Halide ion

2 OXIDATION OF INORGANIC COVALENT SPECIES 2.2.1 Halide ions 

The oxidation of iodide ion by aqueous chromic acid at low acidity is very slow but is subject to marked enhancement in rate on addition of ferrous ion.  

Iodide ion is very slowly oxidised by Fe(III) and the iodide must therefore be oxidised by some species other than Cr(VI) or Fe(III). For a large excess of iodide, the stoichiometry has been reported to be  

This is an example of an induced reaction, which is the subject of the next chapter of this book. That step (11) is involved rather than 

It is assumed that Cr(IV) reacts quickly with Mn(II) but slowly with iodide ion for otherwise the induction factors would not be as found. 

As will be discerned from a perusal of this chapter, the reactions of halide ions, fluoride, chloride, bromide and iodide and the pseudo-halide ions azide, and nitrite, among others, are presented here. In addition, some elimination reactions (hydroxide nucleophile/base) and several organometallic species are discussed. We have also mentioned briefly the phase transfer catalyzed initiation of several addition polymerization processes. 

All four commonly occurring halide ions (fluoride [1-5], chloride [5—11], bromide [5, 8-10], and iodide [5, 7-9, 10, 12-15] have been phase-transferred and in the process, quaternary ions [1, 6-8, 10, 12-15], crowns [2, 4, 8, 9, 13], cryptates [3, 13] and resins [5] have all been utilized. Most of the processes reported are essentially Finkelstein reactions [16]. In a typical phase transfer of fluoride utilizing crown ether as catalyst, an acetonitrile solution of benzyl bromide is stirred with a catalytic amount of 18-crown-6 and solid potassium fluoride. The product, benzyl fluoride (see Eq. 9.1), is isolated in quantitative yield [2]. 

In liquid-liquid phase transfer processes, either a quaternary ammonium ion [6,7] or a crown ether [9] is utilized to solubilize the halide in a nonpolar solution of the substrate. The reservoir in such cases is an aqueous solution of the alkali metal halide. 

One interesting variant of the phase-transfer synthesis of chlorides involves the use of concentrated aqueous HCl as chloride reservoir. Water insoluble primary alcohols were stirred and heated with HCl in the presence of hexadecyltributylphosphonium bromide. The reactions were slow but ultimately yielded well [11]. The data are included in Table 9.1. 

One might legitimately question the advantage of the phase-transfer method in the synthesis of halides, particularly iodides, when the Finkelstein reaction of sodium iodide in acetone with an organic substrate is an efficient, high yield reaction. Where comparative data are available [17], there seems to be little practical advantage to the phase transfer method. It should be borne in mind, however, that the phase transfer technique offers the possibility of using solvents other than acetone or 2-butanone in these reactions and halides other than iodide can obviously be used. The latter point is particularly important in the case of fluoride ion, for which fewer alternatives exist. 

Chloride ions are comparatively weak nucleophiles and do not react with azoles. In general, there is also no interaction of halide ions with azolium compounds. 

Benzimidazole 3-oxides, e.g. (245), react with phosphorus oxychloride or sulfuryl chloride to form the corresponding 2-chlorobenzimidazoles. The reaction sequence involves first formation of a nucleophilic complex (246), then attack of chloride ions on the complex, followed by re-aromatization involving loss of the /V-oxide oxygen (247— 248). 

Reactivity of Five-membered Rings with Two or More Heteroatoms 

A summary of results reported in the literature is given in Table III. Most of these have been derived from analyses of single diffraction curves and only a limited number are based on diffraction data for solutions of different metal ion concentration and halide to metal ratios, which are needed in order to determine structures of individual complexes. The values obtained are, therefore, usually averages over the different complexes, that may be present, and give only limited information on coordination geometry. 

For some heavy metal ions, which form strong halide complexes with relatively well-separated regions of existence, diffraction measurements can be done on solutions in each of which one of the complexes is dominant. The complete structures of the individual complexes can 

Since water molecules also form part of the complexes a more correct formula would be  

Literature values for stability constants (153) have usually been determined for solutions much less concentrated than those needed for diffraction measurements, and the values for these solutions have to be checked by other methods. For the thallium(III) bromide complexes the stability constants for the concentrated solutions used [1-2.6 M in Tl(III)], were derived from Tl-205 NMR shift measurements. The fraction of Tl(III), bonded in each of the complexes calculated from these constants as a function of the chloride concentration (Fig. 17), shows 

For those of the complexes that appear as discrete units also in crystals, T1(H20)63 +, TlBr3(H20)2, and TlBr4 , no significant differences in bond lengths were found between the complexes in the solid and the liquid phase (127, 128). 

Fluoride ion attacks the sulfur atom in 2,3-diphenylthiirene 1,1-dioxide to give ck-1,2-diphenylethylenesulfonyl fluoride (23%) and diphenylacetylene (35%). Bromide or iodide ion does not react (80JOC2604). Treatment of S-alkylthiirenium salts with chloride ion gives products of carbon attack, but the possibility of sulfur attack followed by addition of the sulfenyl chloride so produced to the alkyne has not been excluded (79MI50600). In fact the methanesulfenyl chloride formed from l-methyl-2,3-di- -butylthiirenium tetrafluoroborate has been trapped by reaction with 2-butyne. A sulfurane intermediate may be indicated by NMR experiments in liquid sulfur dioxide. 

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Mercury(II) chloride, HgC, corrosive sublimate, m.p. 280 C, b.p. 302"C. Essentially covalent material (Hg plus CL Hg plus aqua regia). Forms complex halide ions, e.g. (HgCU) (HgCL)" in excess HCl and forms complexes. Very poisonous. 

In general, anions are less strongly hydrated than cations, but recent neutron diffraction data have indicated that even around the halide ions there is a well defined primary hydration shell of water molecules, which, in... 

To maintain charge neutrality, additional halide ions (Cl in our example) have to migrate inside the pit tluis increasing tire local chloride concentration and a chloro-complex is fonned. 

ENTHALPY DATA FOR HALIDE ION FORMATION IN AOLEOL S SOLI DON... 

The formation of halatefV) and halide ions by reaction (11.4) is favoured by the use of hot concentrated solutions of alkali and an excess of the halogen. 

Chlorine, bromine and iodine form halic(V) acids but only iodic(V) acid, HIO3, can be isolated. Solutions of the chloric) V) and bromic) V) acids can be prepared by the addition of dilute sulphuric acid to barium chlorate(V) and bromate(V) respectively, and then filtering (cf. the preparation of hydrogen peroxide). These two acids can also be prepared by decomposing the corresponding halic(I) acids, but in this case the halide ion is also present in the solution. 

The solid anhydrous halides of some of the transition metals are often intermediate in character between ionic and covalent their structures are complicated by (a) the tendency of the central metal ion to coordinate the halide ions around it, to form an essentially covalent complex, (b) the tendency of halide ions to bridge, or link, two metal ions, again tending to covalency (cf. aluminium chloride, p. 153 and iron(III) chloride, p. 394). 

Indication of the presence of a given halide ion can be obtained by the series of tests given in Table 11.4. Confirmatory tests can then be performed. 

Addition of halide ions to aqueous copper(II) solutions can give a variety of halo-complexes for example [CuCl4] (yellow square-planar, but in crystals with large cations becomes a flattened tetrahedron) [CuClj] (red, units linked together in crystals to give tetrahedral or distorted octahedral coordination around each copper). 

The insoluble halides can be prepared by adding the respective halide ion to silver ions ... 

Azide ion Alkyl halide Alkyl azide Halide ion... 

Carbon is partially bonded to both the incoming nucleophile and the departing halide at the transition state Progress is made toward the transition state as the nucleophile begins to share a pair of its electrons with carbon and the halide ion leaves taking with it the pair of electrons m its bond to carbon... 

Step 1 The alkyl halide dissociates to a carbocation and a halide ion... 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

With primary alcohols Ihe nexl slage is an 8 2 reaclion m which Ihe halide ion bro mide for example displaces a molecule of water from Ihe alkyloxonium ion... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

Nucleophilic aliphatic substitution (Chapter 8) Reaction m which a nucleophile replaces a leaving group usually a halide ion from sp hybridized carbon Nucleophilic aliphatic substitution may proceed by either an S l or an Sfj2 mechanism... 

Substitution nucleophilic unimolecular(SNl) mechanism (Sec tions 4 9 and 8 8) Mechanism for nucleophilic substitution charactenzed by a two step process The first step is rate determining and is the ionization of an alkyl halide to a carbocation and a halide ion... 

Halex reaction Halfan [36167-63-2] Half-life data Half-lives Halftones Halide glasses Halide ions Halides... 

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