4-Acetoxybenzoic acid, 98% (Sigma-Aldrich)
Sodium hydrosulfide hydrate, (Sigma-Aldrich)
Thionyl chloride, 99% (Sigma-Aldrich)
Dimethylformamide, 99.5% (Finar chemicals)
Methanol, 99.5% (Finar chemicals)
Dichloromethane, 99.5% (Finar chemicals)
Tetrahydrofuran
 

A 50 mL round bottom flask was charged with a solution of 4-acetoxybenzoic acid (1.0 g, 5.5 mmol) in dry dichloromethane (10 mL) and catalytic dimethyl formamide (0.2 mL). Subsequently, thionyl chloride (0.48 mL, 6.6 mmol) was added at rt dropwise over a period of 3 min. Then reaction mixture was heated to reflux for 4 h. Complete consumption of starting material was indicated on TLC. The solvent was evaporated in vacuo and crude acid chloride (yellowish oil) was used without purification for next transformation.

To a two necked 50 mL round-bottom flask, equipped with a magnetic stirring bar and a reflux condenser was added a solution of sodium hydrogen sulfide (1.24 g, 22.2 mmol) in methanol (12 mL). This solution was heated to reflux temperature and a solution of the crude acid chloride in THF (15 mL) was added dropwise over 30 min. The reaction mixture was heated at reflux for a further 2 h. The reaction mixture was cooled to rt, solvents were evaporated under reduced pressure and the solid residue was diluted with water (50 mL). The pH of the aqueous solution was adjusted to 2 by adding 6M HCl and was extracted with ethyl acetate (3 x 100 mL), dried over sodium sulfate, filtered and evaporated to dryness. Flash column chromatographic purification (2:8 Ethyl acetate:Petroleum ether) gave 4-hydroxythiobenzoic acid (0.541g, white solid, yield: 63%) as a major product and methyl-4-hydroxybenzoate (0.08 g, white solid, yield: 9%) as minor product.

1H NMR (CD₃OD = 4.87 δ, 200 MHz): δ 7.9(d, 2H, J=8.97 Hz), 6.75(d, 2 H, J=8.97 Hz);

MS: EI, M/Z. 154(M+), 121(bp), 93, 65.