Syntheses, characterization and biological activities of two carbamate pesticides

Two carbamate pesticides namely S-ethyi-N-[(methylcarbamoyl)oxy]thioacetimidatc (a) (CH_~-C=NOCONHCH 3 ) and N-phe· nyl-(ethylcarbamoyl)propyl carbamate (b) [C 6 H 5 NII-COO-CH(CH_~)·CO-NH(C 3 H 7 )J were synthesized in the laboratory. With the help of elemental analysis. I R and NMR spectra their structures were elucidated. The biological studies showed that S· ethyi-N-[(methylcarbamoyl)oxy]thioacetimidate (a) acts as insecticide against insects causing damages to potato, tomato, cab· bage plants, while N-phenyl-(ethylcarbamoyl)propyl carbamate (b) has herbicidal character and can be used for cabbage and legume family crops. For estimation of the toxicity of these pesticides LC 50 and L0 50 were determined. The results re· vealed that for fish and rats compound (a) was more toxic than compound (h).

As organochlorine compounds were highly persistent and so strongly dissolved into animal fat, that they get concentrated as they moved upto the food chain causing damaging effects, were banned; therefore, broad spectrum biodegradable short lived chemical compounds such as organophosphates, carbamates and pyrethroids were developed 1 -5 . Carbamate pesticides are mainly used in agriculture as insecticides, fungicides, herbicides, nematocides. They are also used as biocides for industrial purposes. Carbamate-pesticides are also effective against those pests which are resistant to organochlorine and organophosphates.
Oxamyl, a product of E.I. DuPont de Nemours & Co Inc., Wilmington, Delaware, is a broad spectrum insecticide-nematicide with excellent biological efficacy on a variety of crops. To compare efficacy of oxamyl it was considered worthwhile to synthesize following two new carbamate pesticides (I) compound (a) S-ethyi-N-[(methylcarbamoyl)oxy ]thioacetimidate, (2) compound (b) N-phenyl-(ethylcarbamoyl)propyl carbamate and study their physical, chemical and biological properties.

Results and discussion
The physical properties of synthesized compounds a and b, determined by usual methods, are given in Table I. Their ~tructure were c;;onfirmed by element analysis, IR and NMR spectra. For compound a, C 6 H 12 N20 2 S experimental analysis gave: C, 41.02 (41.91); H. 6.80 (6.82); N, 15.76 (I 5.   Preliminary studies suggest that compound a acts as an insecticide and compound bas a herbicide. The result shows that on spraying 100-400 g ai ha-1 of compound a on the Note  crop, the larval population of insect P. xylostella decreased 50 to 72% after seven days of application (Table 2). But after 14 days of application, the larval population increased (27-69%) from lst day of application, indicating the degradation of compound a. Table 2 also denotes that with the increase in concentration from 400 to 1000 g ai ha-l of compound b, the herbicidal activity was increased and more effective in garden legume.
The LC 50 ( Table 3) for fish of compound a was 3.9 ppm while that of compound b was 25.2 ppm denoting that compound a had more aquatic toxicity than compound b.
The oral LC 50 (Table 3) value for male rats of compound a was 31.7 mg kg-1 body weight and compound b was 3018 mg kg-1 body weight.
From these studies it may be inferred that compound a has more pesticidal activity than compound b but later has more stability.

Swdies on insecticidal activity of S-ethyl-N-(methylcarbamoyl)oxy ]thioacetimidate (a) :
The field trial was laid to study insecticidal 6 activity of compound a in five 5 m 2 plots. The cabbage was selected for the field trial. Four aqueous solution of compound a with concentration 100,200,300,400 g ai ha-1 was sprayed ~eparately in four plots (fifth was for blank). Before day one and after l. 7 and 14 days of spraying population counts of insect, Plmella xylostella were recorded. From these data mean larval population on cabbage was estimated. The data are rl.!corded in Table 2.

Eva/uatio11 of herbicidal activity of N-phenyl( ethylcarbamoyl)propylcarbamate) (b) :
For the foliar application test 7 the aqueous solution of compound b was sprayed over the foliage of cabbage, lettuce, garden legume and wheat plants at the spray doses of 400, 800, l 000 g ai ha-1 separately. Spraying was performed with the plants employed at either the two or three leaf stage. The growth inhibition of compounds was evaluated at 400, 800 and 1000 g ai ha-1 . Approximately three weeks after the spraying the herbicidal activity of each dose was as· sessed by visual observation of the treated plants in comparison with the untreate{' controls. According to the extent of the injury of plants htrbicidal potency was assessed. Mean values are recorded in Table 2.

Determination of LC 50 :
To determine the LC 50 of synthesized compounds a and b for fish a solution of 2 to J 0 ppm of compound a and I 0 to 60 ppm of compound b was prepared separately in 20 L of water and stored in aqumia. Twenty five fish were transferred to each aquarium containing test solution, which was Note changed after 24 h to maintain the test concentration. No food was provided during the experimental period. The survival number in each concentration was recorded after 96 h. A control aquaria with equal number of fish was also maintained under similar condition. The data are recorded in Table 3.

Determination of LD 50 :
For determination of LD 50 of synthesized compounds a and b, nine sets of experiments of each compound with 12 male rats were performed separately with suitable blank. The concentration of compound a was from 15 mg kg-1 to IOO mg kg-1 body weight while of compound b was 750 to 12500 mg kg-1 body weight (average body weight of rat was 180 ± 20 g). After 96 h of treatment the mortality number and percentage for each dose was noted. The data are given in Table 3.
Log-dose and probit mortality method 8 was used for calculation of L0 50 . The data are recorded in Table 3.

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IR (KBr) and NMR of both the compounds a and b were also recorded.