The standard method for performing the benzoin condensation begins will benzaldehyde, treated with a catalytic amount of sodium cyanide in the presence of base. The mechanism for this reaction has been well described elsewhere.

Procedure

Obtain 0.175 grams of thiamine hydrochloride, 0.5 mL of water, and place both in a standard sized test tube with a screw on cap. Dissolve the thiamine in the water, which can be better facilitated by crushing the clumps of thiamine with a mortar and pestle before you add it to the test tube. Add 1.5 mL of ethanol to the test tube containing thiamine and water, then cool the thiamine solution on an ice bath.

Obtain 0.5 mL of 10% NaOH and put it in a small test tube. Put the 10% NaOH on an ice water bath as well. Over a period of 6-10 minutes add all 0.5 mL of the cool NaOH at a rate of 1 drop every 30 seconds to the thiamine solution.

Add 1 ml of benzaldehyde to the mixture. Place the test tube containing the prepared solution in a heating block and gently reflux for 1 hour. Make sure the screw on cap is on the test tube, but no screwed so tight that no vapor can escape. Do not bring the solution to a vigorous reflux. It is best to keep the heating block around 80°C.

After removing the solution from the heating block allow it to cool under the hood for 5 minutes, or until it is around room temperature. Then place it on an ice water bath to help induce crystal formation. It may be necessary to scratch the inside of the test tube with a stirring rod to help crystals form. Set up vacuum filtration and use it to collect the newly formed crystals. Rinse the product with two 2.5 mL portions of water.

Place product back in test tube and re-crystallize using a minimum amount of ethanol. Collect the crystals and allow to dry. Determine the mass of waste obtained from both vacuum filtrations in order to calculate the actual E-factor. After the product has completely dried weigh it and find its melting point.

Pre-lab questions

1. What is the limiting reagent?
2. What is the theoretical yield of this reaction (g)?
3. What is the melting point of the product.
4. Calculate the atom economy of this reaction.
5. Estimate the e-factor for this method.
6. Find safety information about thiamine hydrochloride, cyanide, NaOH, and benzaldehyde. Compare the safety data on cyanide, which used to be used to catalyze this reaction, with thiamine, and make note of the improvements in the "greenness" of the reaction.

Literature Sources

Benzoin condensation
Aldrich Chemical Company Search Page
ChemExper Chemical Directory
Acros Chemicals
NIST Chemistry Webbook

References

1. Adapted from the procedure ___, in ___.