Oxidations by the Halogens

Elementary fluorine or chlorine may act drastically on a number of covalent compounds, forming products in which the only bonds remaining are eiement-to-halogen bonds. Such reactions are called halogenolyses the chlorinolysis of carbon diselenide and the fluorinolysis of ethane are typical  

Considering in more detail the oxidation of iodide by bromine, we may assume that it does not proceed through preliminary dissociation of the Br2 molecule into atoms (for this requires 46 kcal per mole, whereas the oxidation takes place without excitation). A course of reaction in which the iodide ion attacks the Br2 directly is far more likely the reaction as represented below is then a pair of nucleophilic displacements by iodide 

The important oxidation of thiosulfate ion to tetrathionate (S 0 ) has been shown to proceed in an analogous fashion  

Thus one might regard the S4OJ- ion as the dimer of the oxidized form of SjO just as I is the dimer of the oxidized form of I-. 

The basic (nucleophilic) reagents in the reactions of the preceding paragraph were the iodide and thiosulfate ions, respectively. If chlorine, bromine, or iodine is dissolved in aqueous alkali, the halogen color disappears, and an apparent self-oxidation takes place. Half of the halogen is converted to the — 1 state, the remainder to the +1 state (as a hypo-halous acid, HOX, or its ion). Such a reaction is similar to those above  

---

Mn ". 1.18 V) is less than that of magnesium, although the lirst and second ionization potentials are closely similar, due to the higher heat of sublimation of manganese. However, manganese is oxidized by the halogens. H+ or even H 0 to the dipositive state. 

The properties of Mo and W are similar. Both have very high melting points and enthalpies of atomization Table 5.2 and Figure 22.2). The metals are not attacked in air at 298 K, but react with O2 at high temperatures to give MO3, and are readily oxidized by the halogens (see later). Even at 298 K, oxidation to M(VI) occurs with F2 (equation 22.27). Sulfur reacts with Mo or W (e.g. equation 22.28) other sulfide phases of Mo are produced under different conditions. 

The metal is slowly oxidised by air at its boiling point, to give red mercury(II) oxide it is attacked by the halogens (which cannoi therefore be collected over mercury) and by nitric acid. (The reactivity of mercury towards acids is further considered on pp. 436, 438.) It forms amalgams—liquid or solid—with many other metals these find uses as reducing agents (for example with sodium, zinc) and as dental fillings (for example with silver, tin or copper). 

During the past 20 y numerous other highly coloured halogen cations have been characterized by Raman spectroscopy. X-ray crystallography, and other techniques, as summarized in Table 17.18. Typical preparative routes involve direct oxidation of the halogen (a) in the absence of solvent, (b) in a solvent which is itself the oxidant (e.g. AsFs) or (c) in a non-reactive solvent (e.g. SO2). Some examples are listed below ... 

Monatomic anions, such as the Cl- ions in sodium chloride and the O2- ions in quicklime (CaO), are named by adding the suffix -ide and the word ion to the first part of the name of the element (the stem" of its name), as shown in Table D.l thus, S2 is a sulfide ion and O2 is an oxide ion. There is usually no need to specify the charge, because most elements that form monatomic anions form only one kind of ion. The ions formed by the halogens are collectively called halide ions and include fluoride (F ), chloride (Cl-), bromide (Br-), and iodide (I ) ions. 

Mercapto-l,2,4-thiadiazoles are oxidized, by 2N nitric acid at 50-60°, by chromic acid mixture, chlorine, or potassium permanganate to the disulfides, which are reconvertible to the thiols by reduction with sodium amalgam and alcohol.71,72,91 168 Use of excess chlorine in aqueous acetic acid results in replacement of the mercapto group by the halogen.168... 

Lithium Arsenide, Li3As, is formed when a mixture of the elements in suitable proportions is heated to redness, or when lithium arsenate is reduced by heating with charcoal in an electric furnace.8 It is a dark brown crystalline substance of great reactivity. It is attacked by the halogen elements at the ordinary temperature with incandescence. It burns vividly when heated in oxygen it reacts violently with nitric acid, and at a comparatively low temperature it reduces many metallic oxides. 

Of the reactions of nitric oxide with the halogens, the one with chlorine has been most extensively studied. Apparently, this reaction was first reported by Gay Lussac shortly before the Battle of Waterloo. Although almost all the early work (prior to 1938) was interpreted in terms of a termolecular reaction to form nitrosyl chloride... 

Since indium has only three electrons in its valence shell, il is an electron acceptor. Indium mhaiides include the triHuoride. trichloride. Uibromide. and triiodide. They can he prepared by heating the metal or oxide in the halogen acid, or in the case uf the trichloride and... 

This compound is of interest not only in analysis, but because by limited oxidation with the halogens it yields a unipositive ion containing tnvalent... 

Atom transfer radical polymerization (ATRP) [52-55]. Active species are produced by a reversible redox reaction, catalyzed by a transition metal/ligand complex (Mtn-Y/Lx). This catalyst is oxidized via the halogen atom transfer from the dormant species (Pn-X) to form an active species (Pn ) and the complex at a higher oxidation state (X-Mtn+1-Y/Lx). 

The reaction of aryl and hetaryl halides with the nitrile-stabilized carbanions (RC -CN) leads to derivatives of the type ArCH(R)CN. Sometimes, however, dimeric products of the type ArCH(R)CH(R)Ar are formed (Moon et al. 1983). As observed, 1-naphthyl, 2-pyridyl, and 2-quinolyl halides give the nitrile-substituted products, while phenyl halides, as a rule, form dimers. The reason consists of the manner of a surplus electron localization in the anion radical that arises upon replacing halogen with the nitrile-containing carban-ion. If the resultant anion radical contains an unpaired electron within LUMO covering mainly the aromatic ring, such an anion radical is stable, with no inclination to split up. It is oxidized by the initial substrate and gives the final product in the neutral form, Scheme 1-7 ... 

Having noted the regularities exhibited by the halogens in their combinations with hydrogen, it is interesting to consider the compounds formed by the halogens and oxygen. Table 11.11 lists the known oxides. 

Hydrogen peroxide is also the oxidant in the halogenations of alkanes under Gif conditions. In these systems, developed by Barton and his coworkers312, alkanes are selectively transformed into alkyl chlorides or bromides by polyhaloalkanes and H202 in the presence of FeCypicolinic acid catalyst in pyridine/acetic acid solvent313-315. It has clearly been established that the reaction mechanism does not involve a free-radical intermediate. 

---