Hydrolysis, sulfonylurea

Sulfonylureas (R—SO2—NH—CO—NHR ) undergo hydrolysis (acid plus water) to form RSO2NH2 -1- CO2 -1- H+. An example of sulfonylurea hydrolysis is Glibenclamide [18]. 

Carboxyhc acid ester, carbamate, organophosphate, and urea hydrolysis are important acid/base-catalyzed reactions. Typically, pesticides that are susceptible to chemical hydrolysis are also susceptible to biological hydrolysis the products of chemical vs biological hydrolysis are generally identical (see eqs. 8, 11, 13, and 14). Consequentiy, the two types of reactions can only be distinguished based on sterile controls or kinetic studies. As a general rule, carboxyhc acid esters, carbamates, and organophosphates are more susceptible to alkaline hydrolysis (24), whereas sulfonylureas are more susceptible to acid hydrolysis (25). 

Sulfonylureas are not directly amenable to gas chromatography (GC) because of their extremely low volatility and thermal instability. GC has been used in conjunction with diazomethane derivatization, pentafluorobenzyl bromide derivatization, and hydrolysis followed by analysis of the aryl sulfonamides. These approaches have not become widely accepted, owing to poor performance for the entire family of sulfonylureas. Capillary electrophoresis (CE) has been evaluated for water analysis and soil analysis. The low injection volumes required in CE may not yield the required sensitivity for certain applications. Enzyme immunoassay has been reported for chlorsulfuron and triasulfuron, with a limit of detection (LOD) ranging from 20 to 100 ng kg (ppt) in soil and water. 

Hydrolysis rates of sulfonylurea herbicides in water are heavily dependent upon pH. In general, acidic conditions promote faster hydrolysis, usually by cleavage of the sulfonylurea bridge. Neutral to alkaline conditions favor the compounds existing in... 

Berger, B.M. and Wolfe, N. L. Hydrolysis and biodegradation of sulfonylurea herbicides in aqueous buffers and anaerobic water-sediment systems assessing fate pathways using molecular descriptors, Anwrort. Toxicol. Chem., 15(9) 1500-1507,1996. 

Huang, C. H., Hydrolysis of Amide, Carbamate, Hydrazide, and Sulfonylurea Agrochemicals, Ph.D. thesis, Johns Hopkins University, Baltimore, MD, 1997. 

Sulfonylureas undergo hydrolysis as shown in the mechanistic scheme in Figure 64 (106). Under acid-catalyzed conditions, water addition leads to loss of an amine and formation of a carbamic acid derivative. Acid-catalyzed loss of CO2 from the carbamic acid derivative yields the corresponding sulfonamide. It was proposed that initial protonation is the rate-determining step in the hydrolysis. 

Hydrolysis appears to be a very significant pathway for the fate of sulfonylurea pesticides in sediments and natural waters. Although these chemicals often have many different functional groups, the principal cleavage occurs at the sulfonylurea bridge. The reaction is highly pH dependent (Equation (17)) ... 

Berger and Wolfe (1996) reported a correlation of hydrolysis data for 12 sulfonylurea herbicides. The use of bond strength or Hammett a constants was impossible because of the complex structures of the compounds. The hydrolysis pathways for this class of compounds are also more complex, but the use of quantum mechanical parameters provided the detailed structural information needed to develop a useful correlation. As a result of the many different functional groups, several reaction pathways are available depending on the substituents. Also, there is a complicating pH effect on the pathways and the kinetics of hydrolysis as shown by product studies. The 12 herbicides used in this study are listed in Table 13.4, and the pseudo first-order hydrolysis rate constants are given in Table 13.5. Figure 13.2 shows the basic structure of these compounds. 

Sulfonylurea Herbicides Investigated for Hydrolysis and Reaction Pathways. 

Pseudo First-Order Hydrolysis Rate Constants with Standard Errors [(k + SE) x 10-2] in Aqueous Buffer Solutions for 12 Sulfonylurea Herbicides at Different pHs and Temperatures. 

Bezemer, E. and Rutan, S.C., Study of the hydrolysis of a sulfonylurea herbicide using liquid chromatography with diode array detection and mass spectrometry by three-way multivariate curve resolution-alternating least squares, Anal. Chem., 73, 4403 4409, 2001. 

Animals. Nicrosulfuron and its metabolites do not bioaccumulate. Hydrolysis of the sulfonylurea bridge and hydroxylation were the main metabolic pathways... 

A15.1.3.1 Sulfonamides, Sulfonylureas, Epoxides, and Aziridines. Sulfonamides (RS02NR"2) are susceptible to acid hydrolysis (RS02NR"2 -1- water -1-acid RSO2OH), but not basic hydrolysis. Primary alcohols react rapidly only with A,A -disubstituted sulfonamides to yield sulfonic esters (RS02NR"2 -1-R OH -I- acid RSO2OR + NHR y. Sulfonamides are not susceptible to oxidation because the sulfur is already fuUy oxidized. 

Berger, B.M. and Wolfe, N.L. (1996). Hydrolysis and Biodegradation of Sulfonylurea Herbicides in Aqueous Buffers and Anaerobic Water-Sediment Systems. Assessing Fate Pathways Using Molecular Descriptors. Environ.Toxicol.Chem., 15,1500-1507. 

Sulfonylureas are systemic herbicides absorbed by the foliage and roots. They act by inhibiting acetolactate synthase, a key enzyme in the biosynthesis of branched chain aminoacids." This results in stopping cell division and plant growth. The most important degradation pathways of sulfonylureas are chemical hydrolysis and microbial degradation. 

As a result, the sulfonylurea herbicides are more rapidly hydrolyzed in acidic waters and soils. Figure 2.11 illustrates the pH-rate profile for the hydrolysis of chlorimuron-ethyl at 45°C in buffered aqueous solution (Brown, 1990). 

The hydrolysis rate constant for chlorimuron-ethyl decreases over 150-fold as the pH increases from 4 to 7. Additionally, ionization affects water solubility. The water solubility of chlorimuron-ethyl increases from 11 to 1200 mg/L when the pH increases from 5 to 7 (Hay, 1990). Consequently, the sulfonylureas are expected to be much more persistent and more readily transported in basic aquatic ecosystems. 

The metabohsm of compounds predominantly results in more polar compounds than the precursor molecules, so for the follow-up of pesticides in the environment in the late 1980s and early 1990s FAB and CF-FAB-MS at this stage of MS development met the need. CF-FAB-MS was appHed by Reiser et al. [171] to unknown sulfonylurea herbicide metabohtes using packed capillary columns. New and unusual heterocychc ring-opened metabohtes and hydrolysis products were identified and metaboHc pathways proposed. The metabohsm of cycloate, a thiocarbamate herbicide, was inveshgated [177]. 

In the soil, there are two major pathways of sulfonylurea degradation [58] (a) chemical hydrolysis and (b) microbial degradation. The breakdown of sulfony-lureas in sterile soils is solely attributable to chemical hydrolysis, whereas breakdown in non-sterile soils is a combination of both microbial degradation and... 

The main soil degradation pathways of sulfonylurea herbicides are cleavage of the sulfonylurea bridge, O- and N-dealkylation reactions, aryl and aliphatic hydroxylation reactions, dehalogenation and ester hydrolysis. It is not within the scope of this chapter to discuss each of these in detail for all of the above-mentioned new sulfonylureas. Instead mesosulfuron-methyl is taken below as a general illustration of commonly found soil degradation pathways established within the sulfonylurea family. 

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