Discussion on the Preparation Process of P-Nitrobenzoic Acid

^{P-nitrobenzoic acid (4-nitrobenzoic acid) is a crucial organic synthetic intermediate. It appears as light yellow crystals or powder and is employed in the production of pharmaceuticals such as Alprenolol, Prilocaine hydrochloride, Procaine hydrochloride, Folic acid, Benzocaine, Cefaclor, and Noscapine. It is also used in the synthesis of dyes and photosensitive materials, including Reactive Brilliant Red M-8B and Reactive Red Purple X-2R. Additionally, it finds application as a light filter, metal surface rust inhibitor, color film toner, and sunscreen.}

^{The primary synthesis method for P-nitrobenzoic acid involves nitration of toluene, specifically oxidizing para-nitrotoluene. Discovering more efficient and practical synthetic methods for the preparation of  P-nitrobenzoic acid and its derivatives holds significant importance for the dye, pharmaceutical, pesticide, and photosensitive material industries. Here, we discuss the following processes:}

^{1、Potassium permanganate oxidation method}

^{P-nitrobenzoic acid  can be prepared by oxidizing para-nitrotoluene with potassium permanganate, a strong oxidizing agent. According to different detailed processes and catalysts, it can be further categorized as follows:}

^{① Using KMNO4 as the oxidizing agent and hexadecyl tributyl phosphonium bromide as the catalyst, the yield can reach over 75%.}

^{② Using KMNO4 as the oxidizing agent, PEG-200 as the catalyst, in an acidic medium, at an environment of 90°C, the yield is approximately 47%.}

^{③ Using KMNO4 as the oxidizing agent, with a dilute sulfuric acid concentration of 1.5mol/L, at an environment of 60°C, the yield exceeds 80%.}

^{④ Using KMNO4 as the oxidizing agent and quaternary ammonium salt A-1 as the catalyst, the yield can exceed 92%.}

^{This method has the advantages of mild reaction conditions and relatively high yields. However, the disadvantages include the generation of numerous by-products, complex operations, and unsuitability for industrial-scale production.}

^{2、Nitric Acid Oxidation Method}

^{① Oxidizing polystyrene with nitric acid using microwave irradiation to convert it to P-nitrobenzoic acid, achieving a yield of 80%.}

^{② Oxidizing para-nitrotoluene with nitric acid at 200°C for 1.5 hours, resulting in a yield of around 98%.}

^{This method significantly reduces costs, is operationally simple, and has a high yield, making it widely adopted by many enterprises. The drawback is that nitric acid can severely corrode equipment, and a substantial amount of wastewater is generated.}

^{3、Sodium dichromate oxidation method}

^{① Oxidizing para-nitrotoluene with sodium dichromate-sulfuric acid, heating under reflux for 1 hour, and using ethanol-water for recrystallization of the crude product, achieving an 80% yield.}

^{② Oxidizing para-nitrotoluene with sodium dichromate-sulfuric acid; adding a small amount of Mn2+ to the oxidizing solution can increase the yield, ultimately reaching 85%.}

^{This method is similar to the nitric acid oxidation method and yields favorable results. However, the drawback remains severe equipment corrosion, making it unsuitable for large-scale industrial production.}

^{4、Sodium Hypochlorite Oxidation Method}

^{Using sodium hypochlorite for the oxidation of para-nitrotoluene, with RuO4 as the catalyst, achieving a yield of 87%.}

^{While this method has mild reaction conditions and a relatively high yield, the costs are excessively high, and severe equipment corrosion makes it challenging to achieve industrial-scale production.}

^{5、Air Oxidation Method}

^{① Using aromatic hydrocarbons as solvents, adding a catalyst, and directly oxidizing para-nitrotoluene with air, achieving a yield of 94%.}

^{② Using o-dichlorobenzene-propionic acid as a solvent, with a catalyst composed of cobalt acetate and bromide complex, oxidizing para-nitrotoluene with air, resulting in a yield of around 90%.}

^{This method features a simple process, minimal environmental pollution, reduced waste generation, and alleviated equipment corrosion. Moreover, it significantly lowers costs, making it conducive to large-scale industrial production.}

^{6、Oxygen Oxidation Method}

^{① Using N-hydroxyphthalimide as a catalyst, glacial acetic acid as a solvent, and oxygen for the oxidation of para-nitrotoluene, achieving a yield of 82%.}

^{② Using tetracarboxylic cobalt phthalocyanine as a catalyst, under alkaline conditions, at an environment of 55°C, and using oxygen for the oxidation of para-nitrotoluene, resulting in a yield of around 87%.}

^{③ Using propionic acid as a solvent, cobalt dichloride, manganese sulfate, and potassium bromide as composite catalysts, and oxygen for the oxidation of para-nitrotoluene, at an environment of 170°C, 0.7 MPa pressure, with a reaction time of 6 hours, yielding approximately 72%.}

^{This method features a simple process, easy product separation, and the oxidation agent is inexpensive and readily available. Although the cost is slightly higher than the air oxidation method, it still holds promising application prospects.}

^{7、Ozone Oxidation Method}

^{① Employing ozone for the oxidation of para-nitrotoluene at a temperature of 95°C, gas flow rate of 30 L/h, ozone volume fraction of 1.0%, achieving a yield of 93%, and the mother liquor can be recycled for further use.}

^{② Using Co(OAc)2KBr as the catalyst for the oxidation of para-nitrotoluene with ozone, resulting in a yield of approximately 96%.}

^{This method can be conducted under atmospheric pressure, with recyclable mother liquor, and yields high purity. However, the catalyst preparation is complex, leading to relatively high costs.}

^{8、Hydrogen Peroxide Oxidation Method}

^{Hydrogen peroxide is a common and excellent green oxidizing agent. It is non-toxic, odorless, decomposes into non-hazardous water and oxygen, and is cost-effective, with mild reaction conditions.}

^{① In a high-pressure vessel, using 5% HBr at an environment of 200°C, employing hydrogen peroxide for the oxidation of para-nitrotoluene, achieving a yield of 84%.}

^{② Using methanol as a solvent, under alkaline conditions, at an environment of 30°C, employing hydrogen peroxide for the oxidation of para-nitrobenzaldehyde, resulting in a yield of around 89%.}

^{This method is highly environmentally friendly, economically efficient, and avoids the environmental pollution and equipment corrosion associated with traditional processes. It represents an excellent technological route in line with modern environmental requirements.}

^{In recent years, the synthesis methods and technologies for P-nitrobenzoic acid  have been continuously improved and enhanced through the research and exploration of scientists worldwide. Currently, the air oxidation method stands out for its minimal environmental pollution, simple operation, and high economic benefits, making it an excellent method for preparing P-nitrobenzoic acid . However, with the development of science and technology, hydrogen peroxide is emerging as a superior oxidizing agent due to its excellent oxidation performance, low cost, and abundant sources. It is an efficient, economical, and environmentally friendly oxidation method, with the potential to become the ideal oxidizing agent.}