12/6/23, 2:41 PM Chemistry 211 Experiment 10

Experiments with the model compound 3,4-dimethylthiazolium bromide have explained how
thiamine-catalyzed reactions work. Part of the chemistry of thiamine is the acidic proton located on
the carbon between the sulfur and nitrogen atoms. Using 3,4-dimethylthiazolium bromide, it was
found that there is a rapid exchange of the C-2 proton for deuterium in the D2O solution. At a pD of
7 (No pH here!), this proton was completely exchanged in seconds!

This experiment indicates that the C-2 proton is much more acidic than one would have expected.
This hydrogen, bonded to the imine carbon has a pKa of 12.7, because the carbanion formed when
the proton is removed is stabilized by the adjacent positively charged nitrogen, yielding the highly
stabilized ylide. An ylide is a compound, or reaction intermediate, with positive and negative formal
charges on adjacent atoms. This ylide can react with an aldehyde to produce an enamine:

The enamine which we will produce, using benzaldehyde, can react with a second benzaldehyde
molecule to produce the desired product, following the acyloin condensation pathway. The enamine
functions much like the enolate partner in an acid-catalyzed aldol condensation. It can condense
with a suitable carbonyl-containing acceptor to form a new carbon-carbon bond. Decomposition of
the intermediate to regenerate the thiamine ylide yields the protonated acyloin, benzoin, in this
reaction. The final product depicted below just needs to undergo deprotonation to produce benzoin.

The reaction pathway outlined above describes the pathway we will follow to produce benzoin using
thiamine as catalyst. The benzoin produced will be used for the sequence of reactions which will be
followed to produce benzil and benzilic acid.

Part 1: Synthesis of Benzoin

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12/6/23, 2:41 PM Chemistry 211 Experiment 10

In this, the first step of our multistep reaction sequence (performed during day one), benzaldehyde
will be condensed, using the thiamine as a coenzyme catalyst, to produce benzoin. The same
reaction can be performed using the cyanide ion (you should include a plausible mechanism for
cyanide catalyst and include it with your report). The reaction mechanism for thiamine catalysis is
described above. (It is imperative that the benzaldehyde be from a newly opened bottle because of
the ease of oxidation, producing benzoic acid, which will interfere with the reaction. The thiamine
hydrochloride must also be from a newly opened bottle, although it is not quite as critical as for the
benzaldehyde.)

The overall reaction for conversion of benzaldehyde into benzoin is:

Procedure
Benzoin is made following using the chemicals listed below:

Add 1.75 g of thiamine hydrochloride to a dry 50-mL flask.

Dissolve the thiamine hydrochloride solid in 5.0 mL of water by swirling.
Add 20 mL of 95% ethanol (remember that standard ethanol liquid is 95%) and cool the
solution for a few minutes in an ice bath.
Very carefully and slowly, add 3.33 mL of 3 M NaOH drop-wise. Mix after every 2-3 drops by
gentle swirling, making certain that the temperature of the solution never rises above 20oC.
(The best way to add such a small volume of NaOH is to place about 10 mL of this NaOH
solution into a 10-mL graduated cylinder; then, withdraw using a Pasteur Pipette liquid down
to about the 6.67 mL marking, thus ensuring that you have added the correct amount of
NaOH.)
To the yellow solution, add 10.0 mL of pure benzaldehyde from a newly opened container,
never a previously opened bottle.

Heat the mixture at 60oC (using a constant temperature water bath set to 60-65oC) for about 1.5
hour. However, an easier and more straightforward procedure is to heat the sample at 60oC for 15
minutes after reaching this temperature (allow about 5 min for warming the sample before starting to
record the time, then incubate for the 15 minutes; going long is not bad.)

Caution: The temperature of this reaction cannot go above 65oC. Constant monitoring of
temperature is paramount during this part of the reaction and must be maintained between 60-65oC.

As mentioned above, you can let the reaction procedure for the full 90 minutes. However, the
following alternative incubation should be used, since it only required about 20-30 minutes of total
incubation time. The 1.5-hour reaction described above could still be used, but the shorter time is
preferred. Therefore, incubate your reaction mixture at 60-65oC for at least 15 min (allow five
minutes to equilibrate temperature). After this minimal incubation time, you will store the reaction
mixture until the next lab period (for at least 24-48 hours) at room temperature. The Erlenmeyer
flask containing the reaction mixture will be sealed using a regular cork to seal the flask. At the
beginning of the next lab period, collect all of your solid material using vacuum filtration as normal.
(Remember that the rate of reaction usually doubles for every 10oC increment in temperature.)

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12/6/23, 2:41 PM Chemistry 211 Experiment 10

At the beginning of the next lab, retrieve your reaction flask from the reagent cart. You should be
able to observed crystals, which are a slight yellow in color. If crystals did not form after storage,
withdraw a drop of the solution on a stirring rod and let it dry to produce a solid on the glass rod;
then, rub it against the inside surface of the flask to induce crystallization.

Collect your crystals via vacuum filtration (wash it free of the yellow mother liquor with a 1:1
mixture of 95% ethanol and water; prepare by mixinf 20 mL of 95% ethanol with 20 mL of DI
water). Since crystals will be moist you cannot do a yield and melt temp. However, your sample
should be dry enough to proceed to the next experiment making benzil.

Optional: Remember, only if your instructor instructs you to perform a re-crystalization, then use
the following procedure, but only if your instructor wants you to do this re-crystallization. In most
cases it is not necessary, just proceed with the Part 2 experiment.

Dispose of the liquid filtrates in the liquid waster container.

Part 2: Synthesis of Benzil

Starting with the α-hydroxyketone benzoin (prepared in Part 1), you will prepare an oxidation
product, benzil, which is an α-diketone. Using the still moist product isolated from the solid material
from Part 1, you may desire to re-crystallize this benzoin using hot 95% ethanol (you will need
about 8 mL of ethanol per gram of product; determine an approximate yield from the filtration
performed above. Let air be drawn through the filter before weighing to enhance the drying of your
benzoin. You will use the amount specified below to do your benzil reaction. Your chemical must be
dry before beginning Part 2 (re-crystallized from ethanol). Typical yield should be about 5-6 g,
although some students recover less than 4 grams. Be sure to record the amount of benzoin isolated,
and dry in the drying oven the benzoin not used in the benzil reaction. The product should be nearly
colorless and of sufficient purity (mp 134-135oC) to use in the next reaction.

This oxidation can easily be done with a variety of mild oxidizing agents, including Fehling's
solution (an alkaline cupric tartrate complex) or copper(II) sulfate in pyridine. In addition, benzoin
could be oxidized by sodium dichromate, but the yield of benzil is lower because some of the
benzoin is converted back into benzaldehyde following cleavage of the bond between the two
oxidized carbon atoms, which is activated by the phenyl rings, producing benzoic acid as the final
product. In this experiment, due to ease of use and consistent results, we will use nitric acid as the
oxidizing agent.

Caution: Concentrated nitric acid is highly corrosive and causes severe burns if spilled onto your
skin. Nitrogen dioxide (NO2) fumes are highly toxic and can damage the lungs due to inflammation.
Do not breathe NO2 fumes, and perform this part of the experiment in the hood.

Procedure
Benzil will be synthesized as follows, and must be performed in a hood:

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