down the walls with 2 mL of water, transfer the water while rinsing the vial to rinse the solid in the Hirsch funnel. Press out the solid in the Hirsch funnel with the bottom of a test tube to press out the water effectively. The filtrate is to be poured into the aqueous waste container.
The solid is then returned to the vial and 2 mL of ether is added. Set up a dry/filter pipet (Na2SO4) and pass the ether solution through it to drain into a tared 25 mL Erlenmeyer flask. Rinse with more ether if necessary. Transfer one drop of the dry solution to a test tube for TLC analysis later. Insert a splinter in the flask and evaporate the ether on the steam bath. Remove the splinter as soon as there is no longer a danger of the ether bumping. While the flask is still hot and no liquid remains, tip the flask on its side and allow the ether vapors to escape. Record the weight of the crude product.
Thin Layer Chromatography (TLC). Place and develop three spots on a silica gel TLC plate: pure reactant, pure product, and product mixture. Use a developing jar with a 9:1 mixture of hexane:chloroform as the developing solvent. Repeat until an good TLC plate is obtained. While you are doing this, carry out a group column chromatography as outlined below.
Column Chromatography. The class will divide itself into teams of 4 (consult instructor) to carry out a column chromatography. Duties can be assigned as follows:
Note: diethyl ether and petroleum ether are not the same thing. Do not substitute one for the other!
1) Obtain six 50 mL Erlenmeyer flasks; wash each with soapy water and use a brush. Thoroughly rinse each flask with distilled water then a small amount of acetone and then diethyl ether; shake out the diethyl ether and then warm each briefly on the steam bath and tip them to allow the ether vapors to escape. The flasks should be clean and dry. Weigh each to the nearest milligram and mark this with a felt pen on each flask.
2) Obtain a clean 12 mm x 30 cm column (or a 10 mL pipette) and place a small (about 1/2 ml when compressed) wad of glass wool at the base of the column, just above the stopcock. Place above this about a 1 cm layer of sand. Rinse the column twice with 10 mL of diethyl ether then 10 mL of petroleum ether. (Petroleum "ether" is really a mixture of alkanes whose boiling point is 30-60 oC.) Be sure all inside surfaces are rinsed and drain the solvent through the stopcock.
3) Prepare a slurry of 6-7 g of silica gel and 20 mL of petroleum ether in a 100 mL beaker. Make sure that you have glass wool at the bottom of your column, else the slurry will slip out. Close the stopcock and transfer the slurry into the column with a spatula. Silica gel remaining in the beaker can be mixed with petroleum ether that is drained through the column, but do not allow the solvent level to drop below the silica gel that settles inside the column. Transfer most of the silica gel via the slurry process. Drain the petroleum ether to just below the top of the silica gel.
4) Add 1/2 ml of chloroform to the crude products of two students and dissolve as much as possible. Transfer the solution directly to the top of the silica gel in the column. Drain the column again to just below the top of the silica gel and repeat the transfer using 1/2 ml of chloroform at a time. Repeat the process once more if necessary, but no more; excess chloroform in the column at this time will make the eluting solvent too polar. Load the column with 1 g of sand to prevent solvents added later from cutting channels into the silica gel. If the solvent level is now above the top of the sand, open the stopcock and allow the liquid to drain just to the top of the sand.
Prepare 10 ml of 9:1 (v:v) pet ether:chloroform and pass this solution through the column. Collect the eluate (the liquid that has passed out the stopcock) with one of the clean, pre-weighed Erlenmeyer flasks. As the final milliliters of 9:1 petroleum ether:chloroform descend to the sand, prepare and add 10 ml of 4:1 pet ether:chloroform (fraction 2). Do not allow the level of the liquid to drain below the sand. For each fraction, remove the solvent by evaporation as described above (steam bath, splinter, remove splinter, remove solvent fumes) and re-weigh. The results are tabulated with headings (left to right): Fraction Number, Solvent, Mass, Remarks (such as "crystals -light yellow, mp of.." or "oil" or "nothing" or "not enough to analyze"). Fraction 3 is eluted with 1:1 petroleum ether:chloroform, and fraction 4 and 5 are eluted with pure chloroform. An extra pre-weighed Erlenmeyer flask was prepared as a back-up and may not be used.
Repeat the chromatography as necessary to obtain a clean separation of each of the components. You should provide a satisfactory accounting of the amounts submitted and obtained from the chromatographic process.
Each group should obtain and fully interpret a CMR spectrum of the pure fractions obtained from the column chromatography.
Some points to be addressed in your report:
1. From the Handbook, the Aldrich Chemical Sales Catalog or another suitable source, find and write out the full structures of fluorene and fluorenone. Structures from either list are referenced to the compound number in the alphabetical list.
2. From the TLC, approximately how much of the reactant was converted to the product in your own reaction?
3. Were the relative Rf values as you expected?
4. In the column chromatography, was the order of elution (i.e., fluorenone before fluorene or vice versa) in the order you expected? Why?
5. From the column chromatography, give a more quantitative figure for the amount of conversion. How did you arrive at this value?
6. Did the masses of the fractions sum to the mass of mixture you submitted to column chromatography?
7. What is the reducing agent? What is the oxidizing agent? How many electrons are transferred in the net balanced equation?
8. Do the CMR spectra support your chromatographic (TLC and column) results? Why or why not?
References:
"Mayo et al.": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991
"Handbook": any of the recent editions of: Weast, R.D. Handbook of Chemistry and Physics; The Chemical Rubber Co.: Cleveland, 1960-present.
"Aldrich Price List": any addition of Aldrich Handbook of Fine Chemicals; Aldrich Chemical Co.: Milwaukee, 1970-present.
Revised by Shane Phillips on 1/5/99.