The general procedure for the synthesis of 3-methyl-5-nitrobenzoic acid from 3,5-dimethylnitrobenzene was as follows: 3,5-dimethylnitrobenzene (30 g, 0.198 mol) was added with stirring to a solvent mixture of pyridine (400 ml) and water (250 ml), and heated to 80°C. Potassium permanganate (KMnO4, 62.7 g, 0.396 mol) was added in batches over a period of 0.75 hr. 0.396 mol) and heating was continued at 85°C-90°C for 1.75 hours. The hot reaction mixture was filtered through diatomaceous earth and the diatomaceous earth was washed with hot water (150 ml). The pink filtrate was decolorized with a few drops of sodium metabisulfite (NaHSO3) and the solution was evaporated to dryness. The residue was dissolved in water (250 ml) and extracted with ether (2 x 90 ml). The aqueous layer was acidified with concentrated hydrochloric acid and extracted with ethyl acetate (3 x 120 ml). The organic extracts were combined, washed sequentially with sodium dihydrogen phosphate (NaH2PO4) solution and brine, and dried over anhydrous magnesium sulfate (MgSO4). Using a mixed solvent of ethyl acetate/dichloromethane/acetic acid (25:25:1), the crude product was eluted by a silica pad to afford 3-methyl-5-nitrobenzoic acid (14.5 g, 40% yield) with a melting point of 171°C-172°C. Nuclear magnetic resonance hydrogen spectroscopy (1H NMR, DMSO-d6) data were as follows: δ 2.51 (single peak, 3H); 8.17 ( single peak, 1H); 8.30 (triple peak, 1H); 8.42 (triple peak, 1H); 13.58 (broad peak, 1H).
