Week 3: Oxidation of 9-Fluorenol to 9-Fluorenone with Polymer-supported Cr (VI)
Cr(VI), in the form of chromium trioxide or sodium or potassium dichromate, is frequently used for
the oxidation of secondary alcohols to the corresponding ketones. The conversion of primary
alcohols to the corresponding aldehydes using the same reagents is difficult to control because of the
ready oxidation of aldehydes under these conditions to the corresponding carboxylic acids (see pre-
lab assignment below)
Chromium compounds are toxic and represent a hazard to the environment, particularly the aquatic
environment. These compounds are used extensively in tanneries and in dyeing houses. The
effluents from these cause havoc with the aquatic life in the vicinity of such industries.
In the laboratory, we will adopt a more prudent approach to the use of Cr(VI) as an oxidizing agent.
The procedure allows the use of the powerful oxidizing properties of Cr(VI) without its bad side-
effects. This is accomplished by binding Cr(VI) to an insoluble polymeric material, which allows for
easy separation of the reagent from the reaction mixture by filtration, and for the reuse of the reagent.
Polymer-bound reagents have been developed for use in a variety of organic syntheses.
In this week’s experiment you will treat 50 mg of 9-fluorenol with 500 mg of Cr(VI) bound on an
insoluble polymer (Amberlyst A-26 ion exchange resin; a gift from Rohm-Haas) suspended in 2.5 ml
of toluene (use a 5 ml vial). The resin contains 2.5 mmole/gm of bound CrO3, which reacts in a 1:1
mole ratio with the alcohol. The contents of the vial are stirred magnetically and heated to reflux (BP
of toluene =110 oC) for about 50 min. You do not need to monitor the temperature.
Check for progress of the reaction at this point by TLC (see previous experiment). If TLC analysis
shows unconverted 9-fluorenol, check with the TA to determine the extra reflux time which may be
necessary to achieve the total transformation of the alcohol to the ketone.
When the reaction is complete, allow the mixture to cool down to room temperature while stirring
before removing the resin by filtration using a small Hirsch funnel. Use two 1-ml portions of
methylene chloride to rinse the reaction flask contents into the Hirsch funnel.
Deposit the filtered resin in the waste container provided in the lab hood for that purpose.
Do not accidentally dispose of the resin in any other manner! Spilled resin may be cleaned up by
blotting it with the sticky side of the tape used to cover TLC plates.
Transfer the filtrates to a small tared beaker and evaporate the solvent by warming under a hood. The
solid yellow residue is crude 9-fluorenone. Weigh the crude product. The product may also be
dissolved in a minimum amount of warm CH2Cl2 and left covered in the desk until next week, to
obtain crystals instead of the sticky adhesive film deposited by evaporation of toluene.
If time permits, recrystallize it from hexane (about 1.5 ml of hexane for 50 mg ketone) using a
disposable Pasteur pipet.
Determine the weight, % yield, and mp of the product, place in a labeled sample vial and turn in.
1. What are the reagent combinations in which Cr(VI) can be used to safely oxidize primary
alcohols to aldehyde stage only? Explain your answer.
2. Show the structural formulas for the two secondary alcohols which can be oxidized to
3. Show a balanced equation for the oxidation of cyclohexanol with potassium dichromate in the
presence of sulfuric acid.