General Aspects

The insecticides belonging to the carbamate family are esters of carbamic acids, with the general structure depicted in Figure 2.6. There is a strong variability in their spectrum of activity, mammalian toxicity and persistence upon chemical modifications in their general structure.181

Carbamate insecticides were developed in the 1950s as a result of investigations on compounds exerting anticholinesterase action on the nervous system similar to that of the organophosphorus insecticides, in an attempt to obtain insecticides with an increased selectively and reduced mammalian toxicity. They are still widely used today.1,182 The structure of these compounds was inspired by the biologically active alkaloids isolated from plants bearing a carbamato moiety.183

As with organophosphorus esters, carbamates exert their toxicological effects against insects by inhibiting nervous tissue AChE found in the synaptic spaces and on the postsynaptic membranes of all neurons. Such activity takes place through a carbamylation reaction in the active site of the enzyme, so a dea-cylation reaction on acetylcholine does not take place.

Therefore the overall effect of suppressing the normal functioning of such enzyme is an impairment of the conduction of signals in the insect nervous system.1 Organophosphorus insecticides react irreversibly with acet-ylcholinesterase (AChE), as indicated in section 2.3.2.4, whereas carbamates act reversibly and their inhibitory effect is so brief that measurements of blood cholinesterase levels in human beings or other animals exposed to them appear to be normal. For this reason, the volume of carbamates needed for getting an insecticidal action exceeds that of organophosphorus insecticides. When used on crops, it is common that several applications of carbamates are required in a growing season.1

Some methyl carbamate insecticides proved to exhibit high acute toxicity in mammals, in some cases comparable to that in insects,65 but they are in general safer than organophosphorus insecticides as a result of the reversibility of their action. The lethal dose of carbamates is much higher than that of organophosphorus insecticides; slight symptoms can appear under long occupational exposure before a dangerous dose is received.184

It was also demonstrated that in the presence of ionic substituents, the carbamates exhibited no insecticidal properties despite being, in some cases, potent AChE inhibitors. This is due to the fact that ionic groups prevent the compound from penetrating through the lipophilic sheaths surrounding the nerves.65

Carbamates are currently among the most popular insecticides, not only in agriculture, but also for home use, both indoors and on gardens and lawns. These compounds are rapidly detoxified and excreted, so their risk to warmblooded animals is less than for other agents.185 They are relatively unstable compounds that break down in the environment within weeks or months, so their persistence is not generally a problem,182 a major difference from previous organochlorine insecticides.

One significant disadvantage of carbamates is that they are toxic against many beneficial insects, especially honeybees. The widespread use of these compounds has increased the residues in water and foods, especially in fruits, and has led to a higher incidence of accidental poisonings.186,187 The clinical symptoms in carbamate-mediated poisonings range from the classic cholinergic syndrome, to flaccid paralysis and intractable seizures.

Much effort has also been devoted to the study of the behavior of carbamates in the environment. The identification of degraded products allows the degradation pathways involved to be established, and thus it provides valuable information for potential ways of protecting the environment.188 Hydrolysis,189 photodegradation190 and microbial-mediated biodegradation (via carbamate hydrolases)191 are the main ways of environmental degradation of carbamate insecticides. With some soil-incorporated carbamate pesticides, microbial degradation processes may be so rapid as to limit pesticide effectiveness in controlling targeted pests.192

In many occasions, degradation reactions studied under laboratory conditions provide data with significant differences from those obtained in field studies. The reason is the presence in field studies of metal ions,193 metal oxides194 or clays195 that can enhance the transformation rates of the organic pollutants.

In addition, numerous analytical methodologies have been developed for the quantification of carbamate content even at low concentrations in environmental samples, such as soil and water.196

The synthesis of carbamates197 usually involves the reaction of isocyanates (prepared by treatment of amines with hazardous phosgene) with alcohols, or alternatively, the phosgenation of alcohols, followed by reaction with amines (see Scheme 2.6). Due to the environmental problems of the phosgene route, significant attempts have been made to explore a phosgene-free synthesis of carbamates.198

Scheme 2.6 Synthesis of carbamate insecticides.

Carbofuran (70) Carbosulfan (71)

Figure 2.6 General structure and classification of carbamate insecticides.

Carbofuran (70) Carbosulfan (71)

Figure 2.6 General structure and classification of carbamate insecticides.

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