Herbicides, urea

Urea herbicides form, along with phenoxy derivatives and triazines, the most important agricultural herbicide group. Since their discovery almost thirty years ago several tens of thousands of urea derivatives have been synthesised in research laboratories, and almost all of the major pesticide manufactures have marketed herbicides with a urea derivative as the active substance. 

Most of the urea derivatives are compounds with herbicidal activity, although some of the aliphatic guanidine derivatives have a fungicidal action and some of the aromatic thioureas a rodenticidal action. 

The first compound with significant herbicidal action discovered among aliphatic urea derivatives, N-butyl-N, N -dimethylurea (Searle, 1954) was never put on the market. Others, for example, halo and polyhaloallyl-N,N-dimethyl ureas, also have herbicidal properties, while symmetric tetraalkyl-ureas are inactive. 

Of the aliphatic urea derivatives a single compound, dichloral urea (DCU), has attained some practical importance (King, 1950), combined with Pyramin it is used in Hungary for weed control in sugar beet. 

Almost all of the urea compounds with good herbicidal action are trisubstituted ureas, containing a free imino hydrogen. According to the receptor theory, this hydrogen plays a role in the formation of the hydrogen bond (for details see the section on the mode of action). 

Deleu et al. [26] have used two-dimensional thin layer chromatography and gas chromatography to separate and identify 10 urea herbicides in concentrations down to 4pg L in river waters (Fig. 14.1). Four different adsorbents were compared. The eluting solutions were  

---

HAs have been issued for bromacil, diuron, and fluometuron no occurrence data are available for tebuthiuron or terbacd (295). Chloroxuron [1982-47-4] fenuronTCA [4482-55-7] andnorea [18530-56-8] also are urea herbicides. 

Liquid paraffin carbamate and urea herbicides stabilization and enhancement spray solution, 20% in toluene, water investigation [241]... 

Urea herbicides give pale yellow and s-triazines blue chromatogram zones on a colorless background. 

These reactions can be opened up to all substances that can yield aniline derivatives in acid or basic medium. Carbamic acid derivatives, numerous variations of which are used as plant treatment agents, provide a striking application. As do urea herbicides and a variety of drug substances e. g. benzodiazepines or phenylbutazone derivatives. 

These high levels were sporadic and transitory. However, some of them were high enough to have caused phytotoxicity, and more work needs to be done to establish whether herbicides are having adverse effects upon populations of aquatic plants in areas highlighted in this study. It should also be borne in mind that there may have been additive or synergistic effects caused by the combinations of herbicides found in these samples. For example, urea herbicides such as diuron and chlortoluron act upon photosynthesis by a common mechanism, so it seems likely that any effects upon aquatic plants will be additive. Similarly, simazine and atrazine share a common mechanism of action. 

El-Fantroussi S (2000) Enrichment and molecular characterization of bacterial culture that degrades methoxy-methyl urea herbicides and their aniline derivatives. Appl Environ Microbiol 66 5110-5115. 

Despite the advances made in high-performance liquid chromatography in recent years, there are still occasionally applications in which conventional column chromatography is employed. These methods lack the sensitivity, resolution and automation of HPLC. They include the determination of urea herbicides in soil, polyaromatic hydrocarbons, carbohydrates, chloroaliphatic compounds and humic and fulvic acids in non-saline sediments. The technique has also been applied in sludge analysis, e.g. aliphatic hydrocarbons and carboxylic acids. 

Spengler and Jumar [90] used a spectrophotometric method and thin layer chromatography to determine carbamate and urea herbicide residues in sediments. The sample is extracted with acetone, the extract is evaporated in vacuo at 40°C and the residue is hydrolysed with sulphuric acid. The solution is made alkaline with 15% aqueous sodium hydroxide and the liberated aniline (or substituted aniline) is steam distilled and collected in hydrochloric acid. The amine is diazotized and coupled with thymol, the solution is cleaned up on a column of MN 2100 cellulose power and the azo-dye is determined spectrophotometrically at 440nm (465nm for the dye derived from 3-chloro- or 3.4-dichloroaniline) with correction for the extinction of a reagent blank. 

Miscellaneous urea herbicides (chlorobromuron (3,4(bromo-3-chlorophenyl)-1 -methoxy-1 -methylurea), Chlorotoluron (3(3 chlorotoluyl) dimethyl urea), Diuron (N-(3,4 chlorophenyl) N,N dimethyl urea), Monolinuron (3(4, chlorophenyl-1-methoxy-1 -methylurea), Linuron (3,(3,4 dichlorophenyl)-1 -1 -methoxy-1 -methylurea) Chloroxuron (3, (3,4 dichlorophenyl-1,1 dimethylurea)... 

The gas chromatographic procedure described by Cohen and Wheals [80] has been applied to the determination of various substituted urea herbicides in soil in amounts down to l-50pg kgr1. 

The method based on immunosorbents coupled on-line with liquid chromatography-atmospheric pressure chemical ionization mass spectrometry [109], discussed in section 9.4.2.1, has been applied to the determination of substituted urea type herbicides. Supercritical fluid extraction with methanol modified carbon dioxide has been applied to the determinants of sulfonyl urea herbicides in soil [261],... 

It is seen from Table 16.4, for example, that TCA a phenoxyacetic acid type of herbicide has a solubility exceeding 500000pg L 1 while chlorotoluron, a phenyl urea herbicide has a water solubility of 70mg L-1. Thus, due to rainfall over a period of time the concentration of TCA in soil will reduce considerably more rapidly than that of chlorotoluron. 

More than 25 different substituted urea herbicides are currently commercially available [30, 173]. The most important are phenylureas and Cycluron, which has the aromatic nucleus replaced by a saturated hydrocarbon moiety. Benzthiazuron and Methabenzthiazuron are more recent selective herbiddes of the class, with the aromatic moiety replaced by a heterocyclic ring system. With the exception of Fenuron, substituted ureas (i.e., Diuron, Fluometuron, Fig. 10, Table 3) exhibit low water solubilities, which decrease with increasing molecular volume of the compound. The majority of the phenylureas have relatively low vapor pressures and are, therefore, not very volatile. These compounds show electron-donor properties and thus they are able to form charge transfer complexes by interaction with suitable electron acceptor molecules. Hydrolysis, acylation, and alkylation reactions are also possible with these compounds. 

Senesi and Testini [147,156] and Senesi et al. [150,153] showed by ESR the interaction of HA from different sources with a number of substituted urea herbicides by electron donor-acceptor processes involves organic free radicals which lead to the formation of charge-transfer complexes. The chemical structures and properties of the substituted urea herbicides influence the extent of formation of electron donor-acceptor systems with HA. Substituted ureas are, in fact, expected to act as electron donors from the nitrogen (or oxygen) atoms to electron acceptor sites on quinone or similar units in HA molecules. 

Liu, J. and Qian, C. Hydrophobic effects of s-triazine and phenyl urea herbicides, Chemosphere, 31(8) 3951-3959, 1995. 

Chlortoluron Phenylurea 15545-48-9 2.79 1994 Herbicide A contact and residual urea herbicide used to control broad-leaved weeds and grasses... 

---