To a stirred solution of 4-ethylphenylthioacetic acid (12 g, 61.1 mmol) and vanadium pentoxide (100 mg) in acetonitrile (50 ml) was slowly added 50% hydrogen peroxide (15 ml) at 0 °C. The reaction mixture was continued to be stirred at room temperature for 2 hours. Subsequently, the reaction mixture was slowly poured into ice water and the aqueous phase was extracted with ethyl acetate. The organic layers were combined and concentrated under reduced pressure to give 10 g of 4-ethylsulfonylphenylacetic acid as a white solid. The structure of the product was confirmed by 1H NMR (DMSO-d6): δ 12.54 (broad peak, 1H), 7.83 (d, J = 8.0 Hz, 2H), 7.55 (d, J = 8.4 Hz, 2H), 3.73 (s, 2H), 3.30 (q, J = 7.2 Hz, 2H), 1.09 (t, J = 7.2 Hz, 3H).
