BOROHYDRIDE RECUCTION OF 9-FLUORENONE TO 9-FLUORENOL
(Adapted from Missouri science and technology organic chemistry lab)
“In this week’s experiment you will be utilizing a metal hydride as a reducing agent. Metal hydrides can
be quite reactive, and therefore difficult, or even dangerous, to handle. Of the many hydrides explored
by H. C. Brown (Nobel Prize, 1979) and co- workers at Purdue, sodium borohydride, NaBH4, (discovered
in 1941), is a mild and selective reducing agent. It is safe to use in the undergraduate organic laboratory.
It will be provided to you as a basic ethanolic solution (100 µL of this solution provides approximately
4.0 mg of NaBH4). The ketone to be reduced is 9-fluorenone and the resultant secondary alcohol is 9fluorenol. You will perform the reduction on about 50 mg of the ketone. The progress of the reduction is
monitored by TLC (Eastman Kodak Fluorescent silica gel sheets ~ 2.5 cm x 10 cm, developed with the
provided 30% acetone in hexane solution, and visualized by UV light).
Weigh about 50 mg of 9-fluorenone to ± 0.001 gm in a tared 5-mL conical vial equipped with an air
condenser and magnetic spin vane. Introduce about 1 mL of ethanol as solvent and stir to bring about
dissolution. The NaBH4 solution is then introduced dropwise (~0.3 ml or 8 drops). After about 10
minutes, a small drop (using a capillary) is placed on the TLC sheet which has already been spotted with
solutions of pure ketone and pure alcohol. (Check first semester notes and MTOL for the proper
procedure for TLC analysis). If the TLC analysis reveals the presence of unreacted ketone, check with the
TA to determine if more hydride reagent is needed. The color of the solution will disappear when all of
the ketone is reduced.
The product alcohol is isolated by first adding ~ 1.5 mL of cold 1M HCl to neutralize the alkaline
reduction medium. The acid should be added dropwise as H2 gas may also be produced by the
decomposition of any excess NaBH4. Check with blue litmus or pH paper to verify that excess acid is
present. Separation of the alcohol as a fluffy precipitate may occur at this stage.
Now you will extract the product alcohol by shaking the reaction mixture successively with one 1.0 mL
and successively, two 0.5 mL portions of methylene chloride, CH2Cl2. Methylene chloride is immiscible
with and more dense than water. If only a single layer is seen, add ~1 ml of saturated sodium chloride
solution to help separate the layers. After each addition and shaking, the layers are allowed to separate
and the lower layer of methylene chloride containing the extracted 9-fluorenol is removed with a
Pasteur pipet into a dry vial. Use a small beaker or Al block to prevent the vial from tipping. If your
combined mixture volume will exceed 5 ml, use a centrifuge tube to perform the extraction.
The combined methylene chloride extracts are then passed through a second Pasteur pipet fitted with a
cotton plug and filled with about 500 mg of anhydrous sodium sulfate into a 50 ml dry tared beaker.
This operation will both dry and filter the extract. Rinse the pipet column afterwards with an additional
0.5 mL portion of methylene chloride. The combined dried eluate will be about 2 mL.
The methylene chloride is then removed by gentle evaporation using a hair dryer under your mini hood.
Weigh the beaker until it reaches constant weight and calculate the yield based on the mass of starting
ketone. Determine the MP and hand in the product in a properly labeled vial.
Procedure Summary
1.
Reduce with NaBH4 solution
2.
3.
4.
5.
6.
7.
Check progress by TLC (see details below)
When complete, neutralize excess NaBH4 with 1M HCl
Extract mixture with CH2Cl2
Dry combined CH2Cl2 extracts with Na2SO4
Remove CH2Cl2
Weigh & find MP of product
TLC
1.
2.
3.
4.
5.
6.
Prepare TLC tank using 30% acetone in hexane, allow vapor space to saturate.
Avoid edges of plate, handle only by edges, draw pencil line ~1/2” from the bottom of the plate.
Spot with reference ketone, alcohol and reaction mix, using a capillary.
Develop, mark solvent front, view under UV, determine Rf’s
When ketone spot is absent, reaction is complete. Reaction mix should also be colorless.
Attach labeled TLC plates to yellow lab book pages with 2” clear tape
Pre-Lab Questions
Compare and contrast the (a) reductive abilities of LiAlH4 with those of NaBH4, and
(b) compatibility of the two reducing agents with protic solvents (water, alcohols,
etc.) (see Solomon’s organic text)
Calculate the theoretical amount of NaBH4 needed to convert 100 mg quantities of benzil to
benzoin, and benzil to hydrobenzoin.
5. Draw the structural formulas for the organic reactants and products in this week’s experiment.
Which compound should move the farthest in TLC? Explain.”[15]
1.
2.
3.
4.
Personal Protective Equipment Required
Dusk Mask
No
Ear Protection
No
Eye Protection
Yes, safety Goggles
Footwear
Yes, closed toed shows
Gloves
No
Required Clothing
Yes, provided lab coat
Safety Consideration
Chemical Reactions,
When adding the HCL dropwise to the NaBH4 + 9-flourenone mixture, this should be done in a vent hood
as H2 gas may be produced if there is any excess NaBH4.
MSDS/SDS
Yes
9-Flourenone
1.
2.
3.
4.
5.
Sodium
Borohydride
Hydro Chloric
Acid
Methylene
Chloride
Anhydrous
Sodium Sulfate
9-Flourenone - http://pubchem.ncbi.nlm.nih.gov/compound/9-Fluorenone#section=Chemical-and-Physical-Properties
Sodium Borohydride -http://pubchem.ncbi.nlm.nih.gov/compound/86616055#section=Synonyms
Hydro Chloric Acid - http://hazard.com/msds/f2/cld/cldfm.html
Methylene Chloride - https://fscimage.fishersci.com/msds/89926.htm
Anhydrous Sodium Sulfate - https://fscimage.fishersci.com/msds/21630.htm
Laboratory Safety Equipment Location and Awareness
Please fill in, by hand, the blank map. Be sure to include, clearly, the locations of fire extinguishers,
telephones, eye wash fountains, safety shower, fume hoods, flammable storage cabinets, first-aid kit,
chemical spill kit and etc.
Blank Laboratory Map
Laboratory Emergency Egress
Emergency egress map shown below. The muster location is outside the building on the lawn nearest
the egress exit. Follow current university practices regarding emergency exit (e.g., close doors but do
not lock them)
Butler-Carlton RM 109 Egress Map http://designconstruction.mst.edu/floorplan/ [15]