Diphenyl Ether → 4-phenoxybenzenesulfonic Acid

To a stirred solution of 42.5 g (0.25 mol) of phenyl ether in 200 mL of dichloromethane at −20° C. under argon was slowly added 23.3 g (0.20 mol) of chlorosulfonic acid. After the addition was complete, the reaction was allowed to slowly warm to room temperature. After 16 hours, 150 mL of isooctane was added and the solution was concentrated to an oily residue. Redissolution in 200 mL of 1:3 dichloromethane/isooctane and reconcentration with cooling to about 100 mL gave a solid. The supernatant was decanted, and the solid triturated with additional isooctane and then dried in vacuo to give 55.2 g of crude 4-phenoxybenzene sulfonic acid. The crude acid was dissolved in 200 mL of dichloromethane, and 34 g (0.25 mol) of oxalyl chloride was added, followed by 2.5 mL of DMF. After 2 days, the reaction solution was poured into 200 mL of ice water, and extracted with 400 mL of hexane. The organic layer was washed with 100 mL of water and 100 mL of brine, dried over magnesium sulfate, and concentrated. Recrystallization of the residue from dichloromethane/isooctane gave 38.5 g of 4-phenoxybenzenesulfonyl chloride as a white solid: mp 41.5° C.