NOVELTY - Synthesis of p-carboxybenzene sulfonamide (I) involves dissolving p-toluenesulfonyl chloride (II) in toluene, dichloromethane, trichloromethane, dimethyl sulfoxide or ethyl ether, adding ammonia source, performing ammonolysis at 20 degrees C for 1-2 hours, adding obtained methylbenzenesulfonamide (III), catalyst, hydrogen peroxide and water to a round bottom flask, stirring, filtering to remove catalyst, washing the catalyst with water, drying, reusing the catalyst, adjusting pH of the filtrate to 2-3, crystallizing, filtering and drying.    USE - Synthesis of p-carboxybenzene sulfonamide (claimed).    ADVANTAGE - The method enables economical synthesis of p-carboxybenzene sulfonamide using nontoxic catalyst and oxidant, under mild reaction conditions, by simple and environmentally-friendly process. The byproduct of synthesis process is water or alcohol.    DETAILED DESCRIPTION - Synthesis of p-carboxybenzene sulfonamide of formula (I) involves dissolving p-toluenesulfonyl chloride of formula (II) in toluene, dichloromethane, trichloromethane, dimethyl sulfoxide or ethyl ether, adding ammonia source, performing ammonolysis at 20 degrees C for 1-2 hours, adding obtained methylbenzenesulfonamide of formula (III), catalyst, hydrogen peroxide and water to a round bottom flask provided with a magnetic stirrer, adjusting pH to 8-14 using hydrochloric acid and sodium hydroxide, stirring at 40-100 degrees C for 3-10 hours, filtering to remove catalyst, washing the catalyst with water, drying, reusing the catalyst, adjusting pH of the filtrate to 2-3 using hydrochloric acid, crystallizing, filtering and drying. The catalyst is metal oxide or heteropoly acid catalyst. The molar ratio of compound (III) and hydrogen peroxide is 1:2-1:10. The ammonia source is concentrated aqueous ammonia, ammonium chloride, ammonium carbonate, or 1-6C primary amine or secondary amine. The molar ratio of p-toluenesulfonyl chloride (II) and ammonia source is 1:2-1:10. The hydrogen peroxide is t-butyl hydrogen peroxide or cumyl hydrogen peroxide. The content of catalyst is 1-5 mol% with respect to the content of compound (III).    R1,R2=H or 1-6C alkyl;and    R=H, (CH3)3C or PhC(CH3)2.