Halides/halide ions

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 ... 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

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... 

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