Detection of the halogen

An effective TLC separation and a simple but sensitive detection of the halogenated synthetic pyrethroids was developed (121). By this method permethrin, cypermethrin, decamethrin, and fenvalerate were detected and distinguished from DDT, DDT metabolites, and HCH isomers. Twenty different solvents and solvent mixtures were checked for the chromatographic separation of the above insecticides. The best resolutions were achieved by /i-hexane benzene (45 55) and n-hexane-chloio-form (60 40), which gave distinct separation of cis and trans isomers of permethrin and cypermethrin (121) (Table 9) as well. 

Few authors have coupled HPLC to MIP-AES [680-682], despite the need for a nonmetal element-specific HPLC detector. Advantages of HPLC-He MIP-MS include improved detection for the halogens. The primary disadvantage is the inability of the plasma to tolerate aerosol introduction. 

The rate of step 4 involving elimination of halosilane also depends on the nature of the halogen atom but in the inverse order F < < Br. For Hal=Br, intermediates (32) are impossible to detect. By contrast, for Hal=F, elimination of Si F is the rate-determining step, which requires a special procedure, for example, refluxing of intermediate (32) (Hal=F) in acetonitrile for 1 h. (Preliminary data show that the microwave technology makes it possible to sharply decrease the time required for completion of step 4 (90)). A decrease in the elimination rate in the series F < < Br is evidence that this reaction is not... 

Many substances are not initially appropriate for gas chromatography because of their relatively high boiling points or insolubility. In such cases it is often possible to modify the compound chemically and render it more amenable to separation. In some instances, the chemical modification is used to enable easier detection of the compound, e.g. the introduction of a halogen for use with electron capture detectors. 

With these compounds the presence of the halogen will have been detected in the tests for elements. Most acid halides undergo ready hydrolysis with water to give an acidic solution and the halide ion produced may be detected and confirmed with silver nitrate solution. The characteristic carbonyl adsorption at about 1800 cm -1 in the infrared spectrum will be apparent. Acid chlorides may be converted into esters as a confirmatory test to 1 ml of absolute ethanol in a dry test tube add 1 ml of the acid chloride dropwise (use a dropper pipette keep the mixture cool and note whether any hydrogen chloride gas is evolved). Pour into 2 ml of saturated salt solution and observe the formation of an upper layer of ester note the odour of the ester. Acid chlorides are normally characterised by direct conversion into carboxylic acid derivatives (e.g. substituted amides) or into the carboxylic acid if the latter is a solid (see Section 9.6.16, p. 1265). 

The boronhalides (BX3) are volatile, highly reactive, monomeric compounds, which show no detectable tendency to dimerize. In this they resemble organoboranes, BR3, but differ sharply from diborane, B2H6. The molecules have a planar trigonal structure with the boron atom (sp2 - hybridization) in the centre. This class of compounds also exhibits partial double bond character due to pT - px interactions of the empty pz-orbital of boron and the filled pz-orbital of the halogen. Because of the incomplete octetstructure of the central boron atom, borontrihalides behave as strong Lewis acids. 

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