Preparation of Benzoic Acid by the Oxidation of Toluene

The chemical formula for benzoic acid is C₆H₅COOH. Its basic structure consists of a benzene ring (hexagonal), C₆H₆ with one of the hydrogen atoms replaced by a carboxyl group (-COOH). There are assorted ways in which benzoic acid can be produced. One popular method is by the simple oxidation of toluene.

History

The concept of producing benzoic acid from alkylbenzenes is not new. Evidence of that is the free, downloadable 1919 thesis publication, "Preparation of Benzoic Acid by Oxidation of Toluol," by DW Grigsby and EE McClung.¹

Starting Materials

Although not limited to the production of simple benzoic acid, that example well illustrates the simplicity of the oxidation of alkyl side-chains to produce carboxylic acids. The starting material in this instance is the readily available methylbenzene, or toluene, C₆H₅CH₃. The methyl group (-CH₃) only is converted by oxidation; the aromatic ring remains unscathed.

The basic reaction is:

2 (-CH₃) + 3 O₂ ---> 2 (-COOH) + 2 H₂O

The source of oxidation in the laboratory is often dichromate or permanganate based, coupling these salts with sulfuric acid. Industrially, oxygen is more often used, in conjunction with a heavy metal-based catalyst. The co-product of the reaction is water, which can be removed by simple drying.

Benefits of the Alkylbenzene Oxidation Technique

Chemical reactions involving organic compounds rarely produce a yield of 100 percent. It is far more typical to obtain yields of 95 percent and less. Frequently yields may dip considerably below 90 percent. Also, reaction conditions may be very complicated, and sometimes even dangerous. Side-reactions may contaminate the yield, forcing the need of a very expensive purification. Contrast this with the production of benzoic acid:

There is only a single, uncomplicated step.
Raw materials are readily available and inexpensive.
The process is efficient; the yield is high.
Co-product presents little difficulty.
Solvents used are recoverable.

Applications

Benzoic acid is widely used as a fungistatic preservative by the food industry, sometimes in the form of the sodium salt. It is also used industrially in plasticizers, such as diethylene glycol dibenzoate, dipropylene glycol dibenzoate, and 1,6-hexanediol benzoate. Benzoic acid is also used in perfumes and flavorings.

¹ toluol: an old name for toluene

References and Resources

Sevas Bioinformatics: Manufacturing Processes: Terephthalic Acid

University of Calgary: Oxidation of Alkyl Benzenes

Official Site State of New Jersey: Right to Know Hazardous Substance Fact Sheet: Benzoic Acid