Chlorophenoxy acids, esterification

The potassium salts of the herbicides are then converted back to their acids by treatment withH2S04. The aqueous solution is acidified with cold 1 3 H2S04 to pH below 2. The chlorophenoxy acids regenerated are then extracted into ether in a separatory funnel by repeat extractions. The aqueous phase is discarded in order to achieve complete esterification of herbicide acids. The ether extract containing herbicides must be completely free from moisture even at trace level. Therefore, add acidified anhydrous Na2S04 to the extract in excess... 

Chlorophenoxy acids after being extracted out from the sample matrix, separated from organic interferences and concentrated down into a small volume of ether, are now converted into their methyl esters. Such esterification of herbicides is essential for their determination by GC. While chlorophenoxy acids themselves show poor response, their ester derivatives produce sharp peaks with good resolution. 

In both these areas, chemical derivatisation has traditionally played a role and with the advent of gas chromatography an even more important role. The reasons for preparing a derivative suitable for GC analysis are many and varied and have been discussed thoroughly in a number of books and reviews (l- >). For convenience they are summarised in Table I. As can be seen, two different types of chemical derivatisation techniques are mentioned under Item 4 of Criteria, There is the chemical derivatisation of a pesticide as a pre-requisite of the method of analysis, e.g. esterification of the chlorophenoxy acids, as well as derivatisation as a method for confirmation of identity. The former must meet all the requirements associated with a practical, viable analytical procedure while for the latter the emphasis is on speed, ease of operation and reproducibility. 

Chlorophenoxy acids are chromatographed after esterification. Methyl esters [498] and propyl esters [499] prepared by transesterification from methyl esters by heating for 5 min with n-propanol and sulphuric acid are often used. For a very sensitive analysis, Mierzwa and Witek [500] proposed the following procedure. They esterified acids with 20% of 2,2,2-trichloroethanol in TFA anhydride in the presence of sulphuric acid by heating at 100°C for 15 min (or several hours at room temperature) and analysed the derivatives on a column packed with 15% of QF-1 plus 10% of DC-200 (1 1) at 195°C. Trichloroethyl esters permit down to hundredths of 1 ppb of chlorophenoxy acids to be... 

Analysis of Chloronhenoxv Herbicides. Many important target environmental pollutants can only be detected via conventional GC methods by first converting them to derivatives that are less polar and more volatile, e.g., the chlorophenoxy herbicides. A standard EPA method (SW-846 8150) specifies soil extraction and alkaline hydrolysis of any esters present followed by (re) esterification via diazomethane and detection and confirmation by GC/MS. The methylation step is required because the free carboxylic acids will not pass through conventional GC analytical columns. Reversed phase chromatography functions equally well to resolve free carboxylic acids or the corresponding esters and thus can eliminate the diazomethylation step. An interlaboratory check sample provided by the EPA of soil spiked with the chlorophenoxy acid herbicides Silvex and 2,4-D was obtained by our laboratory to demonstrate that LC/MS can offer a simpler and effective method for these compounds. 

Esterification of chlorophenoxy acids and of pentachlorophenol has been assessed using tetrabutylammonium hydroxide and methyl iodide in methanol, and has been shown to be as effective as the standard procedure using diazomethane (Hopper 1987). A further development has examined the application of methanol/-benzyl-trialkylammonium for simultaneous extraction and methylation of 2,4-dichlorophenoxyacetic acid in soil (Li et al. 1991) the quaternary ammonium hydroxide may, in this case, also play a significant role in releasing the "bound" analyte. 

In the case where liquid chromatography is not available, acidic herbicides need to be derivatized because they can dissociate in water and are not usually volatile to be analyzed by gas chromatography. The basic methods used for chlorophenoxy acid herbicides are esterification, silylation, and alkylation, as described in a recent exhaustive review.The derivatization step is performed after preconcentration and cleanup. The step consists of the formation of esters and ethers from the carboxyl and phenol groups of the acidic herbicides. A lot of reagents and chemical mechanisms can be used to perform derivatization reactions. The most employed derivatization reagents are diazomethane, methyliodide, trimethylsulfonium (or anilinium) hydroxide, bis (trimethylsilyl) trifluoroacetamide (BSTFA), pentafluorobenzyl bromide, and anhydride acetate. It should be noted that explosive and hazardous diazomethane was replaced by safer agents. Authors also underline that surface water generally contains humic substances, which can interfere with the derivatization reaction. ... 

Catalina, M. I., Dalluge, J., Vreuls, R. J. J., and Brinkman, U. A. T., Determination of chlorophenoxy acid herbicides in water by in situ esterification followed by in-vial liquid-liquid extraction combined with large-volume oncolumn injection and gas chromatography-mass spectrometry, J. Chromatogr. A, 877, 153-166, 2000. 

Esterification of racemic 2-phenoxypropanoic acids with arylalkanols can be doubly enantioselective for both compounds with Candida cylindracea lipase (CCL) in hexane [82]. The enantiomeric ratio E of acids and alcohols depend on the relative structures and, for instance, as shown in Scheme 10, a maximum value E = 108) is reached for the p-chlorophenoxy acid when the alcohol counterpart is 1-phenyl-1-ethanol E = 5.2). 

Enantioselective enzymatic esterifications of trimethylsilyl-substituted alcohols with racemic 2-(4-chlorophenoxy)propanoic acid in water-saturated benzene, catalyzed by the Candida cylindracea lipase OF 360 CCL OF 360 E.C. 3.1.1.3) have been used to prepare (—)-2-(4-chlorophenoxy)propanoic acid76,77. As shown in Scheme 23, the (trimethylsilyl)alkanols 95, 97 and 99 were converted enantioselectively into the corresponding (trimethylsilyl)alkyl (+)-2-(4-chlorophenoxy)propanoates 96, 98 and 100. The enantiomeric purity of the remaining (—)-2-(4-chlorophenoxy)propanoic acid was 95.8% ee (95), 76.1% ee (97) and 77.5% ee (99). 

Analogous results were obtained for the enantioselective enzymatic esterifications of the related t-butyl-substituted alcohols 101 and 103 (carbon analogues of the silanes 95 and 97, respectively). Reaction with racemic 2-(4-chlorophenoxy)propanoic acid in water-saturated benzene yielded the corresponding t-butylalkyl (+)-2-(4-chlorophenoxy)propanoates 102 and 104, respectively76,77. The enantiomeric purity of the remaining (—)-2-(4-chlorophenoxy)propanoic acid was somewhat lower than that observed for the esterification of the analogous silicon compounds [91.1% ee (101), 71.6% ee (103)]. No esterification was observed for the Si/C analogues trimethylsilanol (MesSiOH) and t-butanol (MesCOH). 

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