How to synthesize Benzocaine?

Description

Benzocaine (ethyl p-aminobenzoate), an ethyl ester of p-aminobenzoic acid, is an anesthetic acting as a blocker of nerve impulses and reducing the permeability of the neuronal membrane to sodium iodide; it has become widespread in the pharmaceutical industry having applications as an anesthetic before exams such as endoscopy or as a “retarding” agent when inserted in a condom.

Benzocaine is used in topical anesthesia on the skin and mucous membranes as aerosols or as creams to reduce pain caused by itching, cuts, bites, etc. It begins to work 15–30 sec after application and lasts 12–15 min. It is also used under the names anestezin, dermoplast, and others.

Synthesis

The classic, optimal way of benzocaine synthesis is reducing the nitro group of the ethyl ester of 4-nitrobenzoic acid to benzocaine by hydrogen, which generates directly in the reaction medium by the reaction of iron filings with dilute acids.

The esterification reaction was realized in batch conditions by adding a 50 mL round-bottom flask of the p-nitro benzoic acid (3.12 g, 0.018 mol) in 15 mL of ethanol. The mixture was cooled in an ice bath, and 1.0 ml (0.018 mol) of concentrated sulfuric acid was slowly added. A large amount of precipitate formed but slowly dissolved when the mixture was heated. A reflux condenser connected with a cooler adjusted to 4 °C was attached, and the mixture was heated under 150 °C for 3 h. (monitored by TLC eluted with 30:70 ethyl acetate: hexane). Additional ethanol was added to the reaction vessel to keep the solvent volume during the experiment. The reaction mixture was cooled at room temperature, and the precipitated solid was filtered and washed with hexane (3 × 5 mL) to give the benzocaine (98% yield). A 20 μL aliquot was transferred to a new vial, and 980 μL of ethyl acetate was added and analyzed in the GC-MS.

References

[1] Alexandre de S. França, Rodrigo O. M. A. de Souza, Raquel A. C. Leão. “Two step continuous-flow synthesis of benzocaine.” Journal of Flow Chemistry 10 3 (2020): 563–569.