Selectivity halogenation

The halogen selective microcoulometric and electrolytic conductivity detectors have been applied extensively to the analysis of halogenated compounds in drinking water. Both have adequate sensitivity for the application and sufficient selectivity to allow reasonably accurate identifications with the retention time data. However, these detectors are also best applied to well defined samples where the probability of unexpected compounds is low. 

FID = flame ionization detector GC = gas chromatography HSD halogen-selective detector MS = mass spectometry NR = not reported... 

It is possible to halogenate selectively at the methyl position of alkylpyridines to give halogenoalkylpyridines. Low to moderate yields of fluoromethylpyridines are obtained on fluorination of 2- and 4-methyl pyridines with (V-fluoro-bis[(trifluoromethyl)sulfonyl]imide 52 in the presence of sodium carbonate in dichloromethane <1996T15>. 2-Methylpyridine is fluorinated in 20% yield and 4-methylpyridine in 72% yield under these conditions. Reagent 52 also effects the selective fluorination of 2,4,6-collidine to give the 2-fluoromethylated product in moderate yield (Equation 35). 

The higher homologues of toluene such as ethylbenzene are not usually halogenated selectively and mixtures are often produced (see Chapter 3). In the case of ethylbenzene itself, the major product of chlorination is the 1-substituted product (56%). Bromine is more selective and the 1-bromo derivative 7 is formed exclusively. 

The selectivity of the reaction also makes the peroxidase-catalyzed io-dination a very good tool for the study of the position of proteins within macromolecular structure such as membranes, ribosomes, and micellular polypeptides. Its use in this way is based on the fact that it is a high molecular weight protein and therefore does not readily penetrate these macromolecular structures. - " If the experimental conditions are correct, it catalyzes the halogenation selectively with those groups on the protein with which the enzyme has access. Thus, when the enzyme has access to proteins on only one side of the macromolecular structure such as the cell membrane, only the accessible proteins will be labeled with iodine. The labeled polypeptide of the macromolecular structure can then be isolated and identified. This provides a general method that can be applied to all macromolecular structures. 

In pyrimidines, a 4-alkyl- is deprotonated more readily than a 2-aUcyl-gronp here again one sees the greater stability associated with a y-qninonoid resonating anion. Side-chain radical halogenation selects a pyrimidine-5-methyl over a pyrimidine-4-methyl the reverse selectivity can be achieved by halogenation in acid solntion - presnmably an iV-protonated, side-chain-deprotonated species, i.e. the enamine tantomer, is involved. ... 

Flavones are halogenated selectively in the 3-position on treatment with PhI(OAc)2 and a TMS halide <01SC2101>. The reaction of 3-halogeneochromones with amines is known to be complex the products are dependent on conditions, type of amine and the halogen. It is now reported that secondary amines react with the three 3-halogenochromones 31 in the presence of K2CO3 in DMF to give exclusively 2-aminomethylene-3(2//)-benzofuranones (Scheme 36) <01H(55)881>. 

Element selective detectors Element selective detectors applicable in pesticide residue analysis include electron capture detector (ECD), electrolytic conductivity detector (ELCD), halogen-specific detector (XSD), nitrogen phosphorus detector (NPD), flame photometric detector (FPD), pulsed flame photometric detector (PEPD), sulfur chemiluminescence detector (SCD), and atomic emission detector (AED). To cover a wider range of pesticide residues, a halogen-selective detector (ECD, ELCD, XSD) in conjvmction with a phosphorus- (NPD, FPD), nitrogen- (NPD), and/or sulfur-selective detector (FPD, SCD) is commonly used. A practical approach is to spht the column flow to two detectors that reduces the number of injections however, the reduced amoimt of analyte that reaches the detector must be considered. 

Artman, G.D. Ill and Weinreb, S.M. (2003) An approach to the total s3mthesis of the marine ascidian metabolite perophoramidine via a halogen-selective tandem Heck/carbonylation strategy. Org. Lett., 5, 1523-6. 

Weinreb and colleagues reported the total synthesis of marine ascidian metabolite perphoramidine via a halogen-selective tandem Heck/carbonylation strategy [111]. 

It remains to discuss quantitative aspects of amination by nucleophilic replacement of halogen. Selected data are given in Table 5,24, The results are due mainly to Chapman and his co-workers , 699-70i and the subsequent discussion follows theirs closely. 

Reaction of polyoxyethylene and polyoxypropylene compounds with hydrogen bromide produces mono- and dibromoethane and mono- and dibromopropane, which are easily measured by gas chromatography. Less often, HI is used in place of HBr, giving the corresponding iodoethane and iodopropane compounds. Depending on the product, it may also be possible to determine the hydrophobe component by GC. When coupled with appropriate initial isolation steps, this reaction may even be used for trace analysis, especially if the halogen-selective electron capture detector is used (58-60). A technique for specific determination of AE in wastewater in the parts per billion range is based on GC determination of the alkyl bromides with mass selective detection (61). 

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