Polyhalide formation

Fluorine shows the least tendency for polyhalide formation. The I ions does not exist under normal conditions. Species such as IBrF have been identified, but the fluorine atom never occupies a center position in these structures. 

It follows from the preceding discussion that the unbranched H bond can be regarded as a 3-centre 4-electron bond A-H B in which the 2 pairs of electrons involved are the bond pair in A-H and the lone pair on B. The degree of charge separation on bond formation will depend on the nature of the proton-donor group AH and the Lewis base B. The relation between this 3-centre bond formalism and the 3-centre bond descriptions frequently used for boranes, polyhalides and compounds of xenon is particularly instructive and is elaborated in... 

At(0) reacts with halogens X2 to produce interhalogen species AtX, which can be extracted into CCI4, whereas halide ions X yield polyhalide ions AtX2 which are not extracted by CCLt but can be extracted into Pr O. The equilibrium formation constants of the various trihalide ions are intercompared in Table 17.28. 

The tendency of the halogens to form chain-like polyanions that are stabilized by delocalization of the negative charge [15,34] is a basic chemical principle. Donor-acceptor interactions between Lewis-acidic Br2 and halide anions, but also with polyhalides acting as Lewis bases, give rise to the formation of a variety of homo and heteroatomic adducts. The maximum number of atoms in these chains increases with the atomic weights... 

Although 1, -elimination of o xylene polyhalides with sodium iodide, zinc, copper, or iron metal is a fundamental method for the formation of o-xylylene intermediates, it is difficult to carry out the reaction under mild conditions such as at room temperature. o Trimethylsilylmethyl)-benzyltrimethylammonium halides were devised for this purpose and were shown to generate the o-xylylene at room temperature. However, we have successfully generated o-xylylene at room temperature by the reaction of a, 0,-dibromo o-xylene with metallic nickel(51). The Diels-Alder reaction of... 

The synthesis of polyhalide salts, R4NX , used in electrophilic substitution reactions, are described in Chapter 2 and H-bonded complexed salts with the free acid, R4NHX2, which are used for example in acid-catalysed cleavage reactions and in electrophilic addition reactions with alkenes, are often produced in situ [33], although the fluorides are obtained by modification of method I.I.I.B. [19, 34], The in situ formation of such salts can inhibit normal nucleophilic reactions [35, 36]. Quaternary ammonium chlorometallates have been synthesized from quaternary ammonium chlorides and transition metal chlorides, such as IrClj and PtCl4, and are highly efficient catalysts for phase-transfer reactions and for metal complex promoted reactions [37]. 

The most widely used dihalide is 1,2-dichloroethane. The use of polyhalides (e.g., 2% 1,2,3-trichloropropane) results in the formation of branched or crosslinked products. Sodium tetrasulfide (Na2S4) is generally used as the polysulfide since it contains scarcely any of the monosulfide which reacts with dihalides to form cyclic by-products with unpleasant odors. 

When iodine is dissolved in hydriodic acid or a soln. of a metallic iodide, there is much evidence of chemical combination, with the formation of a periodide. A. Baudrimont objected to the polyiodide hypothesis of the increased solubility of iodine in soln. of potassium iodide, because he found that an extraction with carbon disulphide removed the iodine from the soln. but S. M. Jorgensen showed that this solvent failed to remove the iodine from an alcoholic soln. of potassium iodide and iodine in the proportion KI I2, and an alcoholic soln. of potassium iodide decolorized a soln. of iodine in carbon disulphide. The hypothesis seemed more probable when, in 1877, G. S. Johnson isolated cubic crystals of a substance with the empirical formula KI3 by the slow evaporation of an aqueous-alcoholic soln. of iodine and potassium iodide over sulphuric acid. There is also evidence of the formation of analogous compounds with the other halides. The perhalides or poly halides—usually polyiodides—are products of the additive combination of the metal halides, or the halides of other radicles with the halogen, so. that the positive acidic radicle consists of several halogen atoms. The polyiodides have been investigated more than the other polyhalides. The additive products have often a definite physical form, and definite physical properties. J. J. Berzelius appears to have made the first polyiodide—which he called ammonium bin-iodide A. Geuther called these compounds poly-iodides and S. M. Jorgensen, super-iodides. They have been classified 1 as... 

An important application of alkylation with di- and polyhalides is cyclialkylative ring formation. Of different a,co-dihaloalkanes studied, 1,4-dichlorobutane was shown to react at exceptional rate and with least rearrangement.143 This was rationalized in terms of participation by the cyclic chloronium ion 29 formed with anchimeric assistance by the second chlorine atom in the molecule ... 

In addition to the polyhalide ions discussed previously, which were all anionic, there are comparable cationic species known, although they have been studied considerably less. Many pure interhalogen compounds arc thought to undergo autoionization (see Chapter 10) with the formation of appropriate cationic species ... 

Ethers. Fluoro ethers have been obtained either by replacement, such as the formation of CF3OCH3 from the trichloro derivative,98 or by the action of alcoholic potassium hydroxide, upon a polyhalide. Example are CHBrFCF2OCH3 from CHBrFCF2Br,M and CH2FCF2OCH3 from CH2FCF2Br and methanolic potassium hydroxide.94, It is to be noted that the use of alcohols with longer chains minimizes the importance of the ether formation and favors the elimination of one molecule of halogen acid, with consequent formation of an olefin. 

Adduct ions are formed under some circumstances. Polyhalides will add Cl- to give negative ions, for example this mode of negative ion formation has been used for analytical purposes by Dougherty et al (9,10,11). 

Polyhalide radical anions have recently been reviewed I4- and I6-have been observed in terf-butanol solution, but they are unknown in aqueous solution (127). The equilibrium constant for formation of I2-[reaction (31)] is the link between the reduction potentials of the iodine atom, the diiodine radical anion, and diiodine. Numerous measurements of this equilibrium constant have been made over the years. There are even two reports of the enthalpy of the reaction, obtained from the temperature dependence of the equilibrium constant (35). Published values for the formation constant of I2- are listed in Table IV (32, 36, 128, 129, 149, 314, 318). As noted in Fornier de Violet s review (127) and in Elliot and Sopchyshyn s paper (109), there is a systematic discrepancy between the flash photolysis results and the pulse radiolysis results. Fornier de Violet suggested that the pulse radiolysis results might be in error because of unrecognized adduct formation... 

The heat of solution is -4-68 Cal.,2 and the heat of formation 109-86 Cal.3 A saturated solution boils at 119-9° C. under 760 mm. pressure.4 Caesium chloride resembles the rubidium salt in forming a number of complex polyhalides.5 It is a useful agent in photomicrography, since it forms well-defined double salts with many metals among them—silver, gold, platinum, mercury, lead, copper, and many others.8 It has a decided toxic effect. 

Cation formation is not restricted, as normally observed, to association with metallic species. It can also occm among nonmetals and even the most typical ones such as halogens or interhalogens, which are usually more noted for halide and polyhalide anion formation. 

Alkyl bromides and iodides are converted into the corresponding fluorides in good yield with mercury(I) fluoride.31 36 Fluorinations of polyhalides are complicated by elimination reactions which give a mixture of alkenes and fluorination products. The fluorinating ability ofmercury(l) fluoride can be increased by the addition of bromine or iodine which results in the formation of mercury(II) fluorohalide.37 38 An example of the reaction of mercury(I) fluoride with simple perfluoroalkyl iodides is the formation of 2//.2//-octafluorobutane (12).39... 

Allylations. Catalyzed allylation of a-iodo acetals with leaving group dependence on the Lewis acid has been reported. Formation of N-tosyl allylamine by reaction of an allylsilane with (A -tosylimino)phenyliodinane is promoted by Cu(OTf)j. Halide displacement of a polyhalide" occurs in the presence of CuCl. 

Halide ions often react with molecules of halogens or interhalogens and form polyhalide ions. Iodine is only slightly soluble in water. Its solubility is greatly increased if same iodide ions are present in the solution. The increase in solubility is due to the formation of polyhalide ion, in this case triiodide ion I3 This is stable both in aqueous solution and in ionic crystals. 

Chemi-ionization has been observed54 in the reactions of K2 with HgX2 and SnX4, but with much lower cross sections for ion formation, Q, 0T A2, than in the reactions with halogen molecules, Q-, 3-10 A2. Other polyhalide molecules studied did not yield any observable chemi-ionization giving an upper bound to their chemi-ionization cross sections, < 0001 A2. 

Reactions. - The reactions of diorganoiodophosphines with varying quantities of iodine have been studied by NMR techniques, which indicate the formation of a dialkyldi-iodophosphonium iodide as the initial product, followed by polyhalide anion formation in the presence of excess iodine. A modified McCormack synthesis of 3-silylated phosphol-3-enes (168) from phenyl-dichlorophosphine and 2-silylated 1,3-butadienes has been described. The reaction is carried out in the presence of a small amount of copper stearate in acetic anhydride at 50 °C, these conditions preventing the complete desilylation observed under standard conditions. The consecutive reaction of lithium phenylacetylide with triphenylborane and diphenylchlorophosphine results in the formation of the intramolecularly coordinated phosphino-borane (169). Treatment of the alkoxyvinyldichlorophosphines (170) with Grignard reagents... 

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