MECARBAM

Description

Mecarbam is a pale yellow to light brown oil, bp 144 ?C/0.02 mm Hg. It is slightly soluble in water (<0.1% at 25 ?C) and miscible with most organic solvents except alkanes. Log Kow = 2.6. It is readily hydrolyzed in aqueous media below pH 3.

Chemical properties

Yellow oil.

The Uses of MECARBAM

Mecarbam is an organothiophosphate acaricides used to control powdery mildew of cucumbers.

The Uses of MECARBAM

Mecarbam is an insecticide with some systemic activity which is used to control sucking insects and mites in fruit trees and plant hoppers and leaf miners in rice. Additional uses are the control of whitefly, leafhoppers and thrips on rice and root flies on vegetables.

Definition

ChEBI: An organic thiophosphate that is O,O-diethyl hydrogen phosphorodithioate in which the hydrogen attached to a sulfur is replaced by a 2-[(ethoxycarbonyl)(methyl)amino]-2-oxoethyl group.

Hazard

Highly toxic, cholinesterase inhibitor. Use may be restricted.

Metabolic pathway

Mecarbam in a close chemical analogue of dimethoate and it is photolytically decarboxyethylated to the diethyl analogue of dimethoate. The main route of hydrolytic decomposition in moderately basic solution is via attack on the S-methylene carbon atom rather than on phosphorus or the carbamoyl group, followed by cleavage of the S-C bond yielding O,Odiethyl phosphorodithioate as the main product. Mecarbam oxon has been reported as a metabolite in plants and animals but few detailed studies have been reported on the metabolic or environmental fate of mecarbam.

Metabolism

In mammals, mecarbam is rapidly metabolized by hydrolysis, oxidative desulfuration to the oxon, and degradation of the carbamoyl moiety. O-Deethylation also takes place to a minor extent. In soil, it persists for 4–6 weeks.

Degradation

The rate of hydrolysis and the nature of the products when mecarbam was dissolved in buffered water at pH values between 2.2 and 10 were reported by Lynch et al. (1981). The analysis of mecarbam and its metabolites was by GC-MS with and without methylation by diazomethane and by TLC co-chromatography with reference materials and included the use of chromogenic sprays to differentiate between phosphorothioates (P=S) and phosphates (P=O) . Mecarbam was stable in acid media but was hydrolysed in alkaline solutions with a half-life of about 44 hours at pH 9.2. The principal product of base hydrolysis was O,O-diethyl phosphorodithoate (2), indicating that mecarbam was mainly hydrolysed by nucleophdic attack on the S-methylene carbon atom followed by cleavage of the S-C bond. There was evidence that O,O-diethyl phosphorodithioate (2) was hydrolysed further to give the des-ethyl compound, O-ethyl phosphorodithioate (3). When mecarbam was hydrolysed under much more basic conditions (0.5 M aqueous potassium hydroxide) the main routes of hydrolysis were attack by OH- on the amido carbon atom followed by cleavage of the C-N bond to give S-carboxymethyl O,Odiethyl phosphorodithioate (4) and attack on the phosphorus atom followed by cleavage of the P-S bond to give O,O-diethyl phosphorothioate (5). Initial hydrolysis on the carboethoxy (carbamoyl) group was not observed and nor was reaction product 2 detected, indicating that initial attack on the phosphorus and carbonyl centres was faster in strongly basic solution (Hudson et al., 1991).
The main product of photolysis when mecarbam was absorbed on a filter paper and exposed to 254 nm UV light from a mercury vapour lamp was the decarboxyethylated compound O,O-diethyl S-methylcarbamoylmethyl phosphorodithioate (6) (the diethyl analogue of dimethoate). The rate of &us reaction was potentiated by the surface of certain leaves, with dock leaves being particularly effective.O,O-Diethyl phosphorodithioate (2) was also detected in trace amounts (Lynch et al.,1981). Proposed routes for the photolysis and base-catalysed hydrolysis of mecarbam are shown in Scheme 1.

Toxicity evaluation

Acute oral LD₅₀ for rats is 35–53 mg/kg. Inhalation LC50 (6 h) for rats is 0.7 mg/L air. ADI is 0.002 mg/kg.