Free halogens in halogenated solvents

This simple qualitative test involves the extraction by water of free halogens, followed by the reaction of the halogens with potassium iodide in the presenee of a stareh indieator. The solution color changes to blue in the presence of free halogens. 

Gas chromatography provides many tools for the analysis of solvents. In section 15.1.4, a method was discussed which determines the distribution of boiling points of hydrocarbon mixtures. Many uses of gas chromatography for the determination of purity of different solvents are discussed in Section 15.1.25. In this section, some examples of of gas chromatography are included to show its usefulness in the qualitative determination of solvents mixtures the analysis of solvent impurities, the determination of solvents in a product by direct injection, and the generation of data to evaluate waste materials to determine their hazardous content.  

The relative distribution of aromatic hydrocarbons in xylene products can be quantitatively determined by gas chromatography. A flame ionization or thermal conductivity detector is used with a capillary or packed column containing crosslinked polyethylene glycol as the stationary phase. The peak area of each component is measured and the weight percentage concentration is calculated by dividing the peak area of the component by the sum of the areas of all peaks. 

Ortho-xylene concentration and the concentrations of its admixtures and impurities are measured using a flame ionization detector and a polar fused-silica capillary column. An internal standard (iso-octane) is used to increase precision and a standard mixture is used for calibration. A similar method is used with p-xylene but either n-undecane or n-octane are used as the internal standards. In both methods, peak areas are interpreted relative to the peak area of internal standard. The main impurities in benzene are non-aromatics with less than 10 carbons, toluene, 1,4-dioxane and aromatics containing 8 carbon atoms. The 

---

Reversible formation of ionic intermediates in halogenated solvents has been suggested to be due to the weakly nucleophilic character of the counteranion, the tribromide ion, which should dissociate into nucleophilic bromide and free bromine before reacting with the bromonium ion (refs. 11,25,26). In order to check this hypothesis the product distribution of the c/s-stilbene bromination in chloroform was investigated (ref. 27). In the latter solvent the formation constant of Br3 is considerably lower than in DCE, Kf = 2.77 (0.13) x 10 against > 2 x 107 M 1. (ref. 28). As a consequence, at 10 3 M [Br2] relevant amounts of bromide ions are present as counteranion of the bromonium intermediate. Nevertheless, the same trend for the isomerization of cis- to rran -stilbene, as well as an increase of... 

The mechanistic details of sulphonation have been less closely explored than those of nitration or halogenation. Benzene itself is sulphonated fairly slowly by hot concentrated sulphuric acid, but rapidly by oleum (the rate then being related to its S03 content) or by S03 in inert solvents. The nature of the actual electrophile depends on the conditions, but is probably always S03 either free or linked to a carrier , e.g. H2S04S03 (H2S207) in sulphuric acid. A small concentration of S03 is developed in H2S04 itself through the equilibrium ... 

The Hunsdiecker reaction is a free-radical reaction for the synthesis of an alkyl halide. The starting material comes from the reaction of a silver carboxylate with a solution of a halogen in a solvent such as carbon tetrachloride (see Figure 12-44). The overall free-radical mechanism is shown in Figure 12-45. 

In many cases, the free halogens dissolved in a suitable solvent such as E O (for Br2 or l2>, THF (for I2) or strongly cooled pentane (for CI2) can be used [6,23,120], Generally, the iodinations give the highest yields as there is little competition of addition of I2 to the unsaturated system. The introduction of iodine can even be carried out in liquid ammonia with excellent results [121],... 

Halogenations may also occur by a free-radical mechanism.121,218 Besides taking place in the gas phase, halogenation may follow a free-radical pathway in the liquid phase in nonpolar solvents. Radical halogenation is initiated by the alkene and favored by high alkene concentrations. It is usually retarded by oxygen and yields substitution products, mainly allylic halides. 

Fluoropyridines have been prepared by direct reaction of fluorine diluted in an inert gas and dissolved in a polyhalogenated solvent (91BCJ1081). Presumably these reactions involve attack by free halogen atoms as distinct from the ionic halogenation at lower temperatures which gives p-orientation (cf. Section 3.2.1.4.7). Under similar conditions (Br2,450°C) quinoline gives 2-bromoquinoline. 

The halogenation reaction of ethylene has been modeled by many researchers [170, 172-176], For chlorination in apolar solvents (or in the gas phase), the formation of two radical species requires the use of flexible CASSCF and MRCI electronic structure methods, and such calculations have been reported by Kurosaki [172], In aqueous solution, Kurosaki has used a mixed discrete-continuum model to show that the reaction proceeds through an ionic mechanism [174], The bromination reaction has also received attention [169,170], However, only very recently was a reliable theoretical study of the ionic transition state using PCM/MP2 liquid-phase optimization reported by Cammi et al. [176], These authors calculated that the free energy of activation for the ionic bromination of the ethylene in aqueous solution is 8.2 kcalmol-1, in good agreement with the experimental value of 10 kcalmol-1. 

Photoinduced free-radical halogenation is very useful for the functionalization of electron-deficient alkenes such as vinyl halides. Among halides, the addition of bromine is by far the most useful reaction, as shown in Scheme 3.25. In the first example, a polyfluoroalkene was brominated in a high yield by using a 300 W light bulb as the light source. Of note, the synthesis of l,2,4-tribromo-l,l,2-trifluorobutane (38) was carried out under solvent-free conditions in a near-quantitative yield and on a multigram scale (Scheme 3.25a) [67]. l,2-Dibromo-l,l,2-trichloroethane (39) was likewise obtained in about 250 g amounts by the solar lamp irradiation of neat trichloroethylene to which bromine was continuously added (Scheme 3.25b) [68],... 

Treatment of the tris(dialkyldithiocarbamato)iron(III) complexes in benzene with a controlled amount of concentrated hydrohalic acid affords the black bis(ligand) complexes [FeX(S2CNR2)2] (X = Cl, Br or I).306 For X = Cl the complexes may also be prepared by irradiation of the tris-(ligand) complex in a halogenated solvent. This free radical reaction is believed to proceed via excited-state labilization of one ligand followed by attack of solvent.307 Analogous complexes of pseudohalide ions (X = NCO, NCS- or NCSe ) have been obtained from reaction of the parent tris complex with the appropriate Ag+ salt.380 Representative complexes of this class have been shown by X-ray diffraction methods to have square pyramidal structures (71) in which the sulfur atoms of the two bidentate ligands comprise the basal plane (Fe—S 2.228 2.30 A) with the halide ion in the apical position (Fe—Cl 2.26-2.28 A).309 310 In the cases examined the metal atom sits 0.6 A out of the mean S4 plane in the direction of the apical halide ion. 

---