Process for the preparation of malathion and its intermediate

A process for the preparation of Malathion of formula (I),

• which comprises the steps of:
• i) adding methanol to phosphorous sulfide in organic solvent at 25-50° C. for a period of 1.5-2 hours;
• ii) stirring the above suspension for a period of 5-6 hours at 50-55° C.;
• iii) expelling the H2S gas with nitrogen, after cooling the above suspension to 25-28° C.;
• iv) filtering the above suspension to remove insoluble impurities to obtain crude O,O-DMDTPA;
• v) the crude O,O-DMDTPA is subjected to dissolution in suitable solvent and ammonia gas is purged to precipitate pure O,O-DMDTPA.NH3 salt;
• vi) the above obtain O,O-DMDTPA.NH3 salt is subjected to neutralization with concentrated sulphuric acid to get O,O-DMDTPA;
• vii) the O,O-DMDTPA obtained in the above step is once again purified through ammonia salt formation and neutralization method as mentioned in above steps v) and vi) to get chromatographically pure O,O-DMDTPA of the formula II
• viii) adding the above obtain pure O,O-DMDTPA to diethyl maleate at low temperature of −30 to −25° C. in four lots, each at regular interval of 20 minutes;
• ix) maintaining the above reaction mass at temperature of −30 to −25° C. for a period of 4 hours;
• x) water washing the above mass to remove O,O-DMDTPA;
• xi) treating the above obtain crude Malathion with a sulfur reagent at 5 to 10° C. for a period of 13 hours;
• xii) crystallizing the above obtained Malathion from methanol at low temperature and
• xiii) drying the above obtain mass with anhydrous sodium sulphate in isopropanol to obtain Malathion of formula (I).