Liquid-phase oxidation of 2-chlorotoluene with ozone to 2-chlorobenzoic acid – an intermediate for diclofenac sodium production

Abstract

2-Chlorobenzoic acid is used as an intermediate for diclofenac sodium production. In industry, it is obtained by oxidation of 2-chlorotoluene with potassium permanganate in an aqueous acidic medium. Due to the deficiency and cost of the oxidant as well as the formation of large amounts of toxic wastewater, the known method has no future. Therefore, an urgent task is to develop a new, low-waste synthesis of 2-chlorobenzoic acid using the available and ballastless oxidant – ozone. The kinetics and mechanism of the liquid-phase oxidation reaction of 2-chlorotoluene with ozone are studied. It was found that the oxidation of the substrate with ozone occurs mainly in the aromatic ring with the formation of aliphatic peroxide compounds. Increasing the temperature to 90 °C and introducing the catalyst – cobalt (II) acetate – into the system contributes to an increase of oxidation selectivity by the methyl group from 18.0% to 92.5 %. Among the aromatic products, 2-chlorobenzoic acid (88.0 %) and 2-chlorobenzaldehyde (4.2 %) are formed.The aim of the work is to study the kinetics, the mechanism of the liquid-phase oxidation reaction of 2-chlorotoluene with ozone and to develop a new method of 2-chlorobenzoic acid synthesis basing on the research performed.Materials and methods. Glacial acetic acid manufactured by Sigma, “Ch. P.” qualified; 2-chlorotoluene, 2-chlorobenzaldehyde, 2-chlorobenzoic acid manufactured by company Acros organics company, “Ch. P.” qualified; cobalt (II) acetate, “Ch. P. qualified” were used in the experiments. Studies of the reaction mass quantitative composition were performed by gas-liquid chromatography. 2-chlorobenzoic acid was determined with potentiometric titration. Ozone concentration at the reactor inlet and outlet was determined by the absorption spectrophotometric method in the range of 254–290 nm on a SF-46 spectrophotometer designed to measure the transmission coefficients of liquid and solid transparent substances in the spectral range from 190 nm to 1100 nm.Results. The kinetics and mechanism of the liquid-phase oxidation reaction of 2-chlorotoluene with ozone have been studied. It was found that raising the temperature to 90°C and introducing a catalyst – cobalt (II) acetate – into the system results in an increase in the selectivity of oxidation by the methyl group from 18.0 % to 92.5 %. Among the aromatic products, 2-chlorobenzoic acid (88.0 %) and 2-chlorobenzaldehyde (4.2 %) are formed.Conclusions. During the oxidation of 2-chlorotoluene with ozone in acetic acid, mainly aliphatic peroxide compounds are formed, and 2-chlorobenzoic acid is formed only with a yield of 16.5 %. Increasing the temperature to 90 °С and introducing cobalt (II) acetate into the catalyst system makes it possible to direct the process mainly to the methyl group with the formation of 88.0 % 2-chlorobenzoic acid.