Benzil is a yellow crystaline compound or powder used mainly as a precursor for benzilic acid. It is formed from the oxidation of Benzoin. A variety of reagents can be used to oxidise the benzoin, such as sodium dichromate or even Fehling's solution. In this procedure nitric acid will be used due to the availability and consistent results.
C14H12O2 --HNO3--> C14H10O2
|Benzoin||1000ml round bottomed flask|
|70% Nitric acid||Magnetic stirrer (optional but recomended)|
|Distilled Water||Reflux Column|
|Ethanol||Vacuum filtration apparatus (optional but recomended)|
To a 1000ml round bottomed flask, add 32grams of benzoin. To this, add 115ml of nitric acid. If you are going to use a magnetic stirrer, throw in the stirring bar at this point. Attach a reflux column to this flask. Now, you can either move this outside, or move it under the fume hood. Once the reaction starts, an ungodly ammount of nitrogen dioxide will be evolved. Once it is where you want it to be, start running cold water through the reflux condenser. Begin heating the flask. Once it gets to a certain temperature, the reaction will begin. You will know when it is about to reach the point when the nitric acid starts to get very dark in the flask. Continue heating (but don't raise the temperature) once the reaction starts. Keep refluxing for 5 minutes after the evolution of nitrogen dioxide has stopped. After this point, remove the heat and let the flask cool for 5-10 minutes. Add 600ml of cold, distilled water to this flask. If you have a magnetic stir-bar retriever, use it at this point to remove the stir bar from the mixture. Bring the temperature of the contents of the flask to room temperature while swirling.
Vacuum filter the contents of the flask and wash the solid with distilled water. Keep the vacuum on for 5 minutes to help dry the solid a little bit. This is your crude benzil. In a 200ml beaker, pour 85ml ethanol. Bring this ethanol up to 55C and add the crude benzil. Stir it until it all dissolves, then filter while keeping the ethanol hot. Add water drop by drop until the mixture goes cloudy. It could take a while to reach this point, so it might be wise to use a burrete or similar device to add more than a drop at a time in the beginning. Then set the stop-cock to add it drop-wise, contantly monitoring the ethanol. Once it goes cloudy, set it to the side to let the benzil crystalize out. Vacuum filter this once it has all crystalized out. Yeilds are typically above 25grams, but no more than 31grams.