Phenoxyalkanoates

Phenoxyalkanoics. The phenoxyalkanoic herbicide grouping is composed of two subgroups, the phenoxyacetic acids and the phenoxypropionic acids. The phenoxyacetic acid herbicides include some of the first commercially successhil herbicides, eg, 2,4-D. They continue to be widely used for foUar control of broadleaf weeds. The more heavily functionalized phenoxypropionic acid herbicides are relatively new herbicides compared to the phenoxyacetic acids and are used primarily for selective control of grassy weeds in broadleaf crops (2,296,297). 

The phenoxyalkanoic herbicides are acidic in nature and thus subject to some degree of ionization. The extent to which the herbicide ionizes is controlled by the acid dissociation constant (fQ of the herbicide in question and the soil solution pH (238). The leaching potential is significantly influenced by these reactions. 

Another factor that can influence the environmental distribution of a chemical is the presence of charged groups. Some pollutants, such as the sodium or potassium salts of phenoxyalkanoic herbicides, dinitrophenols, and tetra- or penta-chlo-rophenol, exist as anions in solution. Others, such as the bipyridyl herbicides diquat and paraquat, are present as cations. In either case, the ions may become bound to organic macromolecules or minerals of soils or sediments that bear the opposite... 

In marked contrast to hydrophobic compounds, more polar ones tend to be less adsorbed and to reach relatively high concentrations in soil water. Phenoxyalkanoic acids such as 2,4-D and MCPA are good examples (Figure 4.3). Their half-lives in soil are measured in weeks rather than years, and they are more mobile than OC insecticides in soils. When first applied they are lost only slowly. After a lag period of a... 

PCDDs have been released into the environment in a number of different ways. Sometimes this has been due to the use of a pesticide that is contaminated with them. 2,4,5-T and related phenoxyalkanoic herbicides have been contaminated with them as a consequence of the interaction of chlorophenols used in the manufacturing... 

PCDFs are similar in many respects to PCDDs but have been less well studied, and will be mentioned only briefly here. Their chemical structure is shown in Figure 7.1. Like PCDDs, they can be formed by the interaction of chlorophenols, and are found in commercial preparations of chlorinated phenols and in products derived from phenols (e.g., 2,4,5-T and related phenoxyalkanoic herbicides). They are also present in commercial polychlorinated biphenyl (PCB) mixtures, and can be formed... 

To prevent this contamination, there were several approaches which this mechanistic information suggested. But the one which the manufacturers proposed ( ) was to stop using nitrite to control corrosion of the container. Most dimethylamine phenoxyalkanoic acid products now contain non-detectable NDMA (18). 

Natural systems may be quite complex. For example, the enantiomerization of phenoxyalkanoic acids containing a chiral side chain has been studied in soil using H20 (Buser and Muller 1997). It was shown that there was an equilibrium between the R- and 5-enantiomers of both 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) and 2-(2,4-dichlorophenopxy)propionic acid (DCPP) with an equilibrium constant favoring the herbicidally active / -enantiomers. The exchange reactions... 

Chlorinated phenols are used for the impregnation of timber and the production of phenoxyalkano-ate herbicides that are degraded by dioxygenation to chlorophenols. 

Muller TA, SM Byrde, C Werlen, JR van der Meer, H-P Kohler (2004) Genetic analysis of phenoxyalkanoic acid degradation in Sphingomonas herbicidovorans MH. Appl Environ Microbiol 70 6066-6075. 

Phenoxyalkanoates. There has been considerable interest in the persistence of chlorinated phenoxyalkanoates—and particularly of phenoxyacetates and phenoxypropionates, which have been used as herbicides. This has therefore stimulated studies on the degradation of these aryl-alkyl ethers. Considerable effort has been directed to elucidating the subsequent... 

Buser H-R, MD Muller (1997) Conversion reactions of various phenoxyalkanoic acid herbicides in soil. 2. Elucidation of the enantiomerization process of chiral phenoxy acids from incubation in a DjO/soil system. Environ Sci Technol 31 1960-1967. 

Gintautas PA, SR Daniel, DL Macalady (1992) Phenoxyalkanoic acid herbicides in municipal landfill leachates. Environ Sci Technol 26 517-521. 

Schneiderheinze, J.M., Armstrong, D.W., Berthod, A. (1999). Plant and soil enantioselective biodegradation of racemic phenoxyalkanoic herbicides. Chirality 11, 330-337. 

Table 7. Repeating unit composition of PHAs biosynthesized by P. oleovorans grown with various phenoxyalkanoic acids... 

Phenox process, 10 787 Phenoxyacetic acid herbicides, 13 314 Phenoxyacetic acids, 13 50, 304 Phenoxyalkanoic acids, 13 284 Phenoxyalkanoic herbicides,... 

Phenoxyalkanoic acid herbicides are not amenable to direct gas chromatographic determination because of the high polarity or low volatility of the compounds and must be converted to their... 

There are several routes for nitrosamine contamination in pesticides use of contaminated chemicals during synthesis, side reactions, use of nitrite as a preservative and corrosion inhibitor of metal containers and by reactions with environmental nitrosating agents. Over 300 formulations were shown to be contaminated with nitrosamines however, the main contamination was confined to 2,6-dinitroaniline herbicides, dimethylamino salts of phenoxyalkanoic acid herbicide, diethanolamine and triethanolamine salts of acid... 

The herbicide 2,4-D is itself a potent phytotoxin. However, a number of structurally related but inactive compounds may be converted by plants to 2,4-D following the activation process and thus act as herbicides. These phenoxyalkanoic acids are co-(2,4-dichlorophenoxy) alkanoic acids. The transformation may be viewed as shown in Fig. 12 as 6-(2,4-dichlorophenoxy)hexanoic acid as the parent compound. The sequence is called /1-oxidation because the steps in which two carbons are removed initially involve the oxidation of the /1-carbon to the aliphatic acid moiety. 

Loos MA (1975) Phenoxyalkanoic acids. In Kemey PC, Kaufman DD (eds) Herbicides chemistry, degradation and mode of action, vol 1. Marcel Dekker New York, pp 1-128 Lovley DR (1993) Dissimilatory metal reduction. Annual Review of Microbiology 47 263-290 Macalady DL, Wolfe NL (1983) New perspectives on the hydrolytic degradation of the organo-phosphorothionate insecticide chloropyrifos. J Agric Food Chem 31 1139-1147 Macalady DL, Wolfe NL (1985) Effects of sediment sorption and abiotic hydrolysis. J Agric Food Chem 33 167-173... 

The results of many studies of the degradation of phenoxyalkano-ates using pure cultures of microorganisms have been reported. 

Synthetic auxins Phenoxyalkanoic acids Benzoic acids Pyridinecarboxylic acids Quinolinecarboxylic acids 4... 

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