Preparation method of 3-methyl-4-nitrobenzoic acid

The invention relates to a preparation method of 3-methyl-4-nitrobenzoic acid, which comprises the following steps: by taking 2,4-dimethyl nitrobenzene as an initial raw material, carrying out selective oxidation on 4-position methyl to generate 3-methyl-4-nitrobenzoic acid. Compared with the prior art, the method is characterized in that the oxidation comprises the steps of introducing oxygen under stirring and illumination in the presence of an acetonitrile solvent and a catalyst, conducting reacting for 5 hours at the temperature of 30 DEG C, conducting filtering after the reaction is finished, taking filtrate, carrying out rotary evaporation to recover an acetonitrile solvent, adding water, conducting acidifying until the pH is about 3.8, and conducting filtering to obtain 3-methyl-4-nitrobenzoic acid. The conversion rate of 2,4-dimethyl nitrobenzene is greater than or equal to 98%, and the yield of the 3-methyl-4-nitrobenzoic acid is about 65%. The method is mild in condition, stable in reaction, stable in quality, simple in process and convenient to operate.