Process for synthesis of fipronil

A process for preparing fipronil, said process comprising the following steps:

• a. oxidizing, in a solvent selected from the group consisting of ethylene dichloride, methylene dichloride, carbon tetrachloride, chloroform, dibromoethane, bromobenzene, chlorobenzene and ortho-dichlorobenzene, a reactant mixture containing a compound 5-amino-1-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-trifluoro methyl thio-pyrazole of formula II and concentrated sulfuric acid having concentration in the range of 75% to 98% w/w, with hydrogen peroxide having concentration in the range 40 to 70% w/w, at a temperature in the range of -10° C. to 40° C. to obtain an oxidized product mixture;
• b. quenching said product mixture with water at a temperature in the range of 10 to 25° C.;
• c. heating the quenched product mixture at a temperature in the range of 60 to 70° C. to obtain a biphasic system containing an aqueous phase containing sulphuric acid and an organic phase containing the oxidized product;
• d. isolating said organic phase by separating the aqueous phase;
• e. neutralizing the isolated organic phase containing the oxidized product to obtain crude fipronil;
• and
• f. crystallizing the crude fipronil to obtain crystallized fipronil.