α-Diketones are very useful in the
preparation of cyclic compounds of great value. Benzil, an α-diketone, is
prepared by the oxidation of an α-hydroxyketone, benzoin. This oxidation can
easily be done with mild oxidizing agent. In the experiment, the oxidation is
performed with ironic chloride hexahydrate.
(See also: Preparation of Benzoin - Synthesis of Dilantin )
Reaction
equation
Possible
Mechanism
Step 1
Step 2
Chemical
1.
Benzoin: 1.20g
2.
Ironic chloride hexahydrate:
5.10g
3.
Ethanol
4. Acetic acid: ~6mL
Procedure
1.
Place 6mL of glacial acetic
acid, 3mL of water, 5.1g FeCl3˙6H2O and a stir bar in a 100mL round-bottom flask fitted
with a reflux condenser.
I used a air condenser because the boiling point of the distillate is high
and it also safe more energy
2.
Heat gently the mixture to boil
and shake the mixture occasionally. Stop heating and cool the solution
slightly.
3.
Add 1.20g benzoin to the flask
and continue to heat. Reflux the mixture 1 hour.
4.
Then add 30mL of water to the
flask and heat the mixture again. After boil, cool the solution, yellow solid
separates. Collect the crude product by filtration, and wash it three times
with a small amount of cold water.
After add water
Boil the solution again
Place in an ice bath
5.
Recrystallize the crude product
from 75% aqueous ethanol (about 30mL).
6.
Air dry the product and weight
it.
Get 0.80g
Experimental
Record
Weight of benzoin added
|
1.20 g
|
Weight of benzil after recrystallization
|
0.80 g
|
Theory weight of benzil
|
1.19 g
|
Yield Rate
|
67.2 %
|
Hello, thanks for this idea, I'll try it out.
ReplyDeleteIt would be helpful to have a simple analytical method, such as TLC. When I tried this with Acetonitrile, I could not find a difference between Benzoin and Benzil, so another solvent for TLC would be useful.