A Preparation Process for p-Nitrobenzoic Acid

  ^{p-nitrobenzoic acid (4-nitrobenzoic acid), abbreviated as PNBA, is an important chemical intermediate. It appears as a pale yellow crystal or powder and is mainly used in the pharmaceutical industry. It can be used to produce folic acid, benzocaine, procaine hydrochloride, procainamide hydrochloride, para-aminomethylbenzoic acid, cough suppressants, cephalosporin V, Benadryl, and various other pharmaceuticals. Additionally, it can be used to produce reactive red M-8B, reactive red violet X-2R, optical filters, color film developers, metal surface rust removers, sunscreens, and other chemical products.}

   ^{In previous articles, we discussed various oxidation methods for preparing  p-nitrobenzoic acid from p-nitrotoluene, each with its own advantages and disadvantages. Here, we explore another mild and environmentally friendly preparation process. This method involves using sodium chlorate to oxidize p-nitrotoluene to prepare  p-nitrobenzoic acid  Sodium chlorate does not contain heavy metal ions, and the reaction solution has minimal corrosion to equipment, helping to reduce maintenance costs. Furthermore, the treatment cost of waste liquid is relatively low, making this a preparation process for p-nitrobenzoic acid with industrial prospects.}

^{The process route is as follows:}

Reaction Step: 

^{Add p-nitrotoluene to the reaction vessel, followed by the addition of 98% concentrated nitric acid. Stir in a water bath and heat to about 80°C. Prepare a saturated solution of sodium chlorate in distilled water and add it dropwise slowly. When the yellow-green color in the container fades, continue to add the solution dropwise. Repeat this process several times until the addition is complete. The molar ratio of p-nitrotoluene to sodium chlorate is 1:2.5, and the temperature is maintained at 80°C. React for 4 hours, then continue stirring until the temperature drops to room temperature. Pour the reaction mixture into an ice-water mixture, filter after the ice melts, and wash the residue with water several times. The resulting filter residue is the crude  p-nitrobenzoic acid}

Purification Step: 

^{Add the crude product to distilled water, and slowly add a 15% NaOH solution dropwise until the liquid turns red, then stop adding and let it stand. If the solution decolors, continue adding NaOH until it no longer fades. Then, slowly add anhydrous acetic acid dropwise until the solution decolors and does not revert to the original color within 30 seconds. Filter the solution to obtain the impurities from this purification step as the filter residue. Add 50% dilute sulfuric acid dropwise to the filtrate until no significant precipitate forms. Filter the resulting liquid, and the filter residue is the relatively pure p-nitrobenzoic acid. Repeat this purification step three times. Dry the product to obtain the final product.}

^{This process yields about 81%, making it a relatively ideal preparation route for  p-nitrobenzoic acid.}