December 27, 2009
Benzene is the most fundamental aromatic molecule. It is also a very useful starting material for the amateur chemist. From benzene, it is a short jaunt to chlorobenzene or nitrobenzene and aniline, or perhaps the even more useful synthesis path comprised of meta-dinitrobenzene, meta-diaminobenzene, and rescorniol. I’ve thought up of more than ten possible ways to make benzene at home, but today I rethought my idea of isolating it from benzoyl peroxide. At least in the United States, benzoyl peroxide is found in concentrations of up to 10% in acne cream. This is bound to be a bit of a pricy procedure then, since about 30g of acne cream costs about $10. So after some stoichiometry and math, we’re looking at least $4.50 per milliliter of benzene. That’s crazy expensive, but it still could be a neat proof of concept.
So the idea is that benzoyl peroxide hydrolyzes to two molecules of benzoic acid. I drew up a probable mechanism of this reaction. The weak O-O peroxide bond will cleave with some heat. The resulting radical abstracts a hydrogen atom from water, giving benzoic acid and a hydroxyl radical. Two hydroxyl radicals can then combine to form hydrogen peroxide, which can further decompose into water and oxygen gas.
The benzoic acid formed would then have to be isolated in the solid state and decarboxylated to the desired benzene by heating it with something like sodium hydroxide.
Now I actually peformed this experiment on a very small scale a number of years ago. If I remember correctly, I took about 5g of an acne cream containing 10% benzoyl peroxide and placed in it boiling water for a few minutes. Then, I extracted the filtered solid with acetone and evaporated the acetone off, but I couldn’t detect any benzene as a product although I did notice a strong petroleum-like odor. That might have been due to other ingredients in the acne cream. Let me consider those other inactive ingredients this time. They are:
aluminum hydroxide gel, bentonite, carbomer-940, dimethicone, glyceryl stearate SE, isopropyl myristate, methylparaben, potassium hydroxide, propylene glycol, propylparaben, and water
The aluminum hydroxide, bentonite, and carbomer and dimethiocne polymers should be insoluble in water so they will be removed upon filtration after boiling in water.
The remaining chemicals are probably moderately soluble in acetone so they will be present as impurities when the decarboxylation of benzoic acid with a strong base is performed. All of them are esters except for potassium hydroxide (which is not a problematic impurity) and propylene glycol. These esters will probably saponify into their corresponding alcohols and carboxylic acids under these conditions. So glyceryl stearate will give glycerol and stearic acid, isopropyl myristate will give isopropanol and myristic acid, methylparaben and propylparaben will give methanol and propanol, respectively, and para-hydroxybenzoic acid which will decarboxylate to give phenol.
| Compound | Boiling Point |
| Methanol | 65°C |
| Benzene | 80°C |
| Propanol | 97°C |
| Phenol | 182°C |
| Propylene Glycol | 188°C |
| Glycerol | 290°C |
| Stearic Acid | 383°C |
| Myristic Acid | high |
Looking at the boiling points of all of the chemicals in the mixture from the table above, fractional distillation of the mixture, assuming no azeotropic behavior, would easily separate methanol, propanol, and benzene from the other chemicals. A simple water extraction would then separate the two alcohols which are miscible in water from benzene which is practically insoluble in water.
So from all that, the deduced plan would be to heat the acne cream in boiling water for a while and then filter the solids off. The solids would then be extracted with acetone, and the resulting solution would then be evaporated. The residue containing benzoic acid would then be heated with solid sodium hydroxide to give benzene and impurities. The resulting mixture, after cooling, would then be fractionally distilled and finally washed with water to give pure benzene.