Polyhalides

Chlorides a) Liquids (Listed in order of increasing atmospheric b.p.)  

5 Chlorc orm 61 1 446 1 489 Gives carbylamine test with primary amines 

2-Tetrachloroethylene (Perchloroethylene) 121 1 5055 1 623 With paraformaldehyde + cone HjSO, - a,tt-di-chloro-(3-hydroxypropionic acid 88 9 

18 l-Bromo-3-chloropropane 143 4 1 4861 1 594 I,3-Di-(l-naphthyl)ether. 103-4 l,3-Di-(2-naphthyI) ether, 148-9 1,3-Diphenyl ether. 60 

22 Benzalchloride 207,214 1 5515 1 295 Oxid — benzoic acid, 122, Hydrolysis — benzaldehyde, 2,4-Dinitrophenylhydrazone 237, Semicarbazone, 222 

Numerous compounds of this sort are known. The selection given here—with the exception of KI3 H3O and HICl. 4H3O—is so chosen that to each of the previously described interhalogen compounds there corresponds a polyhalide which yields that compound on decomposition. 

The theoretical quantity of I3 is added to a hot, saturated solution of Kl after the iodine dissolves, the mixture is cooled to 0°C, whereupon Klg HgO crystallizes out. 

Dark brown, hygroscopic prisms which melt in a sealed tube at 38°C and liberate iodine at 225°C, leaving Kl. 

For a discussion of the fact that anhydrous KI3 is unstable at room temperature while the monohydrate is stable, see the references given under 11. 

A solution of 16.9 g. of CsCl in 85 ml. of water is prepared and treated with 8 g. Brg. The solution is then slightly heated in order to hold in solution theCsClBrs that is formed. The solution is then saturated with Clgj glitterir yellow crystals of CsBrClg form. These are filtered, washed with some water, and recrystallized from a small amount of water. A better yield is obtained if the CsCl is dissolved in only 45 ml. of water. The bromine is then added, red crystals of CsClBrg precipitate, and CI3 is then introduced without producing any harmful effects. 

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Polyiodides are specific polyhalides containing species such as I3", I7", Iq". Polyhalides often dissociate on heating the solid complexes. 

INTER HALOGEN COMPOUNDS AND POLYHALIDES There are four types of interhalogen compound ... 

The best known polyhalide is the triiodide ion, Ij", found when iodine dissolves in the aqueous solution of the iodide of a large unipositive cation (usually K ) ... 

Alkyl Halides, Polyhalides, a-Halo Ketones and Esters... 

Iodine Halides and Polyhalides. Iodine forms six weU-defined compounds with the other haUdes (115,116). These compounds are readily formed by direct reaction of the two halogens (117). 

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... 

Pure NI3 has not been isolated, but the structure of its well-known extremely shock-sensitive adduct with NH3 has been elucidated — a feat of considerable technical virtuosity.Unlike the volatile, soluble, molecular solid NCI3, the involatile, insoluble compound [Nl3.NH3] has a polymeric structure in which tetrahedral NI4 units are comer-linked into infinite chains of -N-I-N-I- (215 and 230 pm) which in turn are linked into sheets by I-I interactions (336 pm) in the c-direction in addition, one I of each NI4 unit is also loosely attached to an NH3 (253 pm) that projects into the space between the sheets of tetra-hedra. The stmcture resembles that of the linked Si04 units in chain metasilicates (p. 349). A further interesting feature is the presence of linear or almost linear N-I-N groupings which suggest the presence of 3-centre, 4-electron bonds (pp. 63, 64) characteristic of polyhalides and xenon halides (pp. 835-8, 897). 

The preparative utility of such reactions is, however, rather limited, and neither ICl or IBr has been much used except to form various mixed polyhalide species. Compounds must frequently... 

Typical examples of linear (or nearly linear) triatomic polyhalides are in Table 17.15 the structures are characterized by considerable variability of interatomic distances and these distances are individually always substantially greater than for the corresponding diatomic interhalogen (p. 825). Note also that for... 

Cl-I-Br] the I-Cl distance is greater than the I-Br distance, and in [Br-I-I] I-Br is greater than I-I. On dissociation, the polyhalide yields the solid monohalide corresponding to the smaller of the halogens present, e.g. CsIClj gives CsCl and ICl rather than Csl + Clj. Likewise for CsIBrCl the favoured products are CsCl(s) + IBr(g) rather than CsBr(s) + ICl(g) or Csl(s) + BrCl(g). Thermochemical cycles have been developed to interpret these results. 

Penta-atomic polyhalide anions [XY4] favour the square-planar geometry (D4h) as expected for species with 12 valence-shell electrons on the central atom. Examples are the Rb+ and Cs+ salts of [C1F4], and KBrp4 (in which Br-F is 189 pm and adjacent angles F-Br-F are 90 ( 2°). The symmetry of the anion is slightly... 

Hepta-atomic polyhalide anions are exemplified by BrFe" (K+, Rb+ and Cs+ salts) and IFfi- (K+, Cs+, NMe4+ and NEu+ salts). The... 

The propensity for iodine to catenate is well illustrated by the numerous polyiodides which crystallize from solutions containing iodide ions and iodine. The symmetrical and unsymmetrical 13 ions (Table 17.15) have already been mentioned as have the I5- and anions and the extended networks of stoichiometry (Fig. 17.12). The stoichiometry of the crystals and the detailed geometry of the polyhalide depend sensitively on the relative concentrations of the components and the nature of the cation. For example, the linear ion may have the following dimensions ... 

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... 

Fig. 9. Bromination of aromatic ethers with several polyhalides...

The reaction of acetanilides with tetraalkylanunonium polyhalides, such as TBA Br3, BTMA Bf3, or BTMA Br2Cl in dichloromethane-methanol at room temperature gave bromo-substituted acetanilides in good yields, respectively. These bromination of acetanilides have usually given predominantly the corresponding p-bromo derivatives (Fig. 12) (ref. 17). Furthermore, the reaction of acetanilides with BTMA Br3 in acetic acid in the presence of ZnCl2 gave polybromo-substituted acetanilides which was not obtained from the reactions in dichloromethane-methanol (ref. 18). 

Free-radical mechanisms are mostly found in pyrolyses of polyhalides and of primary monohalides,though they also have been postulated in pyrolysis of certain carboxylic esters/ Much less is known about these mechanisms and we shall not consider them further. Free-radical eliminations in solution are also known but are rare. ... 

Dihalides are reduced by Cr(II) to the corresponding olefin ". Allylic and benzylic halides and polyhalides are reduced more readily than simple alkyl halides, but even the latter are readily reduced by an ethylenediamine complex ofCr(ri) . ... 

Much of the current research into halogen bonding involves organohalogen or inorganic halide acceptors. However, the dihalogens (X2) and inter halogens (XY) continue to attract attention. This review will focus specifically on structural and theoretical studies of such systems over the past decade. While polyhalide anions are closely related to the neutral donor-acceptor concepts that are the focus here, they will be discussed only when necessary to elaborate on the central theme of the review. 

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