Halogenation methods

In acidic medium, potassium bromide and potassium bromate produce bromine stoichiometrically, and this is the basis of a titration method [82], which has been used to determine double bonds in polymethylacrylate. After bromination, excess bromine is estimated by the addition of potassium iodide and the iodine produced by the reaction is titrated with standard sodium thiosulfate to the starch end-point. 

KBrOs + 5KBr + 6HC1 = SBrj + SHjO + 6KC1 

Hensen and Eatough [83] described a direct injection enthalpimetric determination of residual total unsaturation in polymers. This is based on the heat produced during the bromination of double bonds. 

Kolthoff and Mitarb [84] and Boyer [85] discuss the application of bromination to the determination of total unsaturation in di and polyolefins. McNeil [86, 87] discussed methods for the determination of total unsaturation based on chlorination with radiolabelled chlorine ( radiochlorine , Cl). 

Von Houwelinger [8] used coulometric bromination to determine vinyl ester end-groups in polyethylene terephthalate formed by thermal chain scission with the subsequent liberation of acetaldehyde. 

KBrOa + 5KBr + 6HCI = SBrj + 3H2O + 6KCI 

A survey of applications has been given by Hirozawa and co-workers [112]. 

The constant-current generation of bromine is carried out in a medium of dichloroacetic acid, hexafluoroisopropanol, water, potassium bromide and mercury(II) chloride. To this medium an amount of the polymer, previously dissolved in hexafluoroisopropanol and diluted with anhydrous dichloroacetic acid, is added and bromine is generated. The end of the reaction is detected biamperometrically. The suitability of this method was tested against methyl vinyl terephthalate  

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The THM atmospheric concentrations for each halogenation method are then about the same. 

NIOSH. 1994. Hydrocarbons, halogenated - method 1033. In NIOSH manual of analytical methods. 4th ed. Vol. 2. Cincinnati, OH National Institute for Occupational Safety and Health, Centers for Disease Control. 

The aforementioned direct halogenation methods are also applicable to bisthiophenes [8] and terthiophenes [9]. Taking advantage of the a-activation, debromination of... 

Originally conceived as a single review, this work has now been divided into three parts that separate the five-membered rings (Part I), six-membered rings (Part II), and condensed systems (Part III). The section dealing with halogenation methods is introductory to all three parts. 

The halogenated method employs a packed column of 1% SP-1000 on Carbopak-B (60-80 mesh) as its primary analytical column. The column is 8-ft X 0.1-in. i.d. It is operated at a helium flow rate of 40 mL/min under programmed temperature conditions of 45 °C isothermal for 3 min, then 8 °C/min to 220 °C, and then held at 220 °C for 15 min or until all compounds have eluted. An electrolytic conductivity detector operated in the halide-specific mode is used for measurement. 

Nearly all the methods described in Chapter 2 have been applied for the preparation of uranium derivatives. The reaction of metallic uranium with alcohols in the presence of halogenes (method 1) doesn t lead, as it turned out, to homoleptic alkoxides, but is a facile route to alkoxide halides [79, 1669] ... 

This chapter is the second of a three-part series reviewing halogenation of aromatic heterocycles. Part 1 [93AHC(57)291] described halogenation methods and their application to five-membered systems. Part 3 will cover the benz and other fused heterocyclic compounds. Material published since 1978 is emphasized, although earlier references are included where... 

TeUurium (0-Ethyl dithiocarbonate) Halides (Halogen Method)1 2.0 g (5 4 mmol) of tellurium bis[0-ethyl dithiocarbonate] are dissolved in 50 ml of dichloromethane, 2.7 mmol of halogen (0.19 g of chlorine, 0.43 g of bromine, or 0.69 g of iodine) dissolved in carbon tetrachloride are added, the mixture is stirred for 1 h, and then filtered. The filtrate is evaporated under reduced pressure, the residue is washed with boiling light petroleum ether, and recrystallized from chloroform/light petroleum ether yield not reported m.p. not reported. 

Standard enthalpies and entropies for the ketone to enol equilibrium have been determined from data obtained by the kinetic halogenation method (see Section 3) at 5, 15, 25 and 35°C. Since the keto-enol equilibrium constants depend on the absolute rate constant arbitrarily chosen for the diffusion-controlled halogen addition to the enol, only the differences in from one ketone to another must be considered... 

Enol titration Kinetic estimation1 Thermochemical approach Ah initio calculations Kinetic halogenation method ... 

Finally, concordant results have been obtained from a kinetic study of the iodination of acetophenone and acetone at very low iodine concentration (Verny-Doussin, 1979). The procedure used is similar to that followed for the determination of equilibrium constants for enol formation by the kinetic-halogenation method, i.e. second-order rate constants were measured under conditions such that halogen additions to enol and enolate are rate-limiting (43). Under these conditions, the experimental kn-values can be expressed by... 

Recently, the present author (Toullec and Verny-Doussin, 1980) obtained a concordant value for the equilibrium constant of (65 L = H) for [83, n — 3] from kinetic data on amine-catalysed iodination of acetone at low iodine concentrations. The principle of this determination is similar to that described for estimation of the keto-enol equilibrium constants by the kinetic halogenation method (see p. 48). At low iodine concentrations ( 10-6 mol dm-3), the enamine pathway is preferred [see (61)] and iodination of the enamine is rate-limiting. Rate measurements, in a pH range in which only the monoprotonated and diprotonated forms of the diamine exist, made it possible to determine the second-order rate constants k u which include the equilibrium constants, for interconversion of the ketone and protonated enamine... 

Of lesser relevance to this discussion are halogenation methods involving the modification of the carbon skeleton (synthesis and degradation). The Hunsdiecker reaction, as applied to certain heterocyclic acids, has had limited application for the synthesis of halogen derivatives. The preparation of 3-bromo-4,6-dimethyl-2-pyridone from the silver salt of the respective 3-carboxylic acid by treatment with bromine in carbon tetrachloride is a rare example of success.13 The interaction of carbenes with heterocycles also has been employed infrequently, but recent advances in carbene generation may reactivate this approach.14 The Ciamician-Dennstedt ring expansion of pyrrole to / -halopyridines is a case in point18 [Eq. (4)] ... 

Method A, protodesilylation wth acid Method B, reaction of silylchloride and acid Method C, reaction of silylchloride and silver salt Method D, reaction of silane and trytyl salt Method E, reaction of disilane and halogen Method F, reaction of selenosilane and halogen. 

Despite the fundamental importance of halogenation methods in preparative organic chemistry the number of preparative, stereoselective reactions of this type is still very limited. In particular. reagents for the enantioselective introduction of halogens are in short supply. Due to the ongoing activity in this field there is justified hope that this will change in the future. 

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