Process for the preparation of malathion and its intermediate

1. 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 H 2 S 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.NH 3 salt;

vi) the above obtain O,O-DMDTPA.NH 3 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).

2. A process for the preparation of intermediate O,O-dimethyldithiophosphoric acid of formula (II)

for use in the preparation of Malathion, 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 H 2 S 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 ethyl acetate and ammonia gas is purged to precipitate the pure O,O-DMDTPA.NH 3 salt;

vi) the above obtain O,O-DMDTPA.NH 3 salt is subjected to neutralization with concentrated sulphuric acid to get O,O-DMDTPA and 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 product.

3. The process as claimed in claim 1 , wherein said phosphorous sulfide is selected from phosphorus pentasulfide, tetraphosphorus heptasulfide and tetraphosphorus decasulfide.

4. The process as claimed in claim 1 , wherein said organic solvent is selected from hexane, benzene, toluene, diethyl ether, chloroform, ethyl acetate and dichloromethane.

5. The process as claimed in claim 1 , wherein in step v) the duration of the reaction ranges from 1 to 4 hours.

6. The process as claimed in claim 1 , wherein in step (viii), molar ratio of diethyl maleate to O,O-DMDTPA is 1:2.5.

7. The process as claimed in claim 1 , wherein said sulfur reagent used in step xi) is selected from sodium sulfide, potassium sulfide, calcium sulfide, ammonium sulfide and ammonium bisulfide.

8. The process as claimed in claim 1 , wherein in step xii) malathion is crystallized from methanol at temperature in the range of −45 to −25° C.

9. The process as claimed in claim 1 , wherein in step xiii) the drying temperature ranges from 15 to 40° C.

10. The process as claimed in claim 1 , wherein in step xiii) the duration of the drying ranges from 5 to 9 hours.

11. The process as claimed in claim 1 , wherein in step xiii) the water content of isopropanol used is below 0.05%.

12. The process as claimed in claim 1 , wherein said Malathion prepared by the process of claim 1 , after storage at 8-15° C. for a period of 6 months, has the following purity/impurity profile:

i. greater than about 99.5% (w/w) Malathion

ii. less than 0.09% of Isomalathion

iii. less than 0.03% of O,O,S-trimethyl phosphorodithioate

iv. less than 0.002% of diethyl fumarate

v. less than 0.1% of unknown impurities

vi. less than 0.21% of total impurities and

vii. water content is less than 0.02%.

13. The process as claimed in claim 2 , wherein said phosphorous sulfide is selected from phosphorus pentasulfide, tetraphosphorus heptasulfide and tetraphosphorus decasulfide.

14. The process as claimed in claim 2 , wherein said organic solvent is selected from hexane, benzene, toluene, diethyl ether, chloroform, ethyl acetate and dichloromethane.

15. The process as claimed in claim 2 , wherein in step v) the duration of the reaction ranges from 1 to 4 hours.