Benzoic acid (40.0 g, 165 mmol) and H2SO4 (96%; d = 1.84 g mL-1; 80 mL) were heated and stirred at 110–115 °C for 30 minutes. The benzoic acid dissolved within 5–10 minutes, yielding a deep red solution. The mixture was cooled to room temperature and poured into ice water (400 mL). The yellow precipitate was filtered off and washed thoroughly with water until pH = 7. Drying yielded 9-fluorenone-4-carboxylic acid (36.85 g, 99.5%), indicating sufficient purity. Recrystallization from acetic acid (300 mL) yielded a yellow needle-like substance (33.05 g, 89.3%).
