CHEM 321: REDUCTION OF A KETONE
Reduction and oxidation are complementary processes that dramatically change the behavior of certain
susceptible functional groups. For example, nitro groups (-NO2) can be reduced to amino groups (-NH2). Nitro
groups are potent electron withdrawing groups that can oxidize other molecules. On the other hand, amino
groups are good nucleophiles, and compounds with amino groups tend to be easily oxidized.
This experiment illustrates a mild way to carry out reduction of a carbonyl group to an alcohol. The reducing
agent is sodium borohydride, NaBH4. This reagent is particularly convenient because: 1) All four hydrogens are
active; 2) It is not excessively reactive with inexpensive solvents, like water or alcohols; and 3) The reduction is
selective. Functional groups other than aldehydes and ketones will remain unchanged.
Reduction of benzophenone to benzhydrol:
O
OH
NaBH4
borate HO
ester H2O
Another reducing agent that could be considered is LiAlH4. The Al-H bond has more ionic character than the
B-H bond. Therefore, the H atoms in AlH4- are far more electron-rich than those of BH4-. This makes AlH4- far
more reactive and less selective than BH4-. For example, LiAlH4 reacts violently with most protic solvents to
generate H2 gas (which can lead to fire or explosion), but in suitable aprotic solvents, LiAlH4 can successfully
reduce functional groups that NaBH4 cannot.
Experimental Protocol
CAUTION: NaBH4 is corrosive to skin and membranes. Avoid its dust, and wash with lots of water if
you get any on your skin or clothing. Clean up any spilled material immediately with a wet sponge, and
rinse the sponge out thoroughly.
SYNTHESIS: In a 100 mL boiling flask, prepare a slurry of NaBH4 (0.3 g) in 15 mL of 2-propanol. Add
benzophenone (2.75 g) and a medium sized stir bar. Bury the reaction flask in a shallow sand bath that is on top
of a stirring hot plate and reflux gently for 45 minutes. Allow the reaction mixture to cool, and add 15 mL of
10% NaOH (same as 3 M). Swirl the mixture vigorously until all the solid has dissolved (lumps can be broken
up with a glass rod, but be careful that you don't crack your flask!).
RECOVERY: Add 80 mL of DI water into a 250 mL beaker. While constantly stirring, slowly add the entire
reaction mixture to the 80 mL of DI water. Keep stirring until there is no evidence of oily material. The product
should start to become a solid. Place the beaker into an ice-water slush and cool to approx. 5 °C to allow as
much solid as possible to form. Vacuum filter the mixture on a medium size Büchner funnel. While the crude
product is on the funnel, wash it with 50 mL of ice cold water to rid it of as much 2-propanol as possible.
RECRYSTALLIZATION: There is no single good recrystallization solvent for this product, but a
combination of methanol and water can be used to purify it. (This is a mixed-solvent recrystallization.)
Keep the beaker covered at all times unless you need to access its contents! In a 100 mL beaker, dissolve
the crude product in the smallest volume of heated methanol required (try starting with 5 mL if unsure; you may
need less if you don’t have much crude product!) so that, once provided heat, it can completely dissolve. Heat
the mixture until it begins to just barely boil. You will probably need to do a hot filtration to remove insoluble
particles. The course webpage has a good resource for this here. Once all of the solids have dissolved, remove
the beaker from the hot plate onto the bench. With good mixing, slowly add DI water from your squirt bottle to
the warm solution until just persistently cloudy. Then, put the beaker back on the hot plate, and warm until clear.
Cool the mixture slowly to room temp, stirring vigorously and watching carefully for evidence of oil droplets
coming out of solution. If this happens, add a little methanol, warm until everything redissolves, then cool
slowly again. The product will come out as an oil if the solution temperature is above the product's melting
range. Scratching the solution / beaker wall interface with a glass rod can help induce crystals to form if
temperature is below the melting range. After no further crystals form at room temperature, cool to ice
temperature (approx. 5 °C). Vacuum filter on a Hirsch funnel. Fold the crystals into a paper towel (labeled!) to
air dry in your desk drawer for at least one day.
ANALYSIS: The next day (or later) obtain the weight of the product, the melting range of the product, and the
IR spectrum (ATR) of the product; get the IR spectra of authentic benzhydrol and benzophenone from the web.
MAJOR LABORATORY REPORT (no longer than 5 pages, 1.5-2.0 spacing, including figures and tables)
Please pay attention to the general outline, required formatting, and sample for the major lab report in your lab
syllabus. You will have more data to discuss in this report than in the sample report. Guidelines for this report:
 Title
 Source reference
 Introduction containing:
o the experiment objective(s) (what was the purpose of performing the experiment?);
o the context (the experiment that was performed as a means through which the purpose could be met –
pertinent concepts and techniques included);
o the rationale (why is this objective worthwhile?); and
o Figure 1 should show just the above reaction, but balanced (no mechanism necessary this week). Use
ChemDraw. Half reaction work need not be here, but should be shown in the Appendix.
 Experimental methods (refer to previous experiment handouts, and the feedback you received on your first major lab
report (E1 experiment), for examples of writing a concise protocol)
 Results presenting actual data/observations that support whether the goal was met or not (e.g., mp/bp range, IR
spectra, chemical tests, appearance, percent yield, etc.)
 Discussion containing:
o a reiteration of the objective,
o comments on how these data compare to published values and what conclusions can be drawn,
o plausible reasons for percent yield deviations from 100 percent,
o chemistry concepts supporting results/interpretations (including figures when appropriate, such as
reactions for chemical tests), and
o a summary sentence indicating whether or not objectives were met
 References (additional to source reference, and provide appropriate citations throughout the report)
 Appendix containing:
o calculations with work for % yield, and/or additional supporting figures
o work for the balanced reaction: This is a redox reaction. It occurs in alkaline solution, and the inorganic
product is BO33-. Consult a Gen. Chem. text if you have forgotten how to balance redox reactions. Hint:
the balanced reaction does include HO‾ ions, but not 4.Work can be given in molecular formulas, not
structures.
REMEMBER THESE GUIDELINES FOR ALL IR SPECTRA:
Label and attach the IR spectra to your lab report, or insert them into the report document. Your RESULTS
section should direct the reader to IR spectra (labeled as FIGURES):
How do I label an IR spectrum?
 With an appropriate figure caption (see example) and figure number under the figure
 The correct name of the chemical, spelled out, is in the caption
 The molecular structure of the chemical (so the reader may see what functional groups are present –
include on experimental product as well, with the structure of the expected product)
 Circle individual peaks you choose to discuss in your report, and tell what vibration is the likely cause
of each peak. You need to assign enough significant peaks to allow the reader to see that the product is
what it is claimed to be and is pure (or contaminated) as claimed in the report. Label each chosen peak
explicitly with the name of the responsible molecular vibration. Examples: O-H stretch, sp2 C-H stretch,
C=O stretch. These assignments should be chosen to confirm that the molecule is of the class you claim
it to be. You should not use the sp3 C-H stretch unless it is a main significant signature peak.
 Figure caption includes a citation to a bibliographic citation in your REFERENCES section
What are the IR spectra that I should include in my report?
 IR spectrum of experimental product
 Published IR spectrum of expected major product, citation included
 Published IR spectra of any potential contaminants or side products that would show characteristic IR
peaks, citation(s) included
 To decide whether contamination from specific compounds is significant, evaluate the relative
intensities of pertinent peaks in the product spectrum along with intensities of those peaks in the spectra
of the starting materials and/or plausible side products.
Where can I find published IR spectra? (These resources are on the course webpage under “Useful Links” as
well.)
 Spectral Database for Organic Compounds, SDBS. URL: http://sdbs.db.aist.go.jp
 National Institute of Standards and Technology, NIST Chemistry WebBook. URL:
http://webbook.nist.gov./
PRE-LAB QUESTIONS
1. Enter the correct term for each blank (choices can be used more than once): Mixed solvent
recrystallization will be used to ___________ the crude product. In this experiment, a mixture
of two solvents will be used so that the desired product is ___________ in solution at warmer
temperatures and ____________ in solution at lower temperatures. The desired product will be
more soluble in _____________ and less soluble in _____________, demonstrated when the
solution becomes just persistently cloudy. It is important to note that impurities will hopefully
be _____________ in the solvent mixture at warmer temperatures and _____________ in the
solvent mixture at lower temperatures as well.
Isolate
soluble
synthesize
insoluble
purify
methanol
characterize
water
2. What is one thing you can do if you believe you have added too much solvent to your crude
crystals?