Halogen, uses

This IS called the haloform reaction because the trihalomethane produced is chloroform (CHCI3) bromoform (CHBrj) or iodoform (CHI3) depending on the halogen used... 

Table 19-1 shows the electron configurations of the halogens, using the symbols introduced in... 

Story WC, Caruso JA. 1993. Gas chromatographic determination of phosphorus, sulfur and halogens using a water-cooled torch with reduced-pressure helium microwave-induced plasma mass spectrometry. J Anal Atomic Spectrometry 8 571-575. 

This sequence is called the haloform reaction because it results in the production of chloroform, bromoform, or iodoform, depending upon the halogen used. The haloform reaction is a useful method for identification of methyl ketones, particularly when iodine is used, because iodoform is a highly insoluble, bright-yellow solid. The reaction also is very effective for the synthesis of carboxylic... 

Ph3SiSH can be S-halogenated using the 7V-halosuccinimide. The bromo and iodo derivatives decompose to release sulphur in polar solvents, but a structure determination of the bromo derivative shows the Si—S bond to be 216 pm, compared with 215 pm in the parent thiol. Surprisingly, the S— Br bond is 217 pm long29. 

The a-halo substituent is derived from the halogen used, not from the phosphorus trihalide. (g) In the case, bromine is introduced at the a carbon. 

Of similar nature are chiral halogenations using auxiliary groups. Typical examples are the conversion of esters to enantiomerically pure halohydrins (precursors to chiral epoxides) using camphor-10-sulfonic acid derivatives583 and the chiral synthesis of a-amino acid synthons via diastereoselective bromination of TV-acyl oxazolidone derivatives584. 

The carbonyl group in aromatic esters, ketones and acids is transformed into aralkyl bromides or iodides by a one-pot reductive halogenation using LiAlH4/HX reagents (equation 155)988. 

Many - gas sensors based on - solid electrolytes operate under potentiometric conditions [iii]. The sensors for oxygen use oxide -> conductors, such as ZrC>2 -based ceramic, those for halogens use halide conductors (e.g., KAg s), while -> hydrogen sensors use protonic conductors. There are sensors for C02, N02, NH3, S03) H2S, HCN, HF, etc. (see -> lambda probe). 

The oxidizing power of halogens decrease with increasing relative atomic mass Iodine is a mild oxidant, while the iodide ion often acts as reducing agent. Some oxidations with halogens, used in qualitative analysis, are as follows ... 

The methods discussed so far are applicable to aldehydes, ketones, esters and lactones. Hie a-haloge-nation of acids has received relatively little attention, although the traditional Hell-Vollaid-Zelinski conditions are adequate in most instances (equation 2). Alternative conditions have been developed, however, in which the acyl halide may be halogenated using NBS. ( eiKhing the reaction with alcohols or amines offers the opportunity of forming carboxylate derivatives. 

The condensation of two molecules of primary selenoamides proceeds with bromine under extrusion of selenium to give selenadiazoles [93]. The reaction of selenocarbonates, selenothiocarbonates [94], and selenourea [95] with bromine and iodine has been widely tested. The products depend on the amount of halogen used. For example, in the reaction of N-methylthiazoline-2(3H)-selone, the use of one equivalent of bromine gives hypervalent lO-Se-3 complexes 44, whereas two equivalents of bromine cleave the carbon selenium double bond to give product 45. A similar hypervalent compound is formed from 4-imidazolin-2-selone,but the iodination of bis(imidazolin-2-selone)methane gives iodinated product 46. The availability of some of halogen adducts has been tested as a conducting material. 

Increasing the reaction temperature and the amount of halogen used results in the introduction of further halogen atoms. It was noted in Chapter 2 that the halogen atom in chlorobenzene is ortholpara directing, but deactivating to electrophilic substitution as a result of opposing mesomeric (+M) and inductive (-1) effects. Consequently, disubstitution leads to a mixture of the 1,2- and, mainly, 1,4-isomers under conditions similar to those required to attack benzene. 

The halogenopurines react with many other nucleophiles including hydrazines, hydrox-ylamine and alkoxyamines, sulfonic acids and thiocyanates, and compounds with active methylene groups such as diethyl malonate furnish appropriate purine derivatives which offer valuable routes to alkyl and substituted alkyl derivatives. Also aryl groups have been substituted for halogen using aryllithiums (63N224). 

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